Simulation of a Launch and Recovery of an UUV to an Submarine - …/Menu/... · UUV to a submarine. Hence, the goal of this study is to investigate whether a recovery of a UUV at

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AcknowledgmentsI wish to extend the outermost thanks to ASC Pty Ltd and Hans Wicklander for givingme the opportunity to conduct this study at location in Adelaide, Australia. Alsothanks to Patrick Marshallsay, Sean Williams and Per Dahlander who guided me withtheir expertise in the field. Christine Philippou without you I would have been lost inthe office. Richard Hejde, Roger Carlsson, Anders Folbert and Paul Plant for yourinput and friendship.

Jens Fellenius and Lynda Curtis for your hospitality, friendship and above all helpwith my living arrangements. Marucs Leach and Brant Oxlade for teaching me how tosurf. My family for their support throughout my studies with a special thought to mylate father Henry Fedor, who tragically passed away during the course of this study,may you rest in peace.

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AbstractStudies of the flow field surrounding submarines are a common practise, usually to beable to lower the underwater signature of the vessel. In this report the study hasfocused on mapping the forces the flow field and boundary layer exerts on anadjacent, much smaller, vehicle trying to dock with the submarine. From requirementsdefined by ASC Pty Ltd the recovery procedure have to be conducted while at speed.

The submarine was modelled and simulated in a CFD tool with the UnmannedUnderwater Vehicle at different locations along the hull of submarine. The simulationshowed that the boundary layer and vortices surrounding the submarine are highlycomplex. With the CFD code and present computing power available at the time ofthis report it was impossible to accurate map the flow. However it is shown that theforces fluctuate almost chaotically and with current manoeuvrability technology andrecovery systems for Unmanned Underwater Vehicles it is highly unlikely that a safedocking could be conducted at speed.

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SammanfattningStudier av vattnets flöde kring ubåtar är vanligt förekommande, vanligtvis för attminska den akustiska signaturen hos farkosten. Denna studie fokuserar på att försökakartlägga den turbulens och de krafter som uppkommer i gränsskiktet och flödet kringen ubåt när en mindre farkost försöker docka till den. Enligt kravspecifikationendefinierad av ASC Pty Ltd så måste den tänkta dockningen ske i fart.

Ubåten har modellerats och simuleras i ett CFD-verktyg tillsammans med enobemannad undervattensrobot placerad på olika platser utmed ubåtens skrov.Resultatet från simuleringarna visade att gränsskiktet och turbulensen kring ubåten ärmycket komplext. Med den CFD kod och datorkraft som fanns tillgänglig förförfattaren vid tillfället för studien var det omöjligt att kartlägga flödet i detalj. Detvisar sig dock att krafterna har en näst intill kaotisk fluktuation och i relation meddagens manövreringsförmåga hos obemannade undervattensfarkoster samt dedockningssystem som finns tillgängliga är det högst osannolikt att en säker dockningskulle kunna utföras i fart.

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Table of contents1 INTRODUCTION......................................................................................................................... 9

1.1 SCOPE .........................................................................................................................................91.1.1 Limitations ........................................................................................................................ 9

2 APPROACH TO THE FLUID DYNAMIC PROBLEM......................................................... 11

2.1 THE MODEL .............................................................................................................................. 112.2 THE DARPA SUBOFF PROJECT .............................................................................................. 12

2.2.1 Axisymmetric Hull .......................................................................................................... 122.2.2 Sail..................................................................................................................................12

2.3 GOVERNING EQUATIONS ........................................................................................................... 122.4 FLOW AROUND A SUBMARINE ................................................................................................... 13

2.4.1 Boundary layer ............................................................................................................... 132.4.2 Tip and Junction flows at the sail ................................................................................... 13

2.5 CHOICE OF POSITIONS ............................................................................................................... 14

3 CFD AND THE STAR-CCM+ CODE ...................................................................................... 17

3.1 STAR CCM+ (THE CODE) .......................................................................................................... 173.1.1 The mesh ......................................................................................................................... 173.1.2 AMG SIMPLE Solver...................................................................................................... 19

3.2 REYNOLDS-AVERAGED NAVIER-STOKES (RANS).................................................................... 203.3 TURBULENCE MODELS.............................................................................................................. 213.4 DETACHED EDDY SIMULATION (DES)...................................................................................... 223.5 UNCERTAINTY ANALYSIS OF THE PROBLEM AND CODE ............................................................. 22

3.5.1 Scaling of CFD model .................................................................................................... 243.5.2 Presentation and reduction of Data................................................................................ 263.5.3 Grid size and dependence study...................................................................................... 273.5.4 Deciding the Time-step for the DES simulation.............................................................. 293.5.5 Steady modelling of a unsteady problem ........................................................................ 30

4 SIMULATION PROCESS ......................................................................................................... 32

5 NUMERICAL PROCEDURE, RESULT AND DISCUSSION............................................... 335.1 FLOW FIELD IN GENERAL (TIME DEPENDENT ANALYSIS) ........................................................... 345.2 POSITION 1 [X = 0.14LSUB] ......................................................................................................... 345.3 POSITION 2 [X = 0.23LSUB] ......................................................................................................... 365.4 POSITION 3 AND 4 ..................................................................................................................... 39

6 UUVS AND LARS IN SHORT ..................................................................................................45

6.1 THE US NAVY UUV MASTER PLAN (UUVMP) ....................................................................... 456.2 UUV’S ...................................................................................................................................... 47

6.2.1 Navigation ...................................................................................................................... 476.2.2 Guidance and Communication ....................................................................................... 476.2.3 Propulsion and Endurance ............................................................................................. 486.2.4 Stability........................................................................................................................... 486.2.5 Control............................................................................................................................ 496.2.6 Categories....................................................................................................................... 526.2.7 Discussion....................................................................................................................... 53

6.3 LAUNCH AND RECOVERY SYSTEMS .......................................................................................... 536.3.1 Funnel/Cone Recovery Systems ...................................................................................... 536.3.2 Belly mounted Stinger / Buoy Vertical Pole ................................................................... 546.3.3 Universal Launch and Recovery Module........................................................................ 556.3.4 Sea Owl SUBROV........................................................................................................... 566.3.5 Boeing Torpedo mounted retractable arm...................................................................... 576.3.6 Reverse Funnel Recovery – Authors suggestion ............................................................. 58

7 CONCLUSION ........................................................................................................................... 59

8 FURTHER WORK..................................................................................................................... 60

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9 REFERENCES............................................................................................................................ 61

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NomenclatureSymbol Unit Description

[kg/m3] Density[Pa s] Viscosity[–] Under relaxation factor

t [s] Time stepi ju u [m/s] Turbulent inertia tensor

p [r/s2] Angular acceleration roll[r] Euler angle of roll

v , [u,v,w] [m/s] Velocity of fluidx , Lcell [m] Grid/Cell size

B [–] Centre of BuoyancyCD [–] Axial Force CoefficientCFL [–] Courant-Friedrich-LewyD [m] DiameterF [N] ForceFB [N] Buoyancy forceg [m/s2] GravityG, CoG [–] Centre of GravityIXX [m4] Mass moment of inertia, rollK2 [–] Form factor by JacksonLAR, LFR [m] Section LengthLsub [m] Length of submarineLUUV [m] Length UUVM [–] Metacentrep [Pa] Pressurep* [Pa] Guessed pressure

p´ [Pa] Correction pressure,Fluctuating component of pressure

pnew [Pa] Improved pressureRe [–] Reynolds numberSt [–] Strouhal number (~0.2)U∞ [m/s] Free stream velocity of fluidUmax [m/s] Maximum velocity of fluidV [m3] Cell volumev [m2/s] Kinematic viscosityzsail [m] Height of sailδ [m] Boundary layer thickness

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Acronyms and AbbreviationsADCP Acoustic Doppler Current ProfilerAPI Application Programming InterfaceAUV Autonomous Undersea VehicleAUVG Autonomous Undersea Vehicle GliderCAD Computer Aided DesignCFD Computational Fluid DynamicsDARPA Defence Advanced Research Projects Agency (US)DES Detached Eddy SimulationDSTO Defence Science and Technology OrganisationDTMB David Taylor Model BasinDTRC David Taylor Research CenterITTC International Towing Tank ConferenceLARS Launch and Recovery SystemLES Large Eddy SimulationNGS Next Generation SubmarineRAN Royal Australian NavyRANS Reynolds-Averaged Navier-StokesROV Remotely Operated VehicleUUV Unmanned Undersea VehicleUUVMP United States UUV Master Plan

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1 IntroductionThere are two leading trends in the submarine and military industry that are growingrapidly. The first one is to put more of the payloads outside the pressure hull on thenext generation submarines. The second one is to increase the safety of the personnelby using autonomous vehicles on dangerous tasks, such as mine counter measurementand Intelligence, Surveillance and Reconnaissance missions, rather than humans. Dueto this there is an interest in conducting an investigation of where and how anUnmanned Undersea Vehicle (UUV) could be recovered to a submarine.

A submarine displaces thousands of tonnes of water and is surrounded by turbulentfluid while travelling through the water. Why a study of the wake and the fluid aroundthe submarine is needed to fully comprehend the difficulties involved in recovering aUUV to a submarine. Hence, the goal of this study is to investigate whether arecovery of a UUV at speed is feasible, by its own control or with help from a Launchand Recovery System.

The investigation is performed by conducting a series of simulations of an UUVdocking to a submarine using a Computational Fluid Dynamics (CFD) tool. Using aCFD tool is cost efficient compared to full scale experiments but the results from thesimulations needs to be verified to some degree by experimental data.

1.1 ScopeThe goals of the study which is outlined on a requirement specification given byASC Pty Ltd to the author are as follows:

What the effect the boundary layer around the submarine, and the waketrailing the sail, has on the UUV.Where on the submarine is a recovery is most favourable?Whether these forces are so large that the submarine cannot compensate forthem itself?How a Launch and Recovery System should be constructed to accommodatethe need for a safe docking procedure.

The report focuses on the complexity of the flow surrounding a submarine and thedifficulties simulating it. The reader will get a general explanation how the flow fieldaround a generic submarine develops and later presented with the results from fullsized computer simulations with a UUV in close proximity to the Submarine.

A literature study containing detailed information about UUVs and Launch andRecovery Systems (LARS) is presented to the reader. In these chapters different typesof UUVs and LARS are identified and categorised. The study is conducted tohighlight present and past problems with UUV recovery procedures.

1.1.1 LimitationsThere are a few limitations that were decided on during the literature study;

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The submarine model that all simulations are conducted with is decided to besimilar to a normal sized diesel electric submarine or more specifically 87.12meters in length and 10.1 meters diameter.During the literature study it was identified that most submarines cannot, orare reluctant, to travel below 2 knots. At the same time most UUVs cannotexceed 5 knots. Therefore a single recovery speed of 3 knots is chosen.There was originally intended that a set of simulations where to be made fordifferent angles of attack for the submarine to the free streaming fluid.However only one set of simulations is made at a zero degree angle of attack.Because of limitations in the CFD tool only simulations of static problems aremade.

Further limitations and constraints concerning the modelling of the problem in theComputational Fluid Dynamics software will be explained in chapter 3.

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2 Approach to the Fluid Dynamic problemIn order to determine the best location for recovery it is necessary to know how a flowfield around a submarine behaves. One way of achieving this is to construct a scalemodel of the body in question and conduct a series of flow experiments. This is a timeconsuming, expensive but well-established approach. In recent decades however, withthe exponential increase of computing power and improvements in numericalalgorithms, Computational Fluid Dynamics (CFD) has gained increasingly favour.Using CFD an operator can relatively straightforward construct a virtual towing tankor wind tunnel. Used in conjuncture with Computer Aided Design (CAD) the operatorcan construct and modify complex geometries and perform a series of simulationscovering a wide range of flow conditions.

A warning hand is raised though; it is an easy pitfall to think that CFD is the solutionto all fluid dynamic problems. Still even with increasingly computing power a directnumerical solution of a problem will probably not be feasible until earliest 2080,(Spalart, 2000). To overcome this problem a lot of different approaches to model theturbulence and average solutions has seen the light of day.

2.1 The ModelThe DTMB model 5470 configured with bare hull and sail as the only appendage isused as the starting point. The aft control surfaces are omitted from the model as theyare considered not to have any significant effect on the UUV, whereas their inclusionwould negatively impact on the computer resources required.

Figure 1 show the geometry used during this study. The model used in the simulationsis scaled twenty times in order to mimic a diesel electric submarine displacing about7000 tons. Further discussion of the effect of scaling is contained in section 3.5.1. Thequestions asked and answered during this study is:

1. What is the most favourable location on the submarine to launch and recover aUUV?

2. What characteristics does the flow around submarine take at “recoveryspeed”?

3. What pressures and velocities act on the UUV in the turbulent wake andboundary layer of the submarine?

Question 1 and 2 can relatively easily be extracted from a steady-state simulationusing a CFD code. Question three however is more challenging. Ideally one woulduse an overset grid approach, (CFD-Online, the free CFD reference, 2006), whichwould allow the two bodies to move through the computational space independentlyof one another. Unfortunately, such facilities are not yet readily available incommercial CFD codes. Therefore, for each recovery position, a series of steady-statesimulations were performed with the UUV located at various positions along itstrajectory. The process was automated by writing a Java macro script to perform theprocessing required at each point on the trajectory, and running the varioussimulations sequentially in batch mode using a Python script to control the process.The procedure is described further in chapter 4.

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2.2 The DARPA SUBOFF ProjectIt is essential to be able to verify the results gained from the simulations. One way isto compare the results with experiments made in controlled environments, such as awind tunnel or towing basin. Therefore information is available in the public domainfrom experiments performed on hulls, airfoils and submarines. In this study theDARPA SUBOFF Project is chosen for validating purposes.

The Defence Advanced Research Projects Agency (DARPA) is the central researchand development organization for the United States Department of Defence. In theend of the 1980’s there was an initiative taken by DARPA to develop an experimentaldatabase for CFD code validation. The experiments were held at the David TaylorModel Basin (DTMB) in Bethesda, Maryland. Two models were built, DTMB Modelno. 5470 and 5471, which differed only in the location of the surface pressure taps.Model no. 5470 was designed for towing tank experiments while model no. 5471 wasdesigned for the wind tunnel. The details of the models and their configurations aredescribed in (Groves, Huang, & Chang, 1989).

2.2.1 Axisymmetric HullThe DARPA models have an axisymmetric hull with an overall length of 4.356 m anda maximum diameter of 0.508 m. The characteristic length used to reduce the resultsto non-dimensional form is 4.261 m or ~0.978 L. The hull is composed of a fore-body,a parallel middle-body, an after-body, and an aft-body cap of 1.016 m, 2.229 m, 1.111m and 0.095 m respectively, see Figure 1. Full geometrical details are contained in(Groves, Huang, & Chang, 1989). The coordinate system adopted in the present studyis shown in Figure 17.

Figure 1 – DTMB model no. 5470, Hull + Sail configuration

2.2.2 SailThe sail is located on the hull at top dead centre with its leading edge positioned atx = 0.924 m (.2121 Lsub) and trailing edge at x = 1.293 m (.2968 Lsub). A cap isattached to the top of the sail at height of 0.460 m (zsail), from the hull, and is a 2:1elliptical cross-sectional shape. The sail and cap profile are found in (Groves, Huang,& Chang, 1989).

2.3 Governing equationsThe governing equations for fluid flow, which describe the conservation of fluid massand momentum, are the equation of continuity and the Navier-Stokes equations. Thederivation of the Navier-Stokes equations begins with the conservation of mass,

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momentum and energy being written for an arbitrary control volume, and can befollowed in full in (Versteeg & Malalasekera, 2007). If we instead consider that wehave an incompressible Newtonian fluid with constant density, ρ, and constantviscosity, μ, then we can express the Navier-Stokes equations in its most general formby equation (12.1),

2p gtv v v v (12.1)

where v is the flow velocity vector and p the pressure. The Navier–Stokes equationsare strictly a statement of the conservation of momentum and to fully describe thefluid flow you need more information, boundary conditions etc. Regardless of theassumptions made, a statement of the conservation of mass is generally necessary.This is achieved through the mass continuity equation, given in its most general formfor an incompressible fluid in equation (12.2).

0v (12.2)

2.4 Flow around a submarineBefore delving into the numerical approach to fluid dynamics an overview of the flowaround a submarine is explained. When an object travels through water it displaceswater. This in turn constructs complex flow patterns around the submarine as wholebut also the obstacles that are attached to the hull, such as the tower, arrays etc.

2.4.1 Boundary layerIn this study all simulations are conducted at a Reynolds number is in the order ofRe ≈ 150E6 for the submarine, hence the submarine are surrounded by a turbulentboundary layer. The axisymmetric and slender shape of the submarine prevents theboundary layer from separation until it reaches the negative shaped after-body.

2.4.2 Tip and Junction flows at the sailJunction flow occurs when the boundary layer on a surface encounters an obstacleattached to that surface, (Simpson, 2001). The resulting flow fields are complex andfeature three-dimensional separating flow. The stream wise adverse pressure gradientscause the boundary layer to separate and form multiple horseshoe vortices. Figure 2shows a schematic view of a simplified junction flow and wing tip vortex shedding.The separation line that wraps around the sail has its origin at a stagnation point infront of the sail. The stagnation point is the separating point between the relativelyundisturbed flow upstream of the obstacle, and the complex flow region that developsaround and downstream of the obstacle. Generally these vortices are highly unstableand break up to form a highly turbulent wake downstream of the obstacle. Thevortices that trail from the top of the sail arise from separating flow that occurs as aresult of the adverse pressure gradient downstream and upstream flow regions. Thesevortices are referred to in the following discussion as “wing-tip” vortices.

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Figure 2 – Simplified illustration of the flow around the sail

2.5 Choice of positionsInitially three candidate positions where selected. After assessing the output data fromthese simulations, the strength of the tip vortices was found to be greater thanexpected and it became apparent that further information of the wake characteristicswas needed. Therefore the investigation was extended to include one extra recoveryposition aft of the sail, and also to include an un-steady simulation with no UUVpresent, in order to fully understand the wake characteristics and flow aft of the sail.The resulting four recovery positions are shown in Figure 3.

Figure 3 – The positions on the Submarine where recovery simulations where made.

It was concluded at an early stage that it would be inadvisable to consider anyrecovery positions in the aft part of the submarine due to the adverse flowcharacteristics present in this region. As shown in (Huang, et al., 1992) the stern partof the submarine is dominated by a quickly thickening boundary layer and two contra-rotating vortices. The thickening boundary layer is the result of the separation of theboundary layer that occurs due to the adverse pressure that develops from the point

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that separates the middle body and aft part of the submarine, as explained inchapter 2.4.1.

Figure 4 – Presentation of velocity profiles and an approximate illustration of the thickeningboundary layer in the aft region. Based on data extracted from velocity profiles at

x = [75 80 85 90 95] per cent of model length.

Figure 4 further shows that the boundary layer thickness upstream of the afterbody ofthe submarine does not exceed 0.6-0.9 meters, which is in good agreement withpredictions made by the Power-Law theory for turbulent boundary layers on twodimensional flat plates, as formulated by Prandtl (White, 1991). This expressionestimates a boundary layer thickness of about 0.55 meter in this region. The powerlaw relationship takes the form:.

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0.16 0.55mRex

x (12.3)

Further upstream within the wake the sail, the flow is dominated by two contrarotating horseshoe vortices close to the hull, accompanied by two tip vortices. Thenature of the flow within this region is shown clearly in Figure 5, and the existingvortex structures could potentially cause a problem for the recovery of UUV’s withinthis region. Since there are clear logistical advantages in launching and recovering aUUV within this region, a decision was made to investigate the resulting flow forceson a UUV deployed within this region.

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Figure 5 – Vortices generated by the sail at x = 0.5L, illustrated as in-plane velocity i.e. u = 0. Thewhite streamlines together with black arrows define the direction and the colour the velocity of

the fluid.

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3 CFD and the Star-CCM+ codeComputation Fluid Dynamics provides a means of simulating flows of moderatecomplexity using computational methods, generally without recourse to experimentaltechniques. A CFD code comprises three main elements: a pre-processor, a solver anda post-processor:

Pre-processorPre-processing is the part where an operator defines the geometry of theregion; the computational domain of the model. Furthermore a discretizationof the problem is necessary because the partial differential equations thatdescribes a fluid flow are non linear and an analytical solution is almost neverpresent, which is why the domain is divided into a number of smaller sub-domains, often referred to as a mesh (or grid) of cells. The problem can hereonafter be solved numerically over the grid. A selection of what physical orchemical phenomena that needs to be modelled has to be made and finallydefine the fluid properties and appropriate boundary conditions.

SolverThe most well-established numerical solution technique is the finite volumemethod. Its numerical algorithm first discretizes the integral form of thegoverning equations and applies the discrete versions to each cell. Theobjective is to obtain a set of linear algebraic equations, with the total numberof unknowns in each equation system corresponding to the number of cells inthe grid. The resulting equations are then solved by an iterative method.

Post-ProcessorA Post-processor gives the ability to visualize the solution by different kindsof plots, both 2D and 3D. Also many CFD codes include animation tools fordynamic result display.

3.1 Star CCM+ (the code)The code used during this study is the Star CCM+ (version 3.04.008) fromCD-Adapco. Star CCM+ use an “Algebraic MultiGrid Semi-Implicit Method forPressure-Linked Equations solver” (AMG SIMPLE) when solving the discretizedlinear system iteratively. Star CCM+ also provides a powerful semi-automaticmeshing tool which allows the operator to generate both surface and volume mesh.The mesh is automatically generated upon the operator’s inputs and are valid and ofgood quality. Furthermore Star CCM+ has the ability to automatically wrap surfacesin order to ensure a complete closed model.

3.1.1 The meshThe volume mesh is the mathematical description of the space or geometry of theproblem, (CD-adapco, 2008). It consists of three basic mesh entities, vertices, facesand cells. Where, a vertex is a point in space defined by a position vector. A facecomprises an ordered collection of vertices such that they define a surface in three-dimensional space and a cell is an ordered collection of faces that define a closedvolume in space, se figure 6.

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Figure 6 – Illustration of vertex, face and cell respectively

One of the hardest and most time consuming parts of a CFD simulation is the meshgeneration. The refinement of the mesh has a major effect on the accuracy of thesolution and one could say that the greater amount of cells the better chance ofobtaining a good result. Though the denser the mesh the more computing power andtime it takes to generate and calculate the problem. Furthermore the mesh also needsto be valid, no open faces, and of high quality to produce an accurate solution. A nonuniform grid is almost always the optimal one with a denser mesh at complex areasand a coarser one in other areas. An example of a non uniform grid is shown in Figure7.

Figure 7 – Non uniform grid

However, it is getting more common with intelligent meshing tools that are able to,with relatively little human intervention, construct high quality valid meshes.

Star CCM+ offers three different types of volume mesh; tetrahedral, polyhedral andtrimmed mesh. The tetrahedral meshing model use tetrahedral shaped cells and is themodel that is fastest and uses the least amount of memory out of the three provided.However, the tetrahedral model needs approximately five to eight times more cells toproduce the same accuracy as the equivalent polyhedral or trimmed cell mesh.

The polyhedral meshing model use polyhedral shaped cells and provides the operatorwith a balanced solution for complex mesh generations problems. As for thetetrahedral model, the polyhedral mesh model is directly dependant on the quality ofthe starting surface triangulation. In other words, a bad quality starting surface willlead to a bad quality volume mesh.

The trimmed cell mesher provides a robust method of producing a high quality gridthat consists of predominantly hexahedral cells with trimmed cells next to the surface.It combines a hexahedral mesh with automatic curvature and proximity refinementand, most importantly, surface quality independence in a single meshing scheme. Ofthe three models the trimmer meshing model is more likely to produce a good quality

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mesh for most situations, which is why it was chosen for this study. In Figure 8, thethree types of mesh models are illustrated.

Figure 8 – Three types of volume meshing, from left to right; tetrahedral, polyhedral andtrimmer respectively.

Image Copyright© CD-Adapco.

3.1.2 AMG SIMPLE SolverThe SIMPLE solver was originally put forward by Patankar and Spalding in 1972 andis essentially implementing a guess-and-correct procedure. A short description of thealgorithm will now follow, for a complete derivative of the SIMPLE algorithm see(Versteeg & Malalasekera, 2007). A SIMPLE calculation is initiated by first guessinga pressure field p*. Then by using the discretized momentum equation and the guessedpressure field it yields the velocity components v*, where v* is the guessed velocityfield vector in a Cartesian system. After which a correction p’ as the differencebetween the correct pressure field p and the guessed pressure field p* is defined, sothat;

*p p p (13.1)

A similar definition is done for the velocity field with v’. The correct pressure andvelocity fields in the governing equations are substituted for equation (13.1) and byusing the discretized continuity equation the mass fluxes at all faces are calculated.The continuity equation is then rewritten so the pressure correction coefficient p’ canbe extracted. The new pressure field is then calculated by correcting the “old” onewith the newly extracted correction factor. However, the solution is prone to divergeunless some under-relaxation is used during the iterative process so the new,improved, pressure pnew are obtained with;

*newp p p (13.2)

Where, ω is the under-relaxation factor for pressure. Usually the under-relaxationfactor is changed over the total time of simulation. A ω equal to one is often too largewhen the guessed pressure field p* is far from the final solution, which is why theoperator normally start with a low under-relaxation factor and gradually increase it toone. With the new pressure field a corrected mass flux for the faces are calculated,and from the mass flux the corrected cell velocities can be obtained with;

v*new

p

V pv va

(13.3)

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*p p p (13.1)


*newp p p (13.2)


v*new

p

V pv va

(13.3)

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*p p p (13.1)


*newp p p (13.2)


v*new

p

V pv va

(13.3)

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Where, p’ is the gradient of the corrected pressure, vpa is the vector of central

coefficients for the discretized linear system representing the velocity equations and Vis the cell volume.

Star CCM+ uses a Multigrid method spanning the SIMPLE solver to speed up theprocess of finding a solution. Instead of visiting each cell in sequence and updatingthe values of pressure and velocity a Multigrid solver agglomerates cells to formseveral coarse grid levels. It then performs a number of cycles with the SIMPLEsolver over the original fine layer (known as smoothing). After which the solution istransferred to the next in line coarser level (known as restriction) where the cycling isrepeated and yet again the residuals are transferred to the next in line coarser gridlevel. The restriction process continues until the Multigrid solver reaches the coarsestlevel, where it turns and repeats the process of transferring solutions and performingthe cycles, but to the finer level (known as prolongation). The solution is prolongateduntil the finest level is reached and the whole process is repeated until satisfactoryconvergence is reached, se Figure 9.

Figure 9 – Schematics of a Multigrid process

There are two branches of Multigrid methods; Full Geometric Multigrid andAlgebraic Multigrid. Star CCM+ use the latter branch and it has the advantage ofperforming the agglomeration without taking the geometry into account from thefinest level. In other word, the new coefficient matrix representing the coarser levelsconsists of specially chosen coefficients from the original grid which means that nonew discretization is required and the grid does not need to be stored in the virtualmemory.

3.2 Reynolds-Averaged Navier-Stokes (RANS)The numerical solution of the Navier-Stokes equations for turbulence in anincompressible Newtonian fluid with constant viscosity is extremely difficult to solvefor and it take an almost indefinite fine mesh to find a solution which means that thecomputational time becomes infeasible for calculation. To counter this, time-averagedequation such as the Reynolds-Averaged Navier-Stokes equations are used inpractical CFD applications when modelling turbulent flows. The RANS equations canbe obtained by decompose the velocity and pressure into a mean and fluctuatingcomponent, equation (13.4);

p P pv v v

(13.4)

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where v and v´ are the mean (time-averaged) and fluctuating velocity vectors in aCartesian system, and P and p´ are the mean and fluctuating component of pressure.By substituting these expressions into the continuity equation, (12.2), and take thetime average of the entire equation we get (13.5),

0v (13.5)

where, v is the time-averaged velocity component in a Cartesian system. If we nowattempt the same procedure and substituting (13.4) into the nonlinear Navier-Stokesequations, (12.1), and use the time average, we obtain equation (13.6).

2i j

j

p g u ut xv v v v (13.6)

Thus the mean momentum equation has an additional term involving the turbulentinertia tensor jiuu also known as the Reynolds Stress tensor. This term is nevernegligible in any turbulent flow and is the source of the analytical difficulties becauseits analytical form is not known a priori. Essentially the time averaging has added ninenew unknown variables (tensor components) that can be defined only by detailedknowledge of the turbulent structure, which is not known. The problem being that theReynolds stresses are not only related to fluid physical properties but also to localflow conditions such as; velocity, geometry, surface roughness and upstream history,and no physical laws are available to resolve this dilemma, (White, 1991).

One way to get around this dilemma is to model the turbulence by using anappropriate turbulence model.

3.3 Turbulence ModelsAs discussed above, the challenge with acquiring a high-quality solution by using theRANS equations is to model the Reynolds stress tensor satisfactory. This is doneusing turbulence models. It is widely acknowledged that turbulence models areinexact representations of the physical phenomena being modelled and no singleturbulence model is the best for every flow simulation, (CD-adapco, 2008). StarCCM+ come bundled with four major classes of turbulence models

Spalart-Allmaras models are a good choice for applications that has mildseparation and a largely attached boundary layer. A typical example is a flowover a wingK-Epsilon models provide a good compromise between robustness,computational cost and accuracy. Generally well suited for applications thatcontain complex recirculation, with or without heat transfer.K-Omega models are similar to K-Epsilon models but have seen mostapplication in the aerospace industry, and are therefore recommended as analternative to the Spalart-Allmaras models for similar types of applications.Reynolds stress transport models are the most complex and computationallyexpensive models of the four. They are recommended for situations in whichthe turbulence is strongly anisotropic.

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From works by and discussion with Dr. Patrick Marshallsay at ASC Pty Ltd theAbe-Kondoh-Nagano (AKN) Low-Reynolds K-Epsilon model was chosen as theturbulence model for the simulations during this study. The AKN model is developedto be used when calculating complex turbulent flow with separation and heat transfer.The simulations done by Dr. Marshallsay showed that the AKN model performed wellin comparison with other turbulence models.

3.4 Detached Eddy Simulation (DES)RANS together with turbulence models suffers from the inexact representation of thetime-dependent physical phenomena. A different approach is using Detached EddySimulation, which is a hybrid modelling approach that combines features ofReynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES)1. DESuses a RANS solution in regions close to solid boundaries as well as where theturbulent length scale is smaller than the grid dimension and treat the rest of the flowin a LES manner. Therefore a DES model is not as demanding as a pure LES andreduces the cost of computation. However just as in any CFD model DES is not ananswer to all turbulence problems; it must be cautioned that the creation of suitablegrids is a difficult task.

Hence, it needs a lot of careful preparation to run a DES simulation with special caretaking when considering mesh size and time step, further discussed in chapters 3.5.3.2and 3.5.4.

3.5 Uncertainty analysis of the problem and codeThe ITTC (International Towing Tank Conference) recommended procedures from1999, (ITTC, 1999), contain guidelines for a general uncertainty analysis in CFD. It isdivided into four parts:

1. Preparation; which involves the selection of the CFD code and thespecification of objectives, geometry, conditions and available benchmarkinformation.

2. Verification; which amongst other is defined as a process for assessingsimulation numerical uncertainties. This includes a grid dependence study and,for a transient solution, a time-step dependence study.

3. Validation; which is defined as a process for assessing simulation modellinguncertainty by using experimental data as a reference.

4. Documentation; which is a detailed presentation of the CFD code (equations,initial and boundary conditions, modelling and numerical methods),objectives, geometry, conditions, verification, validation and analysis.

This study is sponsored by ASC Pty Ltd and a continuation of a previously conductedwork by Dr. Patrick Marshallsay. Therefore much of the preparatory work containedin the ITTC recommended procedures is taken from the work made by Marshallsay.In (Marshallsay, 2008) Marshallsay compares data extracted from simulating a

1 Large Eddy Simulation model is a time-dependent simulation which implicatesKolmogorov’s theory of self similarity that the large eddies of the flow are dependenton the geometry while the smaller are more universal in character. Thus an explicitsolution of the large eddies is possible while the smaller eddies can be solvedimplicitly using a subgrid-scale model (SGS). (Versteeg & Malalasekera, 2007).

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DTMB model no. 5470, using only a bare hull, with data from experiments in (Roddy,1990) and (Huang, et al., 1992). Two different turbulence models were assessed, thev2-f model and the SST k-ω model. The results, shown in Figure 10 and Figure 11,show that predictions of skin friction and surface pressure coefficients made by theSST k-ω model are in close agreement with the experimental measurementsundertaken at DTRC. The results from the v2-f model on the other hand are clearlyunacceptable. This latter model is known to suffer from realizability problems,(Svenningsson & Davidson, 2003), and is arguably not yet sufficiently mature forindustrial applications.

Figure 10 – Comparison between Skin Friction Coefficient estimated using CFD andexperimental measurements made at the David Taylor Research Centre (DTRC),(Marshallsay,

2008).

Figure 11 – Comparison between Pressure Coefficient distribution estimated using CFD andexperimental measurements made at the David Taylor Research Centre (DTRC),(Marshallsay,

2008).

As explained in chapter 3.3 the author used the AKN k-ε turbulence model describedin (Abe, Kondoh, & Nagano, 1993) rather than the SST k-ω turbulence model. Thiswas decided after personal communications with Dr Marshallsay who found that the

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data collected from the AKN k-ε model simulations are in closer agreement withexperimental data than the data from the SST k-ω model simulations.

3.5.1 Scaling of CFD modelIn order of reducing the scale errors when calculating the forces and moments that acton a UUV when it enters an area in near proximity to the submarine all simulationswas made in as near full size scale as possible. After consulting the supervising groupat ASC Pty Ltd (Dahlander, Williams, & Marshallsay, 2008) it was concluded that anoptimal size is most likely longer than the Collins class, which extends 77 meters, butless than 100 meters long.

Scaling is done either by keeping the Froude’s number or the Reynolds number staticfor both the model and full-scale submarine. The Froude’s number is derived fromwave resistance and therefore not applicable in this case, why scaling using a staticReynolds number is implemented.

Roddy concludes in (Roddy, 1990) that scale effects between models and full-scalesubmarines are negligible if experiments are made at or above a Reynolds number of10E6 – 15E6. Scaling the DTMB model twenty times the size of the submarine reaches87.12 meters, which in turn means that the Reynolds number exceeds 10E6 – 15E6 aslong as you keep the speed of the free flowing fluid above ~0.2 m/s.

Scaling the SUBOFF model twenty times generates a Reynolds number of roughlyRe ≈ 150E6 if one use a inflow speed of u = 1.54333 m/s, which equals 3 knots. Tomaintain Reynolds number for the smaller model an inflow speed of u = 31.618 m/s isneeded. The drag of the two models is compared in order to verify the scaling of thesubmarine model. Table 1 displays the resulting comparison between the twosimulations.

Table 1 – Comparison of Axial Force CoefficientRe ≈ 150E6

Axial Force Coefficient Model Full-scale

)(21 22 Lv

FFC

refref

shearpressureD 0.790E-3 0.799E-3

Figure 12 and figure 13 shows the pressure- and skin friction coefficient distributionover the hull at the lower mean line for both the Full-scale and model simulations,respectively. When compared it is clear that the scaling effects are negligible.

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Figure 12 – Pressure coefficient distribution at the lower mean line for the model and full-scale.

Figure 13 – Skin friction coefficient distribution at the lower mean line for the model and full-scale.

3.5.1.1 Drag predictionsThe drag coefficient extracted from the simulations of the SUBOFF differ with ~5%from the data collected in (Roddy, 1990) during his towing tank experiments. Eventhough Roddy have not presented any uncertainty analysis for the presentconfiguration of the SUBOFF mode does he however give an approximate margin oferror of about 4-10% on the different derivatives of motion on a fully appendedSUBOFF model. Moreover in (Pankajakshan, Remotigue, Taylor, Jiang, Briley, &Whitfield, 2003) a margin of error of 10% is presented.

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The drag coefficient has also been compared to a semi-empirical method proposed in(Jackson, 1992) and differs by a great margin. Jackson’s method is to be used on asubmarine that consists of three sections; a super elliptical bow, a parallel middlesection and a super parabolic stern, which the SUBOFF model clearly not consists off,Figure 1. Further Jackson’s method uses a form factor, K2, to calculate the drag due topressure distribution throughout the hull. The form factor being a function of theshape functions and wetted surface coefficient of the different sections respectively,equation (14.1).

, ,

,

2 ( , where

, and

AR FR AR ws A FR ws F

iiR

i

ws iws,A

i

K L L ) - L C - L CL

LD

AC

D

(14.1)

Where, LAR and LFR is the length of the sections, respectively. Di is the maximumdiameter for the section and Aws,i, is the wetted surface area for the section. Thesubscript ‘i’ denotes the current section. In table 2 a comparison is shown with thedifferent methods used.

Table 2 – Comparison of Axial Force Coefficient.Axial ForceCoefficient Roddy RANS

K-ε AKN Jackson

CD 1.160E-3 1.101E-3 10.69E-3

3.5.2 Presentation and reduction of DataThe data is either presented in tabular form or in a plot. The tabular data is roundeddown to three working decimals. The force and moment coefficients presented inchapter 5, Numerical Procedure, Result and Discussion, are all averaged over fiftyiterations of raw data. This is done because the forces and moments acting on thevehicle did not converge at every single simulation. An example of this is shown inFigure 14 where the top row contain all data collected from one of the simulationsconducted aft of the sail. The lower row on the other hand holds the last 50 iterationsfrom where the mean value was calculated.

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Figure 14 – Example of data reduction for a simulation

3.5.3 Grid size and dependence studyA grid dependence study is necessary for validation that the amount of cells in thegrid yield a close enough solution of the problem. It is however a delicate procedureof choosing just the right amount of cells that the solution is accurate enough but donot take a lifetime to converge. The methodology of a grid dependence study isstraightforward. One only has to increase the density of the grid until no significantdifference is longer observed in the converged solution.

Most of the dependence study has already been done in this study by Dr. PatrickMarshallsay. Marshallsay has in his works concluded that the AKN Low-ReynoldsK-Epsilon model with approximately two million cells gives a sufficient solutioncompared to converging time when conducting simulations on the SUBOFF model.

With this in mind this study instead focuses on the grid on and surrounding the UUV.Position 4d was chosen to where a grid dependence study is carried out. At thisposition the UUV has just entered the heavy turbulent area just aft of the sail. It waschosen because at this position the fluid is very complex and consists of small vorticesthat can only be discovered with a fine grid.

In Star CCM+ one can add volume shapes to the continuum and define gridgeneration rules for them. A block volume was added spanning the area in front andaft of the sail where the length scales of the grid could be varied. Figure 15 showsthis together with a before and after image of the grid.

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Figure 15 – Before and after image of the added volume shape increasing the density of the grid

The characteristic length of the cells in the coarse simulation was set to a cell size of34.82 10 0.42cell subL L meter, while the finer simulation used half that size,32.41 10 0.21cell subL L meter. The result did show a small difference between the

two simulations but judging the fact that the finer mesh used almost twice thecomputing time to converge, the coarser grid size was chosen.

3.5.3.1 Surface grid problems with the UUV Control SurfacesOne problem did arise with the grid. Caution should be taken when using results fromsimulations where the external force acting on the UUV is weak. The surface grid onthe control surfaces of the UUV is not optimal and adds noise in the solution. Asolution to this is to remove the control surfaces but they are needed to give the modelan accurate representation of an ordinary UUV.

3.5.3.2 Deciding grid size for the DES simulationFor the DES to act as a LES and detect all eddies in the wake area of the sail the meshneeds to be fine, but how do one determine what is fine enough? One way of doing itis to analytically estimate the smallest and largest vortices in the wake. It is a rule ofthumb that the largest vortices behind a wing can be as long as the cord of the wing,which can be applied here because it is the area behind the sail that is of interest. To

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determine the smallest ones one can use the Kolmogorov micro scales, which is atheory suggested by Andrey Kolmogorov in 1941, (Versteeg & Malalasekera, 2007).Kolmogorov suggested that the smallest scales of turbulence are universal and thatthey depend only of the fluids viscosity and the average rate of energy dissipation perunit mass. By using the Kolmogorov scale, equation (14.2), the smallest vorticeswould be in the order of 1E-5 m.

1 13 4 19 4

53 3

7.071 10 0.1 1 101.543

vL mU

(14.2)

Where v is the fluids kinematic viscosity, δ the boundary layer thickness on the sailand U∞ is the free stream flow. The maximum boundary layer thickness on the sailwas approximated using both the analytical power law and a visual measurement inthe CFD code to ~0.1 m. The kinematic viscosity used is the default for H20 in theCFD code.

Of course using a mesh size of 1E-5 m would create a mesh so fine that it would not befeasible to solve it in this lifetime. Instead after a couple of tries with different sizes amesh size of 0.14 m in the area aft of the sail was chosen, which created roughly 3.3million cells. Usually, during a steady simulation, the computing power allowed up to5 million cells without the simulation being too time consuming but in this case withmuch iteration over several time steps it was decided that a finer mesh than the onechosen would take too long to solve. This meant that only vortices of a size ~0.14 mor greater was detected by the CFD code.

3.5.4 Deciding the Time-step for the DES simulationJust as choosing a mesh size the selection of time step is an engineering judgment,and a difficult one. The most common approach one would take is suggested in theCFD code user guide, (Cummings, Morton, & McDaniel, 2008), the Strouhal number.The Strouhal number, St, is a dimensionless number for determine cylinder sheddingfrequency. Normally the St is ~0.2 which is valid for most cylinders ranging inReynolds number from 100 to 106. Using this approach and with the added “rule ofthumb” that one need approximate 5 to 10 iterations for every period one would get atime step of 0.4 seconds, (14.3),

0.2 1.543 0.229 4.371.35

St Uf Hz T sL

(14.3)

where St is the Strouhal number, U∞ is the free stream velocity and L is acharacteristic length, in this case the width of the sail. However because thesimulations conducted during this study are run at a Reynolds number of well above106 and that experimental data has showed that the Strouhal number can reach St ≥ 10in high Reynolds flow, (Cummings, Morton, & McDaniel, 2008), other approaches ofdeciding time step where looked at.

(Cummings, Morton, & McDaniel, 2008) discuss different approaches used todetermine an accurate time step. Cummings et al. also proposed an iterative approachor a poor man’s “steepest descent” method, as they wish to call it, for choosing a timestep. In which one compares the wave number (inverse Strouhal number) against

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several converged solutions with different time steps and grid sizes. The problem withCummings et al. approach is the time consumption, their experiment needed sixsimulations before a small enough time step was conceived, all which took 50 CPUhours each to reach convergence, this by using a mere 0.1 million cells and eightparallel processors working together in a cluster. During the present study a cluster offour CPU’s where used and a mesh of roughly 3 million cells, which is whyCummings et al. approach is ruled out as a decider.

Further Cummings et al. discuss the approach made by other researchers in the field.(Spalart, 2001) uses the Courant-Friedrichs-Lewy condition (CFL condition), whichcan be explained as the ratio of the distance a wave-like disturbance travels in a timestep to the grid size, Equation (14.4),

max

max

U t CFL xCFL tx U

(14.4)

where CFL is a non-dimensionalized number, suggested to be approximately one bySpalart for accurate prediction of large eddies. Umax is the maximum velocitymeasured in the area of interest (normally Umax ≈ 2U∞, where U∞ is the free streamvelocity), Δx is the grid size and Δt is the time step needed. Another approachintroducing a non-dimensional time step Δt* ( lUtt* , where l is acharacteristic length of the vehicle, in this case the length of the sail) is used by(Strelets, 2001), (Görtz, 2003) and (Schiavetta, Badcock, & Cummings, 2007) in theirstudies of massively separated flows. Table 3 shows the time step required whenpredicted by the different methods explained above.

Table 3 – Comparison between different methods for time step prediction.

MethodΔt* CFL Strouhal

StreletsΔt*= 0.025

GörtzΔt*= 0.006

SchiavettaΔt*= 0.01

SpalartCFL ≈ 1 St = 0.2

Δt[s] .120 0.029 0.048 0.045 4.37

As shown in table 4 the time step predictions by the different models vary quite a lot.The author choose the method suggested by Strelets because the time step wasdecided to be small enough for accurate prediction of large eddies and still largeenough so that the simulation would not take too long2. However, due to an errormade by the author the time step was set to 0.13 seconds and not 0.12, this wasdetermined not to have any significant effect on the result of the simulation so noattempts for correcting it where made later.

3.5.5 Steady modelling of a unsteady problemSimulating a clearly unsteady problem with a steady model is strongly recommendedagainst in the user manual of the CFD code, (CD-adapco, 2008). However if used theCFD code will average the results in a similar matter to time averaged results thatcould be extracted from a proper transient simulation. The problem arising with

2 It took roughly 350 CPU hours, divided over 4 CPU’s, i.e. just under 4 days, to simulateapproximately 180 seconds.

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simulating an unsteady problem with a steady model is that the result from the steadysimulation will be equivalent to using an extremely inaccurate time step in transientsimulation. Essentially the CFD code will over the iterations use something similar toa local time step that is smaller where the mesh is fine and larger where the mesh iscoarse.

Hence the results presented below from the steady simulations are to some stageincorrect and should not be treated as an absolute fact. The problem lies within thesmall unsteady vortices. Even though the primary vortices could be stable to someextent, the secondary or tertiary vortices are unstable and very difficult to averageover a period of time. Their contribution in terms of force on the UUV is howeversmall compared to the larger vortices. For this reasons, the force- and moment-coefficients extracted from the simulations are determined to be accurate enough forthis investigation.

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4 Simulation processStar-CCM+ has the ability to be started in macro mode without using the graphicaluser interface. It requires that the macro scripts are written in Java and the programcomes bundled with its own Java Application Programming Interface (API). In orderto be more versatile and easier make changes to specific runs several macros iswritten, each with a specific purpose and input data. Spanning this, a Python script iswritten to control the sequence of which positions and what type of simulations isperformed. The process is visualised in Figure 16.

Figure 16 – Overview of the simulation process

Where,1. Translate the UUV to the given position and constructed the grid.2. A less accurate but converged solution is firstly obtained with a first-order

segregated solver and with the under-relaxation factor set to a low value. Thisis a common procedure if a more accurate converged second-order solution isunobtainable at first, (CD-adapco, 2008).

3. After a converged solution is found the script switches to the more accuratesecond-order solver and high under-relaxation factors. Thus resulting in anaccurate and converged solution.

4. This step extracted coefficients from the solution into raw data files.5. The data was lastly imported into MATLAB for post-processing.

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5 Numerical Procedure, Result and DiscussionIn this chapter all results extracted from the simulations is presented and discussed.All simulations used the same Cartesian coordinate system, with its origin situated atthe front tip of the submarine with the X, Y and Z axis directed horizontal positive aft,horizontal positive port and vertical positive up respectively, as shown in Figure 17.All forces and moments coefficients presented below are displayed in this coordinatesystem, with the force and moment coefficients acting on the UUV at its centre ofgravity, which is situated at x = 0.375 LUUV from the front, (Prestero, 2002).

Figure 17 – Coordinate system on the REMUS 600 UUV and the continuum as whole,respectively.

However when the UUVs distance from the hull is presented, the author uses thedistance from body to body. Reason being so the reader can easier tell whether theUUV has entered the turbulent boundary layer or not.

The UUV used during the simulations is based on the REMUS 600 with a total lengthof 3.25 meter and a diameter of 0.35 meter. A total of 26 static simulations have beenconducted divided over the four positions discussed in chapter 2.5. For eachsimulation the UUV had to be placed at its location, after which the continuum wasmeshed and the solution calculated. After the data was extracted to post processingthe UUV was translated to its new location and a new mesh and solution wasgenerated and calculated. All locations simulated at are displayed in Table 4. Forposition 1 and 2 the distance from the hull is non-dimensionalized by the boundarylayer thickness but at positions 3 and 4 the sail height is used. This because at theformer locations the boundary layer is the dominant turbulence factor whiles at thelatter ones the sail is the origin of the dominant turbulence.

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Table 4 – Overview of all simulated locations. Note that position 1 and 2 are non-dimensionalizedby boundary layer thickness while at 3 and 4 the height of the sail is used.

1 2 3 41/δ 1/Zsail

a 1.55b 1.12 1.12c 1.05 1.05d 0.98 0.98e 0.87 0.87f 0.54 0.54g 5.7 0.30 0.30h 4.8 2.9 0.23 0.23i 3.3 1.5 0.11 0.11j 2.5 0.8 0.07 0.07

5.1 Flow field in general (time dependent analysis)The simulations presented below are all conducted so both the submarine and theUUV are fixed in space with the fluid flowing around them and a time-averagedsolution is calculated. The reason, as explained in chapter 3.5.5, is that the CFD codeat the time of this study did not support dynamic meshes and therefore it wasimpossible to simulate a “live” recovery of an UUV. One consequence of this is thatall the calculations of pressure, vorticity and fluid direction is averaged over time,hence it is difficult to draw any definite conclusions how much the force exerted fromthe fluid on the UUV fluctuates over time. Thus an un-steady simulation with only thesubmarine present in the continua was conducted.

The turbulence model chosen was Detached Eddy Simulation, DES, which in somecases can result in the best of both worlds between Reynolds averaged Navier-Stokes(RANS) and Large Eddy Simulation (LES) turbulence models. However thesimulation was conducted with a too coarse grid for the DES to function properly.Instead of showing a vortex shedding and turbulence that changed over time a time-averaged RANS solution appeared.

5.2 Position 1 [x = 0.14Lsub]Position 1 is the only position investigating conditions in front of the sail. This regionconsists of a laminar flow with a small turbulent boundary layer. At this position thesubmarine’s hull is cone shaped which produces a favourable pressure gradient thathinders the boundary layer growth. Furthermore the cone shape increases the fluidvelocity and decrease the pressure at this position. In theory the best solution wouldbe to control the UUV so that the fluid attacks it head on at all time which minimizesthe rudder movements during the recovery procedure. This is however very difficultso the UUV is simulated fixed parallel to the free stream and not the hull, as shown inFigure 18.

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Figure 18 – The direction of fluid, illustrated by black streamlines, at the bow with UUV at closelocation to the hull at position 1.

In conclusion, three simulations where conducted at this position, at increasingdistances from the hull, shown in Figure 19. The distances from the hull to thevehicles centre of gravity and stern are presented in Table 5. The centre of gravity ofthe UUV is not within the boundary layer at any time but the aft of the vehicle entersthe turbulent area in simulation 1c.

Figure 19 – The UUV location at the three simulations, respectively, at [x, y] = [0.14Lsub, 0]

Table 5 – Distances from the hull where the UUV where situated at position 1non-dimensionalized by boundary layer thickness δ.

CoG Sterna 4.8 3.2b 3.3 1.7c 2.5 0.9

Figure 20 shows the resulting force and moment on the UUV. As expected the sideforce- and yaw moments are both very small because of the uniform UUV body. Theincrease is most likely due to noise. The lift force coefficient shows that the UUV isrepelled by the fluid flowing around the bow of the submarine. As the vehiclesapproach each other a squat effect due to the two objects pressure fields give way to adecrease. A similar conclusion can be drawn for the pitch moment coefficient whichincreases as the UUV approach.

36

Figure 20 – Force and Moment acting on the REMUS 600 at Position 1. The dots represent onesimulation each and the Y Axis shows the distance from the hull of the submarine to the CoG of

the UUV. Left and bottom axis are non-dimensionalized.

5.3 Position 2 [x = 0.23Lsub]In these simulations the UUV is positioned next to the submarine’s sail. Except for thehorse-shoe vortices and the turbulent boundary layer the region is dominated by asteady non turbulent flow. The sail is situated at the intersection between the bow andthe parallel middle section which means that the fluid is still flowing with an angleslightly outward from the centre of the hull, see Figure 21. Four simulations areconducted at this position all located along a straight line at 15 degrees from the uppermean line of the submarine, see Figure 22. The distances outwards from the hull arechosen in relation to the boundary layer thickness at this position and are presented inTable 6.

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Figure 21 – Direction of the fluid and its magnitude at position 2 – CoG, viewed from aft. Thearrows represent the direction of the fluid (u = 0) and the background colour represent its

magnitude.

Figure 22 – Locations of the simulations at position 2, viewed from front.

Table 6 – Distances from the hull where the UUV where situated at position 2non-dimensionalized by boundary layer thickness δ.

a 5.7b 2.9c 1.5d 0.8

The reason why the UUV only partly enters the boundary layer is solely because it isnot thick enough and that the UUVs control surface would collide with the hull of thesubmarine before it would enter the turbulent boundary layer area.

38

In Figure 23 where the forces and moments acting on the UUV is shown one can seethat all of the forces and moments are following a distinctive pattern as the smallervehicle approaches the hull of the Submarine. There is a force repelling the UUVwhich is increasing until it enters the boundary layer and one of the horse-shoevortices where a decrease of the force occurs. It is more likely that the decrease is dueto the fluids directional change in the horse-shoe vortex than the interaction betweenthe two objects pressure fields.

Figure 23 – Force and Moment acting on the REMUS 600 at Position 2. The dots represent onesimulation each and the Y Axis shows the distance from the hull of the submarine to the CoG of


The Side Force Coefficient plot’s shape is similar to the Lift Force Coefficient. In thiscase the forces acting on the UUV are inflicted by the horse-shoe vortex which at firstpushes it toward the sail of the submarine but when the UUV get situated inside thevortex the force decrease again.

The Yaw Moment Coefficient is explained by the route the fluid takes around the sailof the hull. The front part of the UUV is located so that the fluid is parallel when itstrikes the smaller vehicle, but when the fluid leaves the aft part of the UUV it isdrawn to the centre of the submarine due to the shape of the sail. This is clearly shown

39

at the aft of the UUV in Figure 24, where the separation of the fluid is clearly largeron the side facing the sail.

The negative Pitch Moment Coefficient is most likely a result of the complexity of thehorseshoe vortex as it propagate throughout the hull of the UUV. A slight decrease inthe negative moment is visible when the vehicle enters the boundary layer which canbe attributed to the pressure field interaction.

Figure 24 – Top view over Pressure and Vorticity distribution, respectively, over the UUV (smallobject) and the sail of the submarine at position 2.

Observe that in the lower right image of Figure 24 the origin of the horse-shoe vortexis visualized as a high level of vorticity.

5.4 Position 3 and 4Position 3 and 4 are both situated aft of the sail of the submarine to answer whether arecovery is plausible at all in this region, maybe at a certain distance aft of the sail.The simulated distances from the submarines hull are directly related to the height ofthe sail and specified in Table 7.

40

Table 7 – Distances from the hull at position 3 and 4, respectively.Non-dimensionalized by sail height (Zsail = 4.41m).

3 4a 1.55b 1.12 1.12c 1.05 1.05d 0.98 0.98e 0.87 0.87f 0.54 0.54g 0.30 0.30h 0.23 0.23i 0.11 0.11j 0.07 0.07

Position 3 is located so the UUVs bow is x = 0.3Lsail aft of the sail and position 4 issituated so the UUV has its centre of gravity amidships the submarine. The region aftof the sail is turbulent and dominated by two pairs of contra-rotating vortices; thehorseshoe- and “wing tip” vortices which both commence from the sail. This isillustrated in Figure 25 which shows an in plane projection of the fluid,[u,v,w] = [0,v,w], at position 3 and 4 respectively.

41

Figure 25 – Direction of the fluid and its magnitude at position 3 and 4 respectively. The arrowsand streamlines represent the direction of the fluid and the background colour represents its

magnitude.

Figure 26 shows the lift force and pitch moment coefficients acting on the UUV. It isvisible that both plots follow a similar pattern but the forces and moments are greaterfor the position closer the sail. The Lift Force Coefficient plots shows that the UUV isaffected by a small downward force when in the free flowing area above the sail. Atthis distance from the hull the attracting force between the objects due to the pressurefield interaction should be next to nothing, therefore the force is most likely due to thenoise created by the UUV’s control surfaces, as explained in chapter 3.5.3.1. Themost interesting observation is when the UUV enters the tip vortices which then exert

42

a repelling force on the smaller vehicle. This force only affects over a short distancebetween circa ~1.05 to ~0.7 the height of the sail. It then changes to an attractingforce which increases as the vehicle approach the submarine. The attracting force isdue to the downward direction of the horse-shoe vortices with a small contributionfrom the pressure field interaction between the objects.

The Pitch Moment coefficient shows a similar behaviour. In theory the UUV shouldattain an increasing positive trim the closer it gets to the submarine, which it alsodoes. An exception exists when the vehicle pass through the tip vortices where anirregular much larger moment is present.

Figure 26 – Lift Force and Pitch Moment at position 3 and 4 respectively. The dots represent onesimulation each and the Y Axis shows the distance from the hull of the submarine to the CoG of


Figure 27 illustrates the interaction between the two objects pressure field. Observethat the dominating low pressure at the stern of the UUV which is causing the positivetrim when the UUV is in close proximity of the submarine.

43

Figure 27 – Interaction of pressure fields between UUV and Submarine in close proximity to eachother at position 4. From left is front, middle and aft of middle-body shown.

44

Figure 28 – Side Force and Yaw Moment Coefficients at position 3 and 4 respectively. The dotsrepresent one simulation each and the Y Axis shows the distance from the hull of the submarine

to the CoG of the UUV. Left and bottom axis are non-dimensionalized.

In Figure 28 the Side force and Yaw moment coefficients is presented and it is clearlyshown that they are very irregular, almost chaotic. This can be attributed to thecomplexity of the vortices which changes characteristic and direction throughout thevehicles body as the UUV descends towards the hull.

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6 UUVs and LARS in shortThese chapters cover the UUV and LARS technology and their basic theory of controland communication. A short presentation of most UUV’s and LARS is presented anda few is discussed in depth. However a lot of information regarding the differentsystem is company and/or Commonwealth proprietary and is not available to thepublic.

There are multiple UUVs on the market today and their numbers are growing. Earlierthere where a clearer distinction between UUV’s for military and civilian use butlately module based systems with the ability to use “off the shelf” products as payloadmodules are becoming more of a standard. A module concept is not only moreversatile but much more cost effective and there are some systems on the markettoday that are able to conduct a wide variety of functions. An example of a modularbased system is the AUV 62 from SAAB Underwater Systems; figure 29 shows thesetup for the UUV and all the interchangeable parts.

Figure 29 – Example of a module based system, AUV 62 Sapphires in basic configuration,Copyright© SAAB Underwater Systems

6.1 The US Navy UUV Master Plan (UUVMP)In 2004 the US navy released an updated version of its UUV Master Plan, (U.S. Navy,2004), in which they describe their vision that an UUV can;

“Attack today’s littoral coverage problem and tomorrow’s advanced threat”

Furthermore an UUV can gather, transmit or act on all types of information, fromanywhere to anyone… Deploy or retrieve devices, anyplace, anytime… Engage anytarget, bottom, volume, air or space. With minimal risk to US force… at an affordablecost. Most importantly is of course the cornerstone where cost is not necessarilymonitored in the monetary value of the UUV but also the cost of human lives. Furtherthe Master Plan categorise UUV/AUV’s into four vehicle classes depending on theirsize, which is shown in Table 8.

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Table 8 – Vehicle classification according to the US UUVMP

Class ManPortable

LightWeightVehicle

HeavyWeightVehicle

LargeVehicle

Displacement 10-50 kg ~250 kg ~1000 kg ~10 000 kg

Endurance 10-20 h 20-40 h N/A N/A

Shape /Diameter N/A Torpedo

12.75 in.Torpedo

21 in. N/A

The Man Portable class is omitted from this study because firstly its endurance isdetermined not sufficient for deployment into littoral zones from a submarine, andsecondly that the Master Plan only identifies the three larger classes as deployable andrecoverable from a submarine. The Large Vehicle class is intended to be used in Anti-Submarine warfare missions thus carrying heavy torpedoes which is why it is simplytoo large and heavy to be carried by a medium/large sized diesel electric submarine.Hence it is omitted from the study.

So, why are UUVs a necessary force multiplier for a Navy? Well, except for beingable to operate in deniable areas there is an obvious important fact;

“Minimizing human casualties during hazardous missions”

Additionally the Master Plan has identified nine functions in which the UUV’s aresuperior in use. Four of them however are solely for the large- and man portable classvehicles and therefore not presented. The five remaining, with a brief explanation, inprioritized order are;

Intelligence, Surveillance and Reconnaissance (ISR)UUV’s are perfectly suited for information recovery due to their ability tooperate undetected in littoral areas extending the reach of their hostplatforms into previously inaccessible areas.

Mine Countermeasures (MCM)It is desirable to minimize risk to a fleet operating in a specific area, to dothis time is paramount and it is proven that using a UUV is far more timeefficient than any human diver.

OceanographyKnowledge of the operating environment is of key importance andconventional data collection is commonly dependent on hull mounted ortowed systems. UUV’s permits characterizations of greater areas at lesscost and also perform reconnaissance in a near shore environment while itshost remain at a safe distance.

Communication / Navigation Network Nodes (CN3)A small vehicle has a greater chance to stay undetected while manoeuvringto the surface and using a discrete antenna to communicate. An UUV canalso provide a link between a submarine and Global Positioning System(GPS).

Information Operations (IO)

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An UUV could be used either as a platform to jam or inject false data intoenemy communication network or secondly as a submarine decoy. Anexample of this is the AUV 62 from SAAB Systems which can be fittedwith a payload module containing noise transmitters and echo respondersto mimic the signature of a submarine.

6.2 UUV’sIn this chapter a brief explanation of how an UUV works is presented and discussed.This is to give the reader a basic understanding of some of the problem involved witha recovery of an UUV to a submarine whilst at speed.

6.2.1 NavigationAll AUVs use an Inertial Navigation System to navigate in submerged mode. Theworks by utilizing motion sensors and computers to keep track of the vehiclesposition, orientation and velocity. Usually it is used together with a GPS or anothermeans of getting an initial position. In this specific case the host’s position is mostlikely be programmed in the AUV before the start of the mission. To easierunderstand the concept of inertial navigation, one can imagine oneself sittingblindfolded in the passenger seat of a car trying to navigate by feeling the movementsof the car.

This system is of course not entirely perfect, far from it, and it does experienceinternal errors. The AUVs then either resurface in intervals to reacquire an absoluteposition by using GPS, or using an Acoustic Doppler Current Profiler (ADCP) theAUV can bounce sound of the bottom and determine its velocity. According to theUniversity of Southampton their Autosub, which uses this system, has a navigationalerror of 0.1% per travelled distance (Underwater Systems Laboratory at the NationalOceanography Centre, 2007). The Kongsberg group on the other hand claims thattheir Hugin and REMUS AUVs have a navigational error less than 0.03% whenfollowing a lawnmower pattern3. This is possible by using a Terrain ContourMatching navigation system that uses an on board contour map of the terrain andcompares it to the image collected by the vehicles sonar system.

Another way of aiding the Inertial Navigation System is the use of acoustic beacons.These need to be delivered beforehand at strategic locations or in some cases there aresuggestions that the AUV could place the beacons before conducting its mission, andto recover them when finished. By using this approach the AUV is constantly updatedwith its absolute position which lowers the navigational errors close to zero. However,the alternative with acoustic beacons might not be optimal if ones intention is to stayundetected.

6.2.2 Guidance and CommunicationWhen in close proximity to the Submarine the AUV communicates by underseaacoustic modems. One would hope that the AUV could be remotely controlled fromthe host ship while conducting the recovery, but the low bandwidth and time delaysinvolved with undersea communications currently makes this very difficult. There arehowever a few Virtual Tether-solutions on the market that claim a bandwidth close to

3 A lawnmower pattern is the name of the movement pattern an AUV follows when it is surveying anarea, the name given because its resemblance of a person’s movement when moving the lawn.

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200 Kbits/s. Whether it is failsafe4 enough to actively remotely control the vehicle ornot is not known but it is fast enough to work as a real time positioning for the AUVrelative the submarine during a recovery procedure.A surfaced AUV can also communicate using the Iridium satellite network or, at acloser range, using a Wireless Local Area Network (WLAN) connection.

6.2.3 Propulsion and EndurancePower consumption is of course directly proportional to the shape of a UUV. A longslender shape is likely to use less energy than a short bulky shape. One considerationhas to be accounted for though: the size of the payload. If the payload is square inshape then it may be of advantage to use an UUV with a box-like shape with roundedcorners. The circular shape body have to be much larger to contain the payload, withadded wetted surface and drag area as a consequence.

Most torpedo shaped UUVs have a single propeller at the aft. Some torpedo shapedUUVs use a setup with two contra rotating motor assemblies, this to exert a zero nettorque in order to give the possibility of controlling the vehicle roll (Stevenson &Hunter, 1994).

6.2.4 StabilityThere are two ways for an UUV to maintain a certain depth during its mission. It fliesin either auto depth or auto altitude mode. In auto depth it uses the depth sensor tocalculate its depth and in auto altitude it uses an ADCP and follows the ocean floorterrain. Auto altitude is by far the most common mission approach.

6.2.4.1 Hydrostatic StabilityFor a surfaced vehicle we consider the metacentre height as a measurement on howstable a certain ship or surfaced undersea vehicle is but as the submarine, or the UUV,descends it is required, for all transversal and longitudinal, that the centre of gravity isbelow the centre of buoyancy. It is the size of this distance, BG, which determines thestability of a submerged vehicle. Figure 30 illustrates the changes of B, G and M in asurfaced and submerged condition.

Figure 30 – Cross-section of UUV to illustrate the changes in B, G and M between surfaced andsubmerged condition.

There is one great advantage with a torpedo shaped vehicle compared to an “odd”shaped vehicle when considering the hydrostatic stability. If a UUV is subjected to an

4 The AUV will have a fallback mode in case the communication fails for military applications.

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external force of i.e. vorticity in the wake after a submarine, let us also assume thatthis force can be simplified as it would be acting on a single point on the UUV.Furthermore this point is directed perpendicular upwards to the longitudinal axis, on abody fixed coordinate system, and situated on the longitudinal centre of gravity. Withthese assumptions this force would only inflict movement in roll terms of the UUV.We are also assuming that it is a static environment so we can make the followingsimplifications to the roll equation of motion for an UUV, formulated by (Nahon,2006), simplified in equation (7.1).

sinassuming static why,

0sin

B XX

B

F BG F x I p

F xpF BG

(7.1)

Where, FB is the buoyancy force, the Euler angle of roll, F and x is the externalforce and its point of action respectively, IXX is the mass moment of inertia in roll forthe UUV and p is the angular acceleration in roll.

We have now determined a relationship between the stability of the submergedvehicle and the external force acting on it. If we now incorporate this to a torpedoshaped and an “odd” shaped UUV and have a look at the differences between them,figure 31, we see that for a torpedo shaped the relation is close to two but for thisparticular “odd” shape it is closer to eight. In this case it would take four times theforce to roll the torpedo shaped UUV than it would for the “odd”-shaped one.

Figure 31 – Difference between righting moments to an external force between a torpedo-shapedand "odd"-shaped UUV.

Figure 31 shows a not so forgiving selection of shape for an “odd”-shaped UUV. Thesame rectangular shape rotated 90 degrees in either direction would increase thestability significantly. The reason why the “odd’-shaped UUV is depicted this way isbecause both of the two “odd”-shaped vehicles discussed in this study have this shape.

Another reasoning that promotes a torpedo shaped UUV is its hull smoothness. Afluid directed toward an object exerts a greater force if the object is flat andperpendicular than if the object is rounded.

6.2.5 ControlBy far most common approach for torpedo shaped UUVs is a propeller at the aft andtwo sets of control surfaces working as rudders, either fitted as a cross or in an

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inverted Y configuration, see Figure 32. Some systems can be fitted with an extra setof control surfaces at the front for extra manoeuvrability. This leads to the conclusionthat there are very few UUVs on the market today with the ability to hover. There arehowever exceptions: The Archerfish, a single shot mine disposal system fromBAE SYSTEMS which instead of a single mounted propeller in the aft has twothrusters mounted on either side of the body amidships. This gives the Archerfish theability to operate in either hover mode or transit mode.

Figure 32 – Example of an inverted Y control surface configuration

Another negative aspect with the rudder configuration as above is the time delayinvolved in controlling the vehicle. The slower the vehicle travels the longer time ittakes for the vehicle to respond to a rudder change. This naturally leads to a problemif the vehicle enters a turbulent area. An example of this is shown in the chapterrelating “Navigation, guidance and control of the Hammerhead AUV” in (Roberts &Sutton, 2006). In which it took the Hammerhead roughly 45 seconds to stabilize on acourse. The Hammerhead was conducting a circle movement at the surface with aconstant rudder angle of 20 degrees and the time is measured from when theHammerhead started its alignment on a specific heading to when it is stabilized. Asshown in figure 33 the rudder command is given at approximately 30 seconds and theAUV stabilizes at around 60 seconds, the spike at roughly 125 seconds is a responseto a change in the vehicles heading due to surface currents.

Figure 33 – Hammerhead AUV controller trial results: (a) rudder deflections generated and (b)Hammerhead heading.

Copyright© the Institution of Electrical Engineers

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An “odd” shaped UUV on the other hand have superior manoeuvrability whencompared. This is necessary if one uses the reasoning mentioned above that it needs alot more control interaction to be stable when conducting its missions. Typically an“odd” shaped UUV uses a set of thrusters at varying locations, instead of apropeller/rudder combination, for control.

6.2.5.1 Ability to maintain depthMost UUV’s incorporate a fixed ballast system i.e. external weights that needs to becalibrated for the salinity and temperature in the mission area. This usually means thatwhen and if the UUV experiences any local variations in the salinity and temperatureor external forces in the water it has to travel with an angle of attack, use thrusters orcontrol surfaces to maintain its depth. This of course affects the endurance of theUUV. Another fact to be taken into account is that most of the UUV’s with fixedballast systems are made slightly buoyant, so if there is a mishap it floats to thesurface. This, of course, is a feature that is not desired if your aim is to stayundetected.

Another way of maintaining altitude or depth is the system that AutonomousUndersea Vehicle Gliders (AUVG) uses. They have the ability of changing theirbuoyancy to descend or ascend and while doing so they use wings to create a liftwhich propels them forward, figure 34. There are two kinds of glider to date, theyboth have a variable buoyancy system but use different technologies.

Figure 34 – Illustration of an Autonomous Undersea Vehicle Glider’s flight through the water.

6.2.5.1.1 Thermal GliderThis type uses the thermal gradient that is present in the ocean. The sun rays keep thewater warm towards the surface but the water gradually gets cooler at depths, down to2-4 degrees Celsius. The “Slocum Thermal Glider” from Webb Research Corporation,(Webb Research Corporation), has wax filled tubes within its body. When the sun andthe surrounding water heats the wax it expands and pushes a mineral oil into bladdersfilling them and thus changing the buoyancy of the glider making it sink. When thewax cools of due to the cooler water the oil retracts from the bladder and the cycle iscomplete.

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6.2.5.1.2 Electric GliderThis system uses a single stroke piston pump which is connected to an electric motor.Then it either uses it to control the amount of seawater in a bladder situated in the freeflooding compartment of the AUVG or to transfer a fluid, usually mineral oil,between one bladder in the pressure hull and one in the free flooding part. The“Slocum Electric Glider” from the Webb Research Corporation, (Webb ResearchCorporation), also has the ability to move its battery pack forward or aft to controlpitch.

6.2.6 CategoriesThere are numerous UUV’s on the market and many of them are very similar in shapeand size. Therefore, as mentioned earlier the UUV’s are divided into two groups,torpedo- and “odd”-shaped UUV’s. In each chapter a brief presentation of the UUV’sfound and an in depth discussion of an UUV of interest is made.

6.2.6.1 Torpedo Shaped UUV’sIn Table 9 a list with the torpedo shaped vehicles found during the literature study arepresented. Generally a torpedo shaped UUV consists of a bow, a parallel middlesection, and an aft section. Most have the same diameter as a torpedo, 21” or in somecases less than that. Very seldom they have the ability to hover but need instead aminimum forward speed of 0.5-1 knot to maintain control. If they don’t maintain thisspeed they rise to the surface due to the natural positive buoyancy. In some cases theyhave a set of forward hydroplanes but most of the time they rely on aft rudders, indifferent configurations. Most of the torpedo shaped vehicles are powered by abrushless energy efficient “off the shelf” DC motor connected to a two bladedpropeller, which makes it very energy efficient and many manufactures claim theirsystem can operate autonomously up to and above 50 hours.

Table 9 – Torpedo shaped UUVs

Name Manufacturer Depl. Size L/D Depth End. Comments[kg] [m] [m] [hrs]REMUS 600

Kongsberg

240 3.25/0.324 600 50REMUS 6000 885 3.84/0.71 6000 22HUGIN 1000 650 4.5/0.75 1000 24HUGIN 3000 1400 5.5/1 3000 60HUGIN 4500 1900 6/1 4500 78Bluefin 12 Bluefin Robotics

Corporation181.5 3/0.33 200 +15

Bluefin 21 330 3.3/0.53 200 18AUV 62 Sapphires SAAB 1500 7/0.53 200 ?Autosub AUV Uni.Southamton 7/0.9 1600 +505

Mullaya DSTO ? ? ? ? As REMUS 6006

6.2.6.2 The REMUS 600There is one UUV in the torpedo shaped group that is of special interest. Not onlybecause it is owned and in use by the RAN but also because it is of a typical torpedoshape and of a size that would be able to dock to an average sized submarine withoutany major modifications of the submarine. One major reason is also that the REMUSwas first developed by scientists from Wood Hole Oceanographic Institution (WHOI)

5 The National Oceanography Centre in Southampton claims that the Autosub have up to 144 hoursendurance in optimal conditions, but Autosub has not yet been on a mission exceeding 50 hours.6 Information gathered from personal communication with Dr. Francis Valentines at the DSTO.

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in Massachusetts, USA. Due to this there is a lot of publicly available information onthe REMUS control characteristics and different lengths. In 2008 KongsbergMaritime A/S acquired the rights to construct and sell the UUV.

6.2.6.3 “Odd”-shaped UUVsThere are still a few “odd”-shaped UUVs on the market today. No detailedinformation, on any of the systems, where possible to apprehend even though severalattempts were made by the author. A discussion with Dr. Francis Valentines at theDSTO revealed that the Wayamba is decommissioned due to instability reasons.Dr. Valentines also believed that “Odd”-shaped UUVs would more or less be replacedwith the torpedo shaped ones. The Talisman from BAE Systems is apparently stillunder development and the information regarding it is mainly speculations. Theinformation available on public domain at BAE Systems, (BAE Systems), reveals ahybrid diesel propulsion system and endurance up to 24-hours. Lastly the author didmanage to sign a non disclosure agreement with SAAB Underwater Systems with thepromise that SAAB would supply with detailed specifications, which wasunfortunately never delivered.

6.2.7 DiscussionIn general an “odd”-shaped UUV is very sensitive for external forces and therefore itscontrol systems and acting thrusters needs to be very responsive. This in turn hindersthe operating time. In a scenario where the operating time is the weighing factor thenthe torpedo shaped UUV has the clear advantage. On the other hand if operating timeis of less importance and the weighing factor is manoeuvrability then an “odd”-shapedUUV with several thrusters, such as the SAAB Double Eagle SAROV, has the upperhand. A torpedo shaped UUV with a rudder configuration in front of the propeller donot react fast enough to external forces to perform a safe recovery.

As of now none of the UUVs are ideal for a recovery procedure and still performoperations at great distances and time from its host.

6.3 Launch and Recovery SystemsA number of underwater docking systems have already been developed by theresearch community and offshore industry. Although most of them are still at aconcept or prototype stage the most significant ones are described in this section.

6.3.1 Funnel/Cone Recovery SystemsIs by far most common approach to construct a recovery system in the offshore andmilitary market is to have a funnel guiding the UUV into its stowed position. Usuallythe UUV homes in on the funnel or cone by either sonar or a transponder systemusing triangulation. All systems on the market today are deployed from surface shipsand used in speeds close to nil. Furthermore according to a representative from theDSTO, who has seen a couple of the systems in action on the annual AUV Fest7,claims that they have a pretty low success rate, down to one successful recovery ofevery ten tries. Another story is told by Stokey in (Stokey, 1997) where it is claimedthat the cone recovery system for the REMUS UUV vehicles works well. Figure 35

7 AUV Fest is an annual gathering arranged by NOAA Office of Ocean Exploration and Research,Office of Naval Research and Naval Undersea Warfare Center to demonstrate advanced technology onAUVs.

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shows an undersea trial and a concept design of a funnel recovery system mounted ona submarine. The concept, developed by the BMT Group, is a dry dock situated on theback of a submarine allowing submariners hands-on handling the UUV whilesubmerged.

Figure 35 – Concepts of funnel recovery systemsCopyright© BMT Group

The advantages with a funnel recovery system is it simplicity and low cost. Thenegative aspects are that it is a relatively large contraption and that it has yet beenproven to work well in non optimal conditions.

6.3.2 Belly mounted Stinger / Buoy Vertical PoleThe Belly mounted Stinger, developed by the Florida Atlantic University’sDepartment of Ocean Engineering, and the Buoy Vertical Pole system, from theWoods Hole Oceanographic institute, works on similar principles. Both systems use astinger or vertical pole slides in and get caught by a scissor like construction.

As shown in figure 36 the Florida Atlantic University’s system have their stinger orpole attached to the underside of the UUV, which in turn interfaces with the recoverysystem itself through a four-petal configuration that lets the vehicle approach fromany direction. The system is construction for ocean floor mounting and first andforemost to be a recharging platform for the Florida Atlantic University’s OceanExplorer UUV.

The system from Woods Hole that is adapted successfully to their Odyssey AUV(Singh, et al., 2001) has on the other hand a nose mounted scissor shaped latch bodythat captures a vertical pole mounted between a buoy and a dead weight.

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Figure 36 – Florida Atlantic University's Ocean Explorer stinger recovery system

6.3.3 Universal Launch and Recovery ModuleThis is a concept formulated by General Dynamics Electric Boat which are, at thetime of this investigation, updating the US Navy’s Ohio class submarines into modernSSGNs, Ship Submersibles with Guided Missiles and Nuclear Powered, carryingTomahawk missiles. The Universal Launch and Recovery Module works as an airlock allowing sailors to put a vehicle into the chamber flood it and eject the vehicleout to sea. When retrieving the UUV the robotic arm would extend grab the vehicleand pull it back on board. Electric Boat prime objective with the system is to let thesubmarine deploy and recover vehicle that are too large to fit in a normal torpedotube. The Vertical Launch System is roughly 2.5 meters in diameter while a torpedotube restricts larger vehicles than 0.53 meter in diameter.

There is no information how the UUV homes in on the recovery system, most likelyUSBL navigation though, nor how the vehicle actually attach itself to the system. Atheory, from the author, is that the vehicle could use something similar to the bellymounted stinger, or a arrestor hook a fighter jet have attached to their empennage forachieving the deceleration needed for landing on aircraft carriers.

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Figure 37 – Illustration of a missile tube recovery systemImage courtesy of Andrew Lightner, GE Electric Boat

6.3.4 Sea Owl SUBROVThe Sea Owl SUBROV is a recovery system intended to be used from a submarinetorpedo tube. Except for being able to recover an UUV the system can also be usedfor inspection, underwater works, Mine Counter Measures and as a platform forCommunications/Surveillance.

The recovery works in that way that the SUBROV, which is controlled by a humanoperator, moves and aligns itself with the incoming UUV. It then uses its grippingtool to dock with the UUV and subsequently steers the vehicle into a torpedo tube forrecovery, se Figure 38.

The advantage with the system is that it can be incorporated onto any submarinewithout prior modifications. The negative aspects is of course the system needshuman interaction and also that it, to the knowledge of the author, has yet to be triedon a real submarine. SAAB Underwater Systems is presently using a mock up on thebottom of Lake Vättern in Sweden to conduct their trials and demonstration of thesystem.

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Figure 38 – Illustration of the SAAB Sea Owl SUBROV systemCopyright© SAAB Underwater Systems

6.3.5 Boeing Torpedo mounted retractable armThe torpedo mounted retractable arm from Boeing Advanced Information Systems isjust as it sounds like an arm that takes hold of a UUV and pulls it into anneighbouring torpedo tube. In figure 39 a drawing of the system is shown. Therecovery system is part of Boeings Long-term Mine Reconnaissance System (LMRS),known as AN/BLQ-11, and is capable of launch and recoveries at speed from the USNavy SSN 688 and NSSN class submarines.

The system was tested successfully in 2007 on the USS Hartford attack submarine butnot without difficulties. The UUV needs to line up directly with the torpedo tube sothat the robotic arm can reach out and grab it.

The advantage of the system is of course that it is works. Negatively, the system isvery expensive, heavy (2 000 kg) and that it use a torpedo tube. In addition, thesystem could probably not be fitted on a Collins class submarine due to the differentpositioning of the torpedo tubes compared to the US navy submarines8.

8 The Collins class have their torpedo tubes situated in the front on a horizontal row while the US Navysubmarines have their coming out on the side.

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Figure 39 – Schematic overview of a torpedo mounted arm recovery system9, where: 1 is theTorpedo tube which the system is installed in, 2 is the torpedo tube which the UUV get recovered

to, 3 is the outer hull of the Submarine, 4 is an extendable cylindrical arm, 5 is the unfoldinggripping tool and 6 is the UUV.

Provided by PatentStorm, (PatentStorm).

6.3.6 Reverse Funnel Recovery – Authors suggestionThe author suggests a recovery system which offers a way around the control issueswith the torpedo shaped vehicle. In this case the LARS system looks similar to afunnel but has its opening towards the front of the submarine. The UUV should in itsrecovery mode position itself next to the submarine and in front of the funnel opening.After which either the submarine could increase/or the UUV decrease its speed so thatthe smaller vehicle would slide into the funnel with its aft first. By being in front ofthe recovery system the UUV is not to experience any of the problems related to aturbulent wake. To navigate the UUV to the designated position a triangulation withi.e. USBL would be used.

9 This system was patented by Lockheed Martin and it is not known whether BOEING a similar or thesame system.

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7 ConclusionPosition 1 would be at first glance a good choice for an engineer to conduct arecovery with an UUV. There is no disturbance in terms of large trailing wakes after asail. There will probably be minor turbulence and wakes due to sonar arrays, openingsand irregularities in the hull. There are though other issues with recovering a UUV atthis position. There are and probably will be control surfaces around the sail and a fewantennas etc. that could get damaged if the recovery procedure of some reason wouldfail.

The greatest force extracted from the simulations was from position 2 and is a spikeforce in the order of roughly 75 Newton. Except from that case the forces at the rest ofthe positions are in the size of 10-30 Newton, which shouldn’t create a problem for anUUV. But as explained earlier these forces represents an average over an unknowntime which gives no knowledge how fast they are fluctuating or the size of themagnitudes.

Problem could arise in this specific case with the REMUS 600 UUV which has itsrudders in front of its propeller thus a relatively long rudder response time. If theturbulent fluid changes direction fast enough to push the UUV repeatedly out of itstrajectory it would have no way of counteract this with its limited control ability. Thesame reasoning applies for all the torpedo shaped UUVs presented above.

Two of the Launch and Recovery System available on the market sounds promising.Neither the Universal Launch and Recovery Module nore the Belly mounted Stingercause vorticity in the area of approach for the UUV.

Finally, even though a torpedo shaped UUV lacks in control the author believes thatits endurance abilities make it the best solution at this stage. However the recoverysystem needs to be constructed and placed so that the UUV is minimally exposed ofturbulent fluid.

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8 Further workSuggestively a person who continues this work should start with accessing moredetailed information about performance data for the different UUVs. If a supplier ofan “odd”-shaped UUV can present data showing that an “odd”-shaped UUV canoperate for 24 hours, and still have enough power to return to its mother vessel andperform a safe recovery, then this type would have the advantage. Furthermore morework on virtual tether systems and USBL triangulations should be conducted so it canbe determined whether a UUV is to have sufficient control and position accuracy tomanoeuvre itself close to its host.

In the CFD part more time should be spent on the grid around the UUVs controlsurfaces, or omitting them completely, and successfully conducting a transientsimulation. Also providing that a new version of CD-Adapco Star CCM+ supports asix degree of freedom simulation and or an overset grid approach such simulationshould be attempted.

However the easiest way of determine whether a UUV could be controlled inturbulence would be to conduct real life experiments. A controlled way of doing socould be obtained in a towing tank or in a cavitation tunnel.

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Simulation of a Launch and Recovery of an UUV to an Submarine - …/Menu/... · UUV to a submarine. Hence, the goal of this study is to investigate whether a recovery of a UUV at