CFD ANALYSIS OF FRICTIONAL DRAG REDUCTION ON THE UNDERNEATH OF SHIP S HULL USING AIR LUBRICATION SYSTEM

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 4, April 2018, pp , Article ID: IJMET_09_04_046 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed CFD ANALYSIS OF FRICTIONAL DRAG REDUCTION ON THE UNDERNEATH OF SHIP S HULL USING AIR LUBRICATION SYSTEM Vijayan.S.N Assistant Professor, Karpagam Institute of Technology, Coimbatore, Sendhilkumar.S Associate Professor, Info Institute of Engineering, Coimbatore, Kiran Babu.K.M Assistant Professor, Annasaheb Dange College of Engineering and Technology, Ashta Duraimurugan G.K and Deepak.P Assistant Professor, Vels Institute of Science Technology & Advance studies, Chennai ABSTRACT Reduction of surface friction between the underneath of ship s hull surface and water surface tends to reduce the fuel consumption and attainment of higher speed in ship. Various researches and methods were going on to reduce the friction for a long time in ship building industry. Among various methods, air lubrication system is most effective and easiest to achieve higher speed with minimum consumption of fuel. Also this method reduces the environmental impacts and an economic one. The present investigation is focused on frictional drag reduction on the underneath of ship s hull by introducing air cavities. Frictional drag has been reduced and increase in speed occurs during the air lubrication. Numerical results are compared with the results obtained by without applying air lubrication system. Keywords: Ship, Air lubrication, Friction, Finite Element Analysis. Cite this Article: Vijayan.S.N, Duraimurugan G.K, Sendhilkumar.S, Kiran Babu.K.M and Deepak.P, CFD Analysis of Frictional Drag Reduction on the Underneath of Ship s Hull Using Air Lubrication System, International Journal of Mechanical Engineering and Technology, 9(4), 2018, pp editor@iaeme.com

2 Vijayan.S.N, Duraimurugan G.K, Sendhilkumar.S, Kiran Babu.K.M and Deepak.P 1. INTRODUCTION Vessels utilize large quantities of fuel to generate the required propulsive power to overcome resistance resulting from their motion across ocean surfaces. The ship s by product such as nitrogen oxide, carbon dioxide and sulphur oxide emissions significantly contribute to global climate change and acidification of ocean releases pollutants. These emissions and pollutants further contribute to environmental problems and surging prices of raw materials, including oil, arising from the economic growth of developing countries. Approximately 60% of a typical ship s propulsive power required to overcome frictional drag. Techniques or practices can significantly reduce ship's frictional resistance which has a substantial impact both economically and environmentally. A number of techniques are available to reduce the viscous drag. The most familiar ones are adding polymers to the flow, the application of structured surfaces and certain coatings. However, most of them cannot be applied efficiently on ships because of various practical limitations, including costs. One of the most promising viscous drag reduction techniques for a ship is so-called air lubrication. Especially when air forms a stable layer that prevent water contact with the hull. The present study focuses on drag reduction by providing air cavities underneath a horizontal surface. The drag reduction by air cavities was acknowledged as a prospective technology for ships. Air Lubrication System is now a technology which is well proved to provide benefits such as reduced carbon emissions and substantial fuel savings thereby increasing the speed and improvement in efficiency of the ship is attained. The air lubrication method, which reduces the resistance of the hull by using air bubbles. The three general approaches are injection of air bubbles along the hull, introducing the air films under the hull and air cavities in the bottom of the hull. 2. LITERATURE REVIEW In the air lubrication system approach both air layer and partial cavity drag reduction could lead to net energy savings of 10 to 20%, with corresponding reductions in emissions (1) with air layers useful in reducing the frictional resistance at specific conditions of air injection in bulk carrier for getting net power savings (2). Determined the reduction in flow resistance based on the bubble coverage around the hull also predicts the intrusion of bubbles on the area of propeller disks, which could deteriorate the performance (3). If properly implemented, it was estimated that air lubrication could lead to net fuel saving between 5 to 20%, with the corresponding reduction in NOx, SOx, particulate and CO 2 emissions(4). Significant decrease in underwater noise can be achieved by using gaseous layers on the ship hull and sound radiation (5). Minimizing the viscous drag and reducing the shipping costs by micro bubble drag reduction technique with the use of hydrophobic plates to trap and retain air layer (6). The effect of air lubrication on resistance of a chemical tanker is investigated numerically and the coefficient of frictional resistance is calculated and the result of air lubricated system is compared with without air lubrication (7). In fully loaded condition net drag reduction is almost zero, then at ballast condition 2% net reduction (8). By reducing the friction improvements of the ship s efficiency of net up to 20% are deemed feasible. 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 which drastically reduces the resistance of ships and thereby reducing propulsive power, fuel consumption and environmental problems (9). Commercial package of Computational Fluid Dynamics (CFD) is being employed to investigate the performance of the system and optimize the parameters (10) editor@iaeme.com

3 CFD Analysis of Frictional Drag Reduction on the Underneath of Ship s Hull Using Air Lubrication System The boundary mixture model is derived to predict the performance of the micro bubble drag reduction(11).the phenomenon of drag reduction by the injection of micro bubbles into turbulent boundary layer has been investigated using two numerical models namely two-fluid inhomogeneous and MUSIG models. Inhomogeneous model, which uses a fixed bubble diameter, shows a very good comparison of the skin-friction co-efficient with the experiment (12). Drag reduction caused by micro bubbles injection within the boundary layer has been investigated in a horizontal channel and the fluctuating velocity components was studied using high resolution PIV technique (13). The injection of bubbles increases the friction coefficient by 50% in the case of laminar channel flows. In the transition region from laminar to turbulent flows, the friction coefficient increases up to two fold because bubbles activate the turbulent flow transition. The increase in the ratio of the friction coefficient matches that of the ratio of turbulent to laminar friction coefficients (14). 3. PROBLEM DEFINITION Ships require large quantities of fuel to generate the propulsive power required to overcome drag and frictional resistance resulting from their motion across ocean surfaces. The exhaust releases by products such as nitrogen oxide, carbon dioxide and sulphur oxide emissions which significantly contribute to the global climate change and acidifications of ocean. Sailing cost will be increased due to large usage of fuel it will affect the profit of the company. All these problems can be eliminated by reducing the power required to overcome drag and frictional resistance without affecting the sailing speed. SOLUTION OF THE PROBLEM Drag and frictional resistance can be overcome by reducing the frictional contact between ships surface and water surface through the introduction of air lubrication in the bottom of hull. The injection of air requires constant pumping power and if the ship sails too slowly it represents a significant part of the propulsive power. The bubble sizes and location of the injection points are important parameters in the persistence of drag reduction. The scope of this work is to compare the resistance of the ship with air lubrication system and without air lubrication system numerically. AIR LUBRICATION SYSTEM Air Lubrication System is a method to reduce the resistance between the ship s hull and seawater using air bubbles. The air bubble distribution across the hull surface reduces the resistance working on the ship s hull, creating energy-saving effects. With the right ship hull design, the air lubrication system is expected to achieve up to 10-15% reduction of CO 2 emissions, along with significant savings of fuel. Air lubrication system can offer reduction in CO 2 emission of up to 35% as compared with conventional container ships. Fuel savings and reduction in carbon emissions is possible through Air lubrication system when combined with other promising green ship technologies. 4. WORKING PRINCIPLE Air Lubrication System works on the simple principle of trapping a layer of air bubbles beneath the ship s hull. An air blower or a dedicated system is used to generate air bubbles to pass them continuously beneath the ship s surface which is shown in figure 1. Air bubble outlets are created at different locations along the bottom of the hull, symmetrically on both the sides of the ship s centre line. The air is blown at a constant rate to form a layer of bubbles, which reduces the drag and resistance between the ship and the seawater editor@iaeme.com

4 Vijayan.S.N, Duraimurugan G.K, Sendhilkumar.S, Kiran Babu.K.M and Deepak.P Figure 1 Air lubrication system Concerns about air lubrication system Air Lubrication System has a few concerns regarding its implementation and performance on ships. The Air Lubrication System (ALS) can only be used for certain types of ships having flat bottoms. Ships having V-shaped hulls, such as certain warships or recreational vessels might not be able to reap the benefits of the air lubrication system. To trap the layer of bubbles beneath the ship s hull is a challenging task. Though solution such as protruding ridges at the edges of the hull can help in trapping the blanket of bubbles, the sucking effect of propeller on the bubbles is difficult to defy. It is also feared that the air cavities made for trapping the air bubbles would affect the handling and stability of the ship at the sea. The air bubbles leaving the hull surface flow into the ship s propeller. This can influence the efficiency, noise, and vibration of the propeller. In order to obtain the desired effect, it is important that air bubbles are of uniform size and are evenly distributed beneath the hull surface. Moreover, a change in air bubble diameter would drastically affect the air bubble distribution beneath the hull. Figure 2 2D view of air lubrication system Different Air Lubrication techniques This section discusses about the available different air lubrication techniques and it is represented in the Figures 3, 4 & 5. The conceptual difference between the various air lubrication techniques is also discussed. The three air lubrication regimes are: editor@iaeme.com

5 CFD Analysis of Frictional Drag Reduction on the Underneath of Ship s Hull Using Air Lubrication System Bubble Drag Reduction (BDR) Transitional Air Layer Drag Reduction Developed Air Layer Drag Reduction (ALDR) Bubble drag reduction In Bubble Drag Reduction (BDR), gas is injected into the boundary layer, usually through a slot, porous material or a perforated plate. The gas is separated into bubbles that reside predominantly in the boundary layer of the hull, which is clearly represented in figure 3. Figure 3 Bubble Drag Reductions The dispersed bubbles act to reduce the density of the air water mixture 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. Transitional air layer drag reduction Figure 4 Transitional Air Layer Drag Reductions Figure 5 Air Layer Drag Reductions When gas is injected beneath a horizontal plate, a transition can take place from a bubbly flow to that of a gas layer. As the gas flux is increased, the friction of the surface covered by clusters of fragmented air layer increases, until finally a continuous layer covers the entire surface. The transitional air layer drag reduction is shown in figure 4 and 5. Partial cavity drag reduction In Partial Cavity Drag Reduction (PCDR), a recess is created on the bottom of the hull that captures a volume of gas and creates a cavity of air between the hull and water surface. A backward-facing step on the upstream end and a gently downward sloping closure on the downstream side normally form the recess that traps the gas, thus forming a ventilated partial cavity. Gas is injected continuously into the cavity to maintain it as some gas is lost due to editor@iaeme.com

6 Vijayan.S.N, Duraimurugan G.K, Sendhilkumar.S, Kiran Babu.K.M and Deepak.P entrainment at the cavity closure; however with proper cavity design, this gas loss is minimized. 5. DESIGN AND ASSUMPTIONS For calculating the resistance, the barge was taken with the following principal particulars: Table 1 Dimensions of ship S.No Description Symbol Value 1 Overall Length, m LOA Beam, m LBP 17 3 Depth, m B Draught, m T Air Bubble diameter, mm d Air Bubble Flow rate, m 3 /s 10 Assumptions The calculation is made without considering the effect of waves. That is the ship is floating in the still water and the effect of the waves are neglected and the calculated drag is only frictional drag due to the frictional force between the ship and the water surface and air bubbles are considered as uniform in size and diameter, flow of air bubbles are also considered as uniform. The two dimensional side and top view of design is represented in figure 6 and its model view is shown in figure 7 used for further analysis process. Figure 6 2D view of Ship with air hole The results reported are based on a model ship navigating in a straight line using double model approximation without considering waves on a free surface. The calculations were performed on the port side only based on the line of symmetry along the hull centreline. The air bubble distribution around the hull surface is believed to be an important parameter for reducing the resistance working on the hull, and must therefore be predicted accurately. Here, the void fraction is the ratio of the air volume to the air fluid mixture. All the air bubbles were assumed to be of a uniform diameter and remain unchanged by the flow. Bubble outlets were created at the bottom of the hull, symmetrically on both sides of the centreline. A editor@iaeme.com

7 CFD Analysis of Frictional Drag Reduction on the Underneath of Ship s Hull Using Air Lubrication System velocity boundary was created at the bubble outlet; the air was blown at a constant flow rate to form the bubbles. Figure 7 Ship Model with air hole 6. RESULTS AND DISCUSSION With the above settings and the assumptions, the following results were obtained and it is tabulated below as follows: S.No Velocity (Knots) Table 2 Velocity vs Drag values Normal Drag Value (N) Buffer Drag Value (N) Decrement in Drag (N) Percentage Decrement (%) Table.2 illustrates the results obtained from the analysis. Drag resistance was analysed for three different velocities such as 15knots, 10knots and 5knots. For these velocities the obtained result were N, N and N respectively without applying air lubrication. After the introduction of air lubrication system on the underneath of the ship, the obtained results were N, N and 45263N respectively for the same velocity. Both drag values clearly shows the decrement in drag. Figure 8 FEA results of frictional resistance on the bottom of hull surface editor@iaeme.com

8 Vijayan.S.N, Duraimurugan G.K, Sendhilkumar.S, Kiran Babu.K.M and Deepak.P Figure 8 shows the low frictional resistance developed on the bottom surface of hull after implementing air lubrication system with low frictional resistance on the front portion of hull. It leads to increase the ship speed. Figure 9 FEA results of frictional resistance on the water surface Figure 9 shows the air flow produced on the underneath of ship s hull which leads to reduce resistance of ship with water. It clearly illustrates the generation of gap between water surface and hull surface due to the injection of air which leads to the generation of low frictional resistance between two surfaces. Figure 9 represents the water velocity during sailing after introducing air lubrication. Red colour indicates the frictional resistance generated during sailing which is very low when compared to without air lubrication system. Ships speed will automatically increase when the frictional drag resistance decreased on the water surface. 7. CONCLUSION Frictional drag reduction resistance was reduced by the introduction of air lubrication system on underneath of ship s hull. The drag value is reduced when compared with the drag values obtained from without introduction of air lubrication system. This lubrication system was a new technology which well proud and provide benefits such as reduced carbon emission and substantial fuel savings and cost savings also can achieve higher speed of ship. REFERENCES: [1] Simo A. Mäkiharju, Marc Perlin, and Steven L. Ceccio, On the energy economics of air lubrication drag reduction, International journal of Naval Architecture and Ocean Engineering, Vol. 4, Issue 4, December 2012, pp [2] Jinho Jang, Soon Ho Choi, Sung-Mok Ahn, Booki Kim and Jong Soo Seo, Experimental investigation of frictional resistance reduction with air layer on the hull bottom of a ship, International journal of Naval Architecture and Ocean Engineering, Vol. 6, Issue 2, June 2014, pp [3] Makoto Kawabuchi, Chiharu Kawakita, Shuji Mizokami, Seijiro Hiasa, Yoichiro Kodan, Shinichi Takano, CFD Predictions of Bubbly Flow around an Energy-saving Ship with Mitsubishi Air Lubrication System, Mitsubishi Heavy Industries Technical Review, Vol. 48, No. 1, March [4] Steven L. Ceccio, Simo A. Makiharju, Air Lubrication Drag reduction on Great Lakes Ships, Great Lakes Maritime Research Institute, February editor@iaeme.com

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