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MINOR PROJECT-2016 DEPT. OF APPLIED MECHANICS, NCW, IIT-DELHI

LIST OF CONTENTS

1. Introduction09

1. Motivation 10

2. Approach 10

2. Types of Air Lubrication Techniques11

1. Bubble Drag Reduction11

2. Air Layer Drag Reduction12

2.3 Partial Cavity Drag Reduction

12

3. Flat Plate Experiment Method13

1. Method 13

2. Cost Benefit Analysis15

3. Calculation of values for Flat Plate16

1. Without ALS 16

3.3.2 With ALS

16

3.4 Results

16

4. Boundary Layer Method for calculating Drag Reduction17

1. Method 17

2. Calculation And Graph Plotting 19

4.3 Results And Conclusions

20

5. CFD Analysis of ALS in a 3D Rectangular Plate22

1. Gambit Modelling22

2. Mesh Modelling23

3. Formulation of problem in CFD 23

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5.4 Calculation And Results25

5.5. Inferences29

6. Seakeeping Aspects of Air Lubrication 301. Basic Computational Method

31

2. Conclusions33

7. Manoeuvring Aspects Of Air Lubrication337.1 PMM Experiments33

8. Scale Effects Of Air Lubrication35

9. Conclusions37

10. Future Scope And Live Projects38

11. References 39

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LIST OF TABLE

Figure 1: Boundary Layer, Viscous And Pressure Drag 11

Figure 2: Eddy Motion And Boundary Layer 11

Figure 3: BDR Technique 12

Figure 4: ALDR Technique 12

Figure 5: PCDR Technique 13

Figure 6: Spanwise Air Volume Fluxes for FD and Transitional

Flow….14 Figure 7: Schematic Drawing of ALS

17 Figure 8: Drag Reduction Predicted By BL Mixture Model 19

Figure 9: Cv vs Drag Reduction 21

Figure 10: Density Ratio Vs Drag Reduction 21.

Figure 11: Gambit Modelling Of Problem 22

Figure 12: Mesh Modelling Of Problem

23 Figure 13: Multiphase Model 24

Figure 14: Drag Reduction for different Ujet and U = 5m/s 26




Figure 18: Heave Response with/without air bubbles 30

Figure 19: Effective Power Reduction 31

Figure 20: Comparison of roll and pitch motion 32

Figure 21: Lateral forces measured on air lubricated ships 34

Figure 22: Lateral forces along ship’s length 34

Figure 23: Overall lateral forces 35

Figure 24: Measured Drag Force 36

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LIST OF TABLE

Table 1: Model Parameters Used 15

Table 2: Operating Parameters Used

15 Table 3: Density of Reynolds number on Microbubble DragReduction 19

Table 4: Drag Reduction for different Ujet and U = 5m/s

25 Table 5: Drag Reduction for different Ujet and U = 1m/s



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ABSTRACT

For the majority of current ships sailing, the dominant part of the resistance is due to friction with the surrounding water. Addressing this part of a ship’s resistance means to improve ship’s performance on top of what is achievable by “traditional” optimisations, such as shape optimisation and minimising the radiated waves. By reducing the friction improvements of the ship’s efficiency of net up to 20% are deemed feasible. There is currently no other technique in naval architecture that can promise such savings. A promising technique to address the frictional resistance of a ship is insulating the ship from the water by actively providing an air-layer between ship and water to drastically reduce the resistance of ships and thereby reduce

propulsive power, fuel consumption and CO2 production.

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1. INTRODUCTION :Shipping is vital for global commerce, as it is generally one of the most economical and environmentally friendly transportation methods. In addition. Since approximately 60% of a typical ship’s propulsive power is required to overcome frictional drag, any technique that could significantly reduce a ship's frictional resistance might have a substantial impact both economically and environmentally.Frictional drag stems from the velocity of a fluid on a solid surface being the same asthe velocity of the surface due to the no-slip condition. Momentum is transferred from free stream to near-wall-region by structures in the boundary layer and shear. Methods proposed for frictional drag reduction (FDR) are based on reducing the density or viscosity of fluid near the wall (air lubrication), alter the momentum transport in the boundary layer (air or polymers) or “violate” the no slip condition (can be encountered in microscopic MEMS scale devices). Throughout the last two centuries, various methods to reduce the frictional component of drag have been proposed.We will consider only Air lubrication. Successful application of air lubrication to both existing and new craft would save fuel and reduce exhaust emissions. If successful,air lubrication has been estimated to lead to fuel saving between 5 and 20%.The drag resistance of the hull can be split up into two main parts. One being frictional or viscous drag and the other being pressure drag.Pressure drag is the drag that is created by a flow field around the hull. In this flowfield, water particles show eddying motions, which in this case means that the particles flow around the hull with different velocities. These eddying motions cause resistance and are created by the passage of the hull itself. When a ship has a large block coefficient it will cause a lot of these eddying motions and thus a lot of pressure resistance.Viscous drag is the resistance that is caused by the creation of a boundary layer. This boundary layer consists of water particles clinging to the hull. These water particles are dragged around by the ship. A part of this resistance can be seen when we lookat the stern of the ship. Sometimes it is visible that a bit of water stays with the ship and is only slowly refreshed. It needs no explanation that dragging around water uses extra energy and is therefore extra resistance. When a ship is streamlined or has a low block coefficient, the greater part of the resistance is caused by viscous drag. Pressure drag is mainly caused by the hull form.

Air lubrication aims to reduce the frictional or viscous resistance of the ship. By inserting air in the boundary layer of the flow around the hull, contact between water particles and the hull is being avoided. When there is less or no more contact, the clinging of water particles on to the hull is being avoided. In this way, a lot of resistance is avoided. As mentioned above, the shape of the hull influences the ratio of pressure versus frictional drag, but on average in shipping, the frictional or viscous resistance takes up 80% of the total drag resistance.

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1. Motivation:

The data available on drag reduction by air lubrication is highly scattered. There is a requirement of collating the data in one place and carrying out numerical, analytical and CFD analysis to know the extent to which the process can be implemented on naval as well as marine vehicles. The broader aim is to determine the efficacy and suitability of retrofitment and concept design for the future ships. If proved feasible and economical, this can go a long way in defining the maritime future. There is a requirement for long term analysis of the operational effectiveness of the same and hence determine the more operational effective option from the given design variables in terms of drag reduction.

2. Approach:

For realisation of the objectives, the following approach was adopted:

(a) Data collection: A literature review was carried out and data was collected.

(b) Development of generalised model: Based on the literature survey a generalised model was developed for carrying out the study

(c) Application of result: A preliminary software tool was used for calculation of operational effectiveness after the fitment

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Figure 1: Boundary layer, viscous and pressure drag

Figure 2: Eddying motions and boundary layer

2. TYPES OF AIR LUBRICATION TECHNIQUES :2.1 Bubble drag reduction

In Bubble Drag Reduction (BDR) small bubbles are injected into the boundary layer . The dispersed bubbles act to reduce the bulk density and to modify turbulent momentum transport. The technique is sometimes referred to as micro bubble drag reduction, when the bubbles are very small compared to the boundary layer thickness or wall units. This technique is subject of many studies and some discuss whether the drag reduction mainly comes from modification of effective viscosity, density change, turbulence modification, or change in momentum transport. However, many of the8 | P a g e


early and most promising studies were conducted at the laboratory scale and questions remain regarding the technique's suitability to ship scale; how much gas injection is needed, what is the maximum possible FDR, how far downstream frominjection site will FDR persist, how important is the bubble size, performance in salt water, what is the best injection method

Figure 3: Bubble Drag Reduction Technique

2.2 Air Layer drag reduction

In Air Layer Drag Reduction (ALDR) gas creates a seemingly continuous lubricating layer between hull and liquid. Surface devices (small backward step for instance) may be used to enforce boundary layer separation upstream of the injection point to aid in the initial formation of the layer. In ALDR, as in BDR, no effort is made to re-circulate the injected gas. Air is injected beneath the hull of a ship, forms a film on the flat (horizontal) part of the hull and reduces the frictional drag on the area covered by in excess of 80%.

Figure 4: Air Layer Drag Reduction Technique

2.3 Partial cavity drag reduction

In Partial Cavity Drag Reduction (PCDR) gas creates a lubricating layer between the hull and liquid. Drag reduction is achieved by filling a recess, much thicker than the ship-hull boundary layer thickness, with gas. To apply PCDR on a ship's hull, the bottom of the hull needs to have indentations, which are to be filled with gas, usually air. A backward-facing step (BFS) on the upstream end of the recess and a gently9 | P a g e


downwards sloping closure on the downstream side normally form the recess which traps the gas, thus forming a ventilated partial cavity. Gas is injected continuously into the cavity to make up for that which is lost to entrainment, but with proper cavity designthe gas loss is minimized. In addition to the single wave partial cavity, a multi wave partial cavity drag reduction may be possible, which would enable there to be multiple ideal operating speed ranges where the cavity would be closing on the beach with low air loss. With a properly designed closure and within a design speed range(s), only a minimal amount of the introduced gas is lost at the cavity closure. The gas separates the solid surface from the liquid resulting in more than a 95% decrease in frictional drag for the area covered.

Figure 5: Partial Cavity Drag Reduction Technique

3. FLAT PLATE EXPERIMENT METHOD : 3.1) METHOD :We consider the flow beneath a horizontal flat surface. Gas is injected near the leading edge of a horizontal surface of length L and width b. The free-stream flow beneath the surface has velocity U, and the depth (i.e. draft) of the surface is d. The baseline flowoccurs without injection, and we assume that the flow is of sufficient Reynolds number that a fully developed turbulent boundary layer exists along the entire

length, L. The baseline friction drag coefficient on the smooth plate, CFBS

F is the friction force, and ρ and μ are the density and viscosity of the liquid, respectively. The baseline friction coefficient on the rough surface is CFBR = 0.004 overthe entire speed range, which corresponds to a fully rough surface with k/L ~2x10-5, with k the representative length of the surface roughness and L ~ 10 m. This corresponds to the conditions reported in Elbing et al. (2009). Hence, we are assuming that the baseline drag coefficient for the rough plate is constant with ReL. The baseline power needed to move the fluid across the surface of the plate is

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The power needed to inject a given quantity of air beneath the surface of the hull is given by,

where qA is the gas volume flux per unit width at depth d, pA is atmospheric pressure, g is the gravitational acceleration, and ηA is the pumping efficiency. Note that the pumping work increases with roughly the square of the draft, since work is required to overcome the hydrostatic head of the flow at the point of injection, and the presence of a higher pressure at the plate surface leads to compression of the gas (i.e. for relevant ship drafts, ρgd/pA >> 1). As the draft increases, the gas mass flux must increase to deliver the required gas volume flux at the depth of the plate surface.

Figure 6. The spanwise air volume fluxes required for transitional and fully developed air layers on the smooth and rough surfaces as a function of flow speed, U.

We use these data to estimate the required gas flux for the formation of air layers. The following relationships are employed. To create an air layer on a smooth surface producing an average %DR = 80%, the required minimum flux is:qALS = 0.0002U2 + 0.0063U − 0.0234This equation and the two that follow are determined via curve. To create an air layeron a rough surface producing an average of %DR = 80%, the required minimum flux is:qALR = 0.0004U2 + 0.0058U − 0.0003And, to create the boundary for the formation of a transitional air layer on a smoothsurface producing an average %DR = 20%, the required minimum flux is:

qALT = 0.0002U2 + 0.0043U − 0.0233

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These data were acquired on a test model with L ~ 10 m over a speed range of 6 m/s< U < 15 m/s. Once air layers were formed, there was no appreciable change in their appearance by the downstream termination of the test model. However, we do notknow the ultimate persistence length of the air layer beyond which the layer either breaks up or requires additional injection of gas. We now compare the power required to produce a transitional or developed air layer on the horizontal surface with the propulsion power that will be saved. Of course, the break-even points are influencedstrongly by our assumed propulsion and pumping efficiency.

Table 1. Model parameters used in the calculation

Table 2. Operating parameters used in the calculation

The air-flux data were collected for L ~ 10m over a speed range of 6 m/s < U < 15 m/s. Hence,we extrapolate these data beyond their measured range as we conduct the cost-benefit analysis.

3.2) COST BENEFIT ANALYSIS :

The relationships presented above suggest that the air flux required to form air layers increases as ~ U2, while the propulsive power increases as U3. Hence we would expect that at some speed, the energy cost balances the energy saved. Also, the required pumping power will increase as ~ d2. Finally, the relative benefit will increase with increasing L, as the cost of pumping the gas into one location along the surface yields drag reduction along the entire length of the surface. Hence, three parameters will drive the cost-benefit comparisons: speed, draft, and length.

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Therefore, with 100% drag reduction, and negligible air pumping power, the savings would be 100%. But, with less drag reduction and increased pumping power, we can reach the break-even condition where %PS = 0%. And, it is possible to have a negative value of %PS when the required pumping power exceeds any realized savings in propulsive power.

3.3) CALCULATION OF VALUES FOR A FLAT PLATE :

Given Plate Dimensions:

Length

Breadth

Speed Of Advance

: 20 m

: 5 m

: 10 m/s

3.3.1) WITHOUT ALS :Using ITTC FORMULA, power required to move the plate (without ALS)

P1 = 711.816 watts

3.3.1) WITH ALS :Power required to move to plate is given by

P2 = 377.735 watts

Also, for 80% drag reduction, power required to pump the air is given by

P3 = 0.667 watts

Total power required using Air Lubrication System: P2 + P3

= 378.402 watts

3.4) RESULTS :Reduction in power value = [P1-(P2+P3) ] / P1

= 46.8%

Hence, reduction in power for a rectangular flat plate of given dimensions using Air Lubrication System (ALS) is 46.8%

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4. BOUNDARY LAYER METHOD FOR CALCULATING DRAG REDUCTION:

4.1) METHOD :

Figure 7: Schematic drawing of a microbubble injecting system (plate on top).

The injected microbubbles are assumed to be distributed uniformly across the boundary layer. The air volume fraction Cv is defined as the ratio of the injected air flow rate divided by the summation of the air flow rate and the water flow rate within the boundary layer,

where Qa is the injected air flow rate, and Qw is the waterflow rate within the boundary layer of the plate. Based on the turbulent boundary theory, the water flow rate within the boundary layer of the plate can be calculated by

where b is the width of the plate, U0 is the inflow velocity, and δ is the boundary layer thickness,

which is defined as the distance from the wall where the velocity is 0.99U0. A seventh power velocity distribution is assumed for the velocity distribution across the boundary layer

And the displacement thickness δ∗ is defined as

The Schlichting boundary thickness formula is used to estimate the thickness of the boundary layer. The water flow rate can be calculated by

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Then the air volume fraction Cv in can subsequently be calculated with measured injected air flow rate and the water flow rate. For a flat plate without injected bubbles, the frictional resistance Df of a flat plate with length l and width b can be derived as

For the water-bubble mixture of the boundary layer with injected bubbles, the mixture

density ρb can be calculated by the linear combination of the density of air and the density of water according to the air volume fraction Cv, and is given by

where ρa is the density of the injected air. The dynamic viscosity of the water-bubble mixture can also be calculated by using the same approach as

The frictional resistance of a flat plate with a water-bubble mixture boundary layer Dfb

can be calculated by using the same approach as

The ratio of the frictional resistance of water-bubble mixture to the frictional resistance of the water is then expressed as

If the frictional resistances Dfb and Df are normalized by the dynamic pressure of the inflow (1/2)ρwU2 and the surface area of the flat plate b∗l, then the same result will be had for the nondimensional resistance coefficients as will be

Equation predicts the nondimensional frictional resistance of water-bubble mixture of the plate from the nondimensional frictional resistance of flat plate in water. The dragreduction ratio DR predicted by the boundary layer mixture model can be calculated by

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4.2) CALCULATION AND GRAPH PLOTTING :

Table 3: Density and Reynolds number effect on the microbubble drag reduction.

Figure 8: The drag reduction effect predicted by the boundary layer mixture model

Cv ρ b / ρ w R e b l ^ (0 . 2 ) / R e l ^ ( 0 . 2 ) C fb /C f D R0 1 1 1 0

0.1 0.9 1 0.9 0.10.2 0.8 1.001 0.801 0.1990.3 0.7 1.001 0.701 0.2990.4 0.6 1.002 0.602 0.3980.5 0.501 1.003 0.502 0.4980.6 0.401 1.004 0.402 0.5980.7 0.301 1.006 0.303 0.7970.8 0.201 1.011 0.203 0.8970.9 0.101 1.024 0.103 0.997

0.99 0.011 1.174 0.013 0.987

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Figure 8 shows the ratio of non dimensional resistance coefficients of the bubble- water mixture to the pure water with the parameter of the air volume fraction and the drag reduction ratio. The effect of the density of the mixture and Reynolds number onthe microbubble drag reduction technique is shown in Table 3. The effect of the Reynolds number is very small when compared with the effect of density of the mixture. The density of the bubble mixture becomes the key parameter for the microbubble drag reduction technique. The ratio of the frictional resistance of the water-bubble mixture boundary layer to the water boundary layer is almost directly proportional to the density ratio.

4.3) RESULTS AND CONCLUSIONS:

A boundary layer mixture model was derived to predict the drag reduction effect of microbubble drag reduction technique in the flat plate.From the prediction of the boundary mixture model, the test results, and the discussions, the following conclusions can be drawn.

(1)The drag reduction effect predicted by the boundary mixture model is almostdirectly proportional to the density ratio of mixture and water.

(2)The maximum drag reduction effect of the microbubbles in water tunnel is about 80%, and the frictional resistance coefficient is in good agreement with the value predicted by the boundary mixture model. The maximum drag reduction effect of the microbubbles in the towing tank is only about 30% and drag reduction is much smaller than that predicted by the boundary mixture model.

(3)The drag reduction effect increases monotonically with increasing the air flow rate in the water tunnel. However, an optimal air flow rate exists for each velocity in the towing tank.

(4)The different drag reduction effect in the water tunnel and in the towing tank may be due to the different bubble behaviors produced by the different velocity gradient. Future study is required to examine the details of the developing process of the injected bubble in the towing tank.

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0

0.1

0.199

0.299

0.398

0.498

0.598

0.797

0.897

0.997 0.987

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

DR

DR

Figure 10 : densityb/densityw vs DRAG REDUCTION

Figure 9 : Cv vs DRAG REDUCTION

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5. COMPUTATIONAL FLUID DYNAMICS (CFD) ANALYSISOF BUBBLE LUBRICATION IN A 3-D RECTANGULAR PLATE

: 3m: 1.5 m: 0.5m: 0.2m

PLATE DIMENSIONLength Breadth ThicknessDiameter of hollow opening

DOMAIN DIMENSIONLength

Breadth Thickness

: 9m:4.5 m:1.5m

5.1) GAMBIT MODELLING OF THE PROBLEM:

Figure 11: Gambit modelling of the problem

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5.2) MESH MODELLING OF THE PROBLEM

Figure 12: Mesh modelling of the problem

5.3) FORMULATION OF THE PROBLEM IN CFD:In ANSYS FLUENT, Euler-Euler multiphase modelling was adopted. The other one is Euler – Langrangian approach. There are three different models available for this kind of modelling

1.) The VOF Model

The VOF model is a surface-tracking technique applied to a fixed Eulerian mesh. It is designed for two or more immiscible fluids where the position of the interface between the fluids is of interest. In the VOF model, a single set of momentum equations is shared by the fluids, and the volume fraction of each of the fluids in each computational cell is tracked throughout the domain. Applications of the VOF model include stratified flows, free-surface flows, filling, sloshing, the motion of large bubbles in a liquid, the motion of liquid after a dam break, the prediction of jet breakup (surface tension), and the steady or transient tracking of any liquid-gas interface.

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2.) The Mixture Model

The mixture model is designed for two or more phases (fluid or particulate). As in the Eulerian model, the phases are treated as interpenetrating continua. The mixture model solves for the mixture momentum equation and prescribes relative velocities to describe the dispersed phases. Applications of the mixture model include particle- laden flows with low loading, bubbly flows, sedimentation, and cyclone separators. The mixture model can also be used without relative velocities for the dispersed phases to model homogeneous multiphase flow

3.)The Eulerian Model

The Eulerian model is the most complex of the multiphase models in FLUENT. It solves a set of n momentum and continuity equations for each phase. Coupling is achieved through the pressure and interphase exchange coefficients. The manner in which this coupling is handled depends upon the type of phases involved; granular (fluid-solid) flows are handled differently than non-granular (fluid-fluid) flows. For granular flows, the properties are obtained from application of kinetic theory. Momentum exchange between the phases is also dependent upon the type of mixture being modeled. FLUENT’s user-defined functions allow us to customize the calculation of the momentum exchange. Applications of the Eulerian multiphase model include bubble columns, risers, particle suspension, and fluidized beds.

Figure 13: Multiphase model

SST K-Omega Turbulence Models

The SST k-omega turbulence model is a two-equation eddy-viscosity model that is used for many aerodynamic applications. It is a hybrid model combining the Wilcox k- omega and the k-epsilon models. A blending function, F1, activates the Wilcox

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model near the wall and the k-epsilon model in the free stream. This ensures that the appropriate model is utilized throughout the flow field:

The k-omega model is well suited for simulating flow in the viscous sub-layer. The k-epsilon model is ideal for predicting flow behavior in regions away from the

wall.

OTHER VALUES USED:

Number of cells

Reynolds number

Under relaxation factor

: 4,98, 143

: 6.6 * 106

: 0.2

Multiphase flow model was decided upon so that the interpenetrating two phases can be taken

5.4) CALCULATION AND RESULTS:

Table 4: Drag reduction for different jet velocities and body speed 5m/s

U (m/s) Ujet (m/s) FP (N) FF (N) FD (N) Drag reduction (%)0 53.54 589.9 643.44 -

0.1 52.61 535.18 587.79 8.640.5 50.45 348.89 399.35 37.931 52.05 218.23 270.29 57.99

2.5 73.44 170.63 244.07 62.065 5 120.21 153.03 273.25 57.53

7.5 164.23 146.79 311.02 51.6615 188.13 141.46 329.59 48.7730 381.62 137.04 518.66 19.39100 520.24 137.42 657.67 -2.21

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Figure 14: Drag reduction for different jet velocities and body speed 5m/s


U (m/s)

Ujet (m/s)

FP (N) FF (N) FD (N) Drag reduction (%)

0 3.48 5.29 8.7 -0.001 3.44 5.09 8.53 1.970.005 3.6 4.52 8.13 6.57

1 0.01 3.95 3.94 7.9 9.230.5 11.93 2.55 14.49 -66.531 20.43 2.13 22.57 -159.310 72.37 2.52 50 -474.42

-10

70

60

50

40

30

20

10

00 20 40 60 80 100 120

DR

U JET

Drag reduction (%) -

Drag reduction (%) - Linear (Drag reduction (%) -)

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U (m/s)

Ujet (m/s) FP (N) FF (N) FD (N) Drag reduction (%)

0 57.27 58.53 115.8 -1 65.26 15.53 80.79 30.23

4 2 64.47 13.77 78.24 32.434 60.83 12.94 73.78 36.28

8 68.47 12.35 80.79 30.2325 117.38 11.23 128.62 -11.06

-20

-40

-60

-80

-100

-120

-140

-160

-180

40

20

0-0.2 0 0.2 0.4 0.6 0.8 1 1.2

DR

U jet

-- Linear (-)

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U (m/s) Ujet (m/s) FP (N) FF (N) FD (N) Drag reduction (%)0 824 684 1508 -1 890 224 1115 26.065 805 164 969 35.7415 613 149 762 49.46

15 30 539 137 677 55.150 525 128 654 56.63

100 727 122 846 43.89200 914 123 1037 31.23300 1100.925 127.2 1228.12 18.55500 1406.849 144.18 1551.03 -2.85

36.2832.43

30.2330.23

-20

-10

30

20

10

00

40

0 5 10 15 20 25

-11.06

30

DR

U jet

Drag reduction (%)Drag reduction (%) Linear (Drag reduction (%))

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5.5) INFERENCES:

A computational fluid dynamics approach for estimation of drag reduction using air jets for underwater axisymmetric vehicles has been presented and reasonably validated with other numerical work. Maximum drag reduction can be obtained if the influencing parameters are selected in their optimal ranges. Drag reduction with air jets has been demonstrated to be an effective potential method of drag reduction. Experimental validation is required for further meaningful research in this area.

-10

0 100 200 300 400 500 600

DR

U jet


Drag reduction (%)60

50

40

30Drag reduction (%)

20

Linear (Drag reduction (%))

10

0

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6.) SEAKEEPING ASPECTS OF AIR LUBRICATION

Tests were performed with the segmented model for the condition without air lubrication, with air chambers and with air bubbles, in regular and irregular waves, at two speeds in wave directions ranging from bow to stern quartering. The purpose of the tests was to establish whether the air lubrication remained effective in waves, if lost air quantities would appear in the propeller disk area to disturb the propeller efficiency and loading and to which extend the ship motions would be affected by air lubrication. In general terms the results of the tests were as follows:

•Ship motions appeared not to be significantly affected by air lubrication. An exception is beam seas for which the model with air chambers showed significantly more roll. This is due to the change in stability.• Although power savings reduce when operating in waves, for most conditions withair bubbles power reductions remain possible, while for some low speed conditions with air chambers more power is required with air lubrication than without air lubrication• The model with air chambers showed substantial loss of air volume due to shipmotions and waves at the lower speeds and substantial quantities of air occasionally arrived in the propeller operating area causing thrust and torque variations

Figure 18: Heave response with and without air bubbles

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Figure 19: Effective power reduction in waves with air chambers

6.1.) BASIC COMPUTATIONAL MODEL

To study the effects of a seaway on the efficiency of air lubrication and vice versa the effects of air lubrication on the seakeeping characteristics of ships, a computational method is required that combines traditional ship hydrodynamics with a description of air chamber flows. PANSHIP is a time domain panel method that uses linearised free surface boundary conditions. The geometry of the ship hull is represented by a number of quadrilateral source and doublet panels. By using a transient Green function description for the fluid potential and including the effect of ship motions and incident waves in the boundary condition at each panel, the pressure acting on each panel is obtained. PANSHIP has two modes of use: a semilinear one and a semi- nonlinear one.The first mode is the time domain equivalent of frequency domain based panel methods in which the nonlinear effects are limited to the Froude-Krylov forcecomponents and the determination of the steady trim and sinkage. The second mode determines besides the Froude-Krylov forces also “added mass” and “damping” forces on the actual submerged hull form. The only linearization applied is the use of linear free surface boundary conditions. A relatively new development in PANSHIP is the use of the lifting surface option to account for so-called viscous reaction or manoeuvring forces when operating in beam or quartering waves. Traditionally, such force components are described by means of empirical formulations based on experimental results. At high speed viscous forces other than friction are assumed to be relatively small so that the side forces acting on a hull operating at drift angle can be described by modelling the hull as a low aspect ratio lifting surface. This would eliminate the use of empirical models which are inherently of limited validity. PANSHIP determines motions in six degrees of freedom and can handle all wave directions. The fundamental nature of the computational method and the use of panels to

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represent the hull shape makes PANSHIP a suitable tool for use for ships with air chambers. Air chambers can be well represented by surface panels while replacing the water fluid flow by an air flow enables the computation of the pressures on panels in air chambers.

Figure 20: Comparison roll and pitch motions

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6.2.) CONCLUSIONS:The following conclusions were drawn with respect to the investigations for the Seakeeping characteristics of ships with air lubrication:

•The seakeeping characteristics of the investigated ship are not significantly affected by the use of either bubbles or air chambers for air lubrication. An exception is the roll motion in beam seas for the model with air chambers.•Power savings remain largely intact with lubrication by means of air bubbles but may become negative when using air chambers at a relatively low speed. Also, lost air volume from air chambers may affect the propeller thrust and torque.•Predictions for air chambers dynamics such as the variations of air volume, pressures and air losses seem to be in qualitative agreement with expectations.•Predictions for the motions in waves agree well with experimental data. This is the case for the ship operating with and without air chambers. Only when motions aresmall, relative predictions for the ratio of motions with and without air chambers are inaccurate.

7.) MANOEUVRING ASPECTS OF AIR LUBRICATIONThe primary purpose of the manoeuvring tests was to determine the effect of the air lubrication on the manoeuvring characteristics and to obtain validation data for manoeuvring simulation models. Secondary objectives were:

• judge the difference in manoeuvring characteristics between the concepts• verify compliance with relevant criteria posed in IMO Resolution A.751• compare the behaviour of the ship to other ships

The manoeuvring tests comprised captive planar motion mechanism (PMM) test and free sailing manoeuvring tests. During the manoeuvring tests, the hull without air lubrication (designated conventional or bare hull), with micro-bubbles and with air cavities are examined.

7.1) PMM EXPERIMENTS:

The PMM tests were conducted to investigate the forces on the hull form during manoeuvring conditions. Furthermore, data was obtained for the determination and validation of mathematical manoeuvring models for air lubricated ships. It is seen that a slight reduction in the side forces is found for the ship with micro-bubble lubrication, compared to the conventional (bare-hull with air cavities closed and without lubrication) configuration. Note that the relatively large force on the aft segment is caused by the method of non dimensionalisation: the local forces are made nondimensional using the lateral area of the segment, which in the case of segment 1 is30 | P a g e


much smaller than the lateral areas of the other segments. Judging from the test results and underwater video observations, it was found that the amount of air lubrication using micro bubbles or air cavities reduced as a function of the driftangle. In general, the micro-bubbles followed the flow direction and left the ship's bottom at about the drift angle. With air cavities, air left the air chambers due to pressure build-up or reduction at the side-walls of the chambers. For increasing drift angles, this effect was more pronounced. This means that during extreme manoeuvres, the air lubrication is not effective anymore.

Figure 21: Lateral forces measured on an air-lubricated ship, two speeds, two cases (with micro-bubbles (markers) and without (lines)), pure drift, drift angle 2.5°

Figure 22 Distribution of lateral force along ship length at 9 deg drift angle

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Figure 23: Comparison of overall lateral force.

8.) SCALE-EFFECTS OF AIR-LUBRICATIONThe de-formability of micro-bubbles is a necessity for achieving drag reduction. This is also supported by findings of this project being discussed and described in Thill (2004) and Thill (2005). As on model scale the counter pressure at model draught is much too less to compress the ambient air similar to full-scale, tests should be conducted in depressurised conditions. This can only be achieved in MARIN’s depressurised towing tank, which was used during all calm water model tests. Similarity of the air compressibility is achieved when the pressure is lowered as:

where the indices M and S, respectively, represent the ship and the model, whereasλ is the scale ratio.For the tests we assumed a ship of 120m long being represented by the two models,resulting in ambient pressures of down to 50 mbar. As it is very difficult to compress such highly diluted air, a reservoir of nitrogen was taken along with the carriage. This nitrogen was 99.999% clean nitrogen, in other words, water or other pollutants, which could congest the air supplying porous medium were not expected. However, expanding the nitrogen from the 200 bar inside the vessel to the ±97 mbar and ±50 mbar, respectively, in the tank goes hand in hand with an undesired fall in temperature, which might cause the air supplying porous medium to freeze.Therefore, the nitrogen flow was heated up twice to 500 Celsius while stepwise expanding from 200 to 5 bar.The gas flow was controlled and measured by industrialMass Flow Controllers (MFC), having an assured accuracy of ±1 per cent. The expansion of the nitrogen over the MFC was accounted for by assuming isotherm expansion to the tank water temperature, seen the large heat capacity of the

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surrounding water. No conclusive scale effects of micro bubbles could be derived from the model tests the two ship models. Therefore, drag reduction by micro bubbles was tested at almost full-scale Reynolds number in the large cavitation tunnel (UT2) of Berlin Technical UniversityAn effective flat plate length of about 10m and tunnel speeds of up to 10m/s allowed testing of Reynolds numbers of up to about 8·107. Still drag reductions of about 30 per cent were found, indicating that micro bubbles deliver the desired drag reductionseven at Reynolds numbers close to that of full scale ships. Compressed air was injected in two upstream positions, 4.1m and 2.5m upwards of the centre of gravity of the sensitive plate. Within the achievable accuracy of the measurement, hardly any relaxation effect was measurable with this set-up. Besides air flow and relaxation length, the ambient pressure inside of the tunnel was varied between atmospheric pressure and 125 mbar. It turned out that the dominant effect of the pressure variation was the expansion of the gas. Converting the controlled mass flow of the MFC into a volume flow by the ideal gas law showed that the achieved drag reductions where almost identicaI. This way, the measured drag forces to the sensitive plate of 300mm by 1000mm in size is exaggerated as the flow measured by the MFC indicate the mass flow at atmospheric (standard) condition.

Figure 24: Measured drag force at 30 / 30 l/min, in Newton versus speed in [m/s]

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9.) CONCLUSIONS:Important overall conclusions from the investigations of PELS are:

•In calm water net effective power reductions of 3-10% were achieved. Note, this improvement can always be added to achieved improvements of traditional optimisations.

•In increasing wave heights an increasing part of this advantage is lost. However, the system does not cause an increase in added resistance or an otherwise deterioration of seakeeping at normal operating speeds.

•The application of drag reduction imposes some changes in the design of the ship, especially the bottom of the hull. When microbubble drag reduction is used, the resistance of the ship against rotation or sway motion is reduced. This deteriorates the manoeuvring performance of the ship. Therefore, additional measures should be taken to maintain the desired manoeuvring characteristics.For example, skegs are to be applied in the aft ship to improve the controllability. When using air chambers, the sidewalls of the chambers produce sufficient cross-flow dragto obtain the required manoeuvrability.

•No severe scale effects were found which could question the principle of air- lubrication in general.

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10. FUTURE PROJECTS AND SEA TRIALS

Currently at least four large projects investigating the various air lubrication techniques are ongoing worldwide, and air lubrication looks to be especially promising for ships with flat bottoms and with high length-to-beam ratios. Of the two sea-trial producing net energy savings, the Pacific Seagull had L/B = 5.9 and Mitsubishi's ship had L/B = 4.3. A Great Lakes 1000 footer has L/B ratio of 9.6, which combined with high box coefficient and shallow draft make the 1000 footer a perfect candidate for implementing air lubrication.

There have been two sea trials where the flow was likely in the BDR-transitional-ALDR region. One such sea trial on the Pacific Seagull yielded 5 to 10% net energy savings while a second sea trial by Mitsubishi Heavy Industries achieved 8 to 12% net energy savings. As for PCDR, a scale test by MARIN recently showed 15% net energy savings (Foeth, 2011) and for a 1:12th scale test by STENA they reported resistance reduction of 20 to 25% (Surveyor 2011). The potential net energy savings predicted for ALDR are slightly higher than observed in the sea trials. This is likely explained by a combination of the following: the sea trials may not have had sufficient air flux supplied to achieve a true air layer, air entrainment into the propulsor, presence of flow perturbations in the open ocean or the area fraction of the wetted hull covered for these ships was less than assumed in the current analysis.

PCDR requires more modifications to the bottom of the hull than BDR or ALDR, but could potentially offer larger frictional drag reduction with a lesser gas flux. Hence, the capital cost would probably be higher than for ALDR, but the operating cost may be lower. The tradeoff between upfront cost and operating cost will be a ship specific consideration. Also, air layers probably offer a more flexibility in the operating speed range, while PCDR may be significantly more economical for a narrow operating speed range(s). Hence the suitability of each of these techniques for a given ship or barge is also affected by the intended use of the vessel.

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11.REFERENCES1. Amromin, E. and Minize, I. "Partial Cavitation as Drag Reduction Technique

and Problem of Active Flow Control" Marine Technology, Vol. 40, No. 3, pp. 181-188. 2003

2. Amromin, E., Kopriva, J., Arndt, R. E. A., and Wosnik, M. “Hydrofoil Drag Reduction by Partial Cavitation.” Journal of Fluids Engineering, 128, pp. 931- 936. 2006

3. Butuzov AA. "Artificial cavitation flow behind a slender wedge on the lower surface of a horizontal wall." Fluid Dyn. 3. pp.56–58. 1967

4. Butuzov, A., Sverchkov, A., Poustoshny, A. and Chalov, S., “State of art in investigations and development for the ship on the air cavities.” International Workshop on Ship Hydrodynamics, China, pp. 1-14. 1999

5. Ceccio, S.L. "Friction Drag Reduction of External Flows with Bubble and Gas Injection," Annual Review of Fluid Mechanics, Vol. 42, pp. 183-203. 2010

6. Ceccio, S.L., Perlin, M. and Elbing, B.R., “A cost-benefit analysis for air layer drag reduction” Proc. Int. Conf. On Ship Drag Reduction- SMOOTH-SHIPS,Istanbul, Turkey. 2010

7. Elbing, B. R., Winkel, E. S., Lay, K. a, Ceccio, S. L., Dowling, D. R., & Perlin,M. "Bubble-induced skin-friction drag reduction and the abrupt transition to air- layer drag reduction". Journal of Fluid Mechanics, 612, 201-236, 2008.

8. Foeth, E.-J. “Projects prove that air cavities reduce ship resistance.” report, MARIN’s news magazine, 16. August, 2011

9. Latorre, R., “Ship hull drag reduction using bottom air injection.” Ocean Engineering, 24(2), pp. 161-175. 1997

10.Lay, K.A., Yakushiji, R., Mäkiharju, S., Perlin, M. and Ceccio, S.L. “Partial cavity drag reduction at high Reynolds numbers,” Journal of Ship Research, v.54, n.2, pp. 109-119. 2010

11. Mäkiharju, S., Elbing, B.R., Wiggins, A., Dowling, D.R., Perlin, M. and Ceccio, S.L., “Perturbed Partial Cavity Drag Reduction at High Reynolds Numbers” Proc. 28th Symposium on Naval Hydrodynamics, Pasadena, CA, 2010.

12.Mäkiharju, S., “The Dynamics of Ventilated Partial Cavities Over a Wide Range of Reynolds Numbers and Quantitative 2D X-ray Densitometry for Multiphase Flow,” PhD thesis, University of Michigan, 2012.

13.Madavan, N.K., Deutsch, S., and Merkle, C.L. “Measurements of local skin friction in a microbubble modified turbulent boundary layer.” Journal of Fluid Mechanics, 156:237–256, 1985.

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