Drag reduction through air lubrication report

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

LIST OF CONTENTS

Introduction09Motivation10Approach10

Types of Air Lubrication Techniques11Bubble Drag Reduction11Air Layer Drag Reduction122.3 Partial Cavity Drag Reduction12

Flat Plate Experiment Method13Method13Cost Benefit Analysis15Calculation of values for Flat Plate16Without ALS163.3.2 With ALS163.4 Results16

Boundary Layer Method for calculating Drag Reduction17Method17Calculation And Graph Plotting 194.3 Results And Conclusions20

CFD Analysis of ALS in a 3D Rectangular Plate22Gambit Modelling22Mesh Modelling23Formulation of problem in CFD 23

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

5.4 Calculation And Results255.5. Inferences29

Seakeeping Aspects of Air Lubrication30Basic Computational Method31Conclusions33

Manoeuvring Aspects Of Air Lubrication337.1 PMM Experiments33

Scale Effects Of Air Lubrication35

Conclusions37

Future Scope And Live Projects38

References 39

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

LIST OF TABLE

Figure 1: Boundary Layer, Viscous And Pressure Drag11 Figure 2: Eddy Motion And Boundary Layer11 Figure 3: BDR Technique12 Figure 4: ALDR Technique12 Figure 5: PCDR Technique13 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 Model19 Figure 9: Cv vs Drag Reduction21 Figure 10: Density Ratio Vs Drag Reduction21. Figure 11: Gambit Modelling Of Problem22 Figure 12: Mesh Modelling Of Problem 23 Figure 13: Multiphase Model24 Figure 14: Drag Reduction for different Ujet and U = 5m/s26 Figure 15: Drag Reduction for different Ujet and U = 1m/s27 Figure 16: Drag Reduction for different Ujet and U = 4m/s28 Figure 17: Drag Reduction for different Ujet and U = 15m/s29 Figure 18: Heave Response with/without air bubbles30 Figure 19: Effective Power Reduction 31 Figure 20: Comparison of roll and pitch motion32 Figure 21: Lateral forces measured on air lubricated ships34 Figure 22: Lateral forces along ships length34 Figure 23: Overall lateral forces35 Figure 24: Measured Drag Force36

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

LIST OF TABLE

Table 1: Model Parameters Used15 Table 2: Operating Parameters Used 15 Table 3: Density of Reynolds number on Microbubble DragReduction 19Table 4: Drag Reduction for different Ujet and U = 5m/s 25 Table 5: Drag Reduction for different Ujet and U = 1m/s 26 Table 6: Drag Reduction for different Ujet and U = 4m/s 27 Table 7: Drag Reduction for different Ujet and U = 15m/s 28

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

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 ships resistance means to improve ships 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 ships 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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MINOR PROJECT-2016DEPT. OF APPLIED MECHANICS, NCW, IIT-DELHI

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

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.Approach:

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

Data collection: A literature review was carried out and data was collected.Development of generalised model: Based on the literature survey a generalised model was developed for carrying out the studyApplication 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 dragFigure 2: Eddying motions and boundary layer

2. TYPES OF AIR LUBRICATION TECHNIQUES :2.1 Bubble drag reductionIn 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 the

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

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 methodFigure 3: Bubble Drag Reduction Technique2.2 Air Layer drag reductionIn 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