Evaluation of Parametric Study on Submarine Using CFD

Transcription

1 Evaluation of Parametric Study on Submarine using CFD 53 Evaluation of Parametric Study on Submarine Using CFD Mohd Zubair Nizami 1 and Mohammed Ahmed Khan 2 1,2 Research Scholar, Department of Mechanical Engineering, Lords Institute of Engineering and Technology, Hyderabad, India, Nizami777@Gmail.com. ABSTRACT: Submarine is a stream lined war ship design to operate under the sea for long periods. As the submarine moves with high loads and under high pressures the design of the warship is complicated and crucial. The factors that are to be considered to design therefore needed to be critically studied and analyzed. The inability to deal with complete Navier Stokes formulation and analysis of such problems are generally limiting the design aspects in the design room and experiment more. There by increasing design expenditure. After the advent of super computers high speed processors and Numerical methods an entirely new branch of science known as Computational Fluid Dynamics emerged which is helping us in not only studying the problem parametrically but also with high accuracy and understanding in design intricacies. In the present work a submarine is analyzed using a commercial CFD software Fluent 6.2 to understand the flow behavior and other design parameters, the results therefore obtained using CFD are strikingly matching with the experimental values as the problem can be substantiated. Keywords: Parametric study, pressure, temperature, density, viscosity, modeled behavior under the water. 1. INTRODUCTION Roots of fluid are extended to almost every aspect of science and engineering. Civil engineering, for example, is developed primarily from the need for fluid systems and structures. Mechanical engineering studies fluid in combustion, lubrication, and energy systems. Aeronautical engineering studies gas flow to produce energy and to provide lift on flying structures. Electrical engineering uses fluid to cool electronic devices with air flow. the study of fluid is essential for the chemical engineer, because the majority of chemical processing operations are conducted either partially or totally in the fluid phase. Flow processes in the human body, cardiac and cardiovascular system, blood flow and respiratory system are few examples from the discipline of bio-fluid in the human body. When an object is submerged in a fluid then we can observed the parametric behavior, the compression force is always a normal force, that is, it is always perpendicular to the surface of the object, independent of the orientation of the object. Computational fluid dynamics (CFD) is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Numerical analysis is the study of algorithms for the problems of continuum mathematics. Computers are used to perform the millions of calculations required to simulate the fluids and gases with the complex surfaces used in engineering. even with high-speed supercomputers only approximate solutions can be achieved in many cases. Ongoing research, however, may yield software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial validation of such software is often performed using a wind tunnel with the final validation coming in submarine test. 2. CFD MODELLING There is growing evidence of benefits accruing from the combined knowledge of FDM and FEM. Finite Volume Methods (FVM), because of there simple data structure, have become increasingly popular in recent years, there formulations being related to both FDM and FEM. The flow field-dependent variations (FDV) methods also point to close relationships between FDM and FEM. Therefore in this book we are seeking to recognize such views and pursue the advantage of studying FDM and FEM together on an equal footing. CFD has been an important tool in air, marine and space industry or vehicle design for a long time where it has to a large extent replaced time-consuming and expensive with tunnel experiments. Yet, while in this applications the flow behavior on a submarine is analyzed which can be used to design the structure of submarine. Therefore CFD applications have gained broad attention only during the last decade since increasing computational power available has enable computations previously considered infeasible. Still, most literature reports are limited to flows on submarine.

2 54 Mohd Zubair Nizami and Mohammed Ahmed Khan The behavior of any fluid flow can be determined using commercial CFD software as GAMBIT and FLUENT Geometry and Mesh Generation Gambit is the preprocessor in which the fluid dynamics problem is modeled under certain boundary conditions and the modeled problem is meshed into finite number of element. This element may be either volume or vertex or edge. Geometry of the submarine can be prepared using CAD or GAMBIT. Here the complete structure of the submarine is drawn in a wireframe model. The main reason of analyzing the fluid parameters over the submarine is only to change the dimension of the submarine to much better extent for best efficiency. A mesh is a discretization of a geometric domain, e.g., the fluid around a wing, into small simple shapes called elements. A structured mesh is usually a warped grid of boxes, while an unstructured mesh is typically a triangulation. Some advantages of structured meshes that hold generally over most applications are simplicity, availability of code, and suitability for multi-grid and finite difference methods. On the other hand, unstructured meshes conform to the domain more easily and allow element sizes to vary more dramatically. Structured meshes are currently more popular, but unstructured are catching up, especially in the more academicallyinclined community. These meshed elements are exported to the FLUENT 6.3 software to solve the meshed cells. Now the fine elements are iterated and the behavior of every fluid parameter over the surface is studied. Some mesh generation goals vary with the application. For example, long skinny elements, aligned with flow, can be quite useful in computational fluid dynamics. Moving features, such as shock fronts and vortices, require changing meshes. Fluent is general-purpose computer program for modeling fluid flow, heat transfer, and chemical reaction. Using FLUENT you can quickly analyze complex flow problems even if you do not have prior expertise in computational fluid dynamics or computer programming. FLUENT enables you to apply computer simulation methods to analyze and solve your practical design problems. FLUENT incorporates up-to-date modeling techniques and a wide range of physical models for simulating numerous types of fluid flow problems. These are accessible to you through an interactive graphical user interface for problem definition, computation, and graphical post-processing. When required, FLUENT can also be customized to your specific modeling needs and and/or interfaced to your in-house CAD system. 3. CASE DESCRIPTION The complete project is divided into two cases and the details of each case is (flow input, boundary condition details, grid details, various plots with results and discussion is presented Case: Flow Analysis on a Submarine (Unsteady State) The aim of our case is to simulate the flow on the submarine computing the static pressure, turbulence and total energy Flow Inputs Fluid Velocity in x-direction Velocity in y-direction Velocity in z-direction Saline water m/s 6m/s 0m/s Turbulent Viscosity m 2 /s Temperature Boundary Conditions Pressure far field Pressure far field Pressure outlet Sea water Submarine Velocity inlet 300k Outlet Fluid Inlet 3.2. Material Properties Material: Water-Liquid (Fluid) Property Units Method Value(s) Density kg/m3 constant Cp (Specific Heat) j/kg-k constant 4182 Thermal Conductivity w/m-k constant 0.6 Viscosity kg/m-s constant Molecular Weight kg/kg mol constant

3 3.2.2 Material: Air (Fluid) Evaluation of Parametric Study on Submarine using CFD 55 Property Units Method Value(s) Density kg/m3 constant Cp (Specific Heat) j/kg-k constant Thermal Conductivity w/m-k constant Viscosity kg/m-s constant e-05 Molecular Weight kg/kg mol constant Material: Aluminum (Solid) Property Units Method Value(s) Density kg/m3 constant 2719 Cp (Specific Heat) j/kg-k constant 871 Thermal Conductivity w/m-k constant Force Report Force Vector: (1 0 0) 4. RESULTS Zone name Pressure Viscous Total Pressure Viscous Total force (n) force (n) force (n) coefficient coefficient coefficient pressure_far_field pressure_far_field Submarine wall e e net Grid and Analysis Details The model is created in GAMBIT. Unit of length is in meters. Tetrahedral mesh is generated for the fluid domain with Boundary layer on the submarine with appropriate spacing. The number of cells is for this model, the flow inputs and Boundary conditions are described in above table. These meshed elements are exported to processor FLUENT 6.3 to solve. All the three continuum equations viz continuity, momentum, and energy equations are solved with iterations until they get convergence. Then the results are tabulated and presented as shown in Figures by post-processing Post-Processing of the Results The fluid domain on the submarine is shown in Fig****the middle plane on the domain is x-y-z plane. The Lift and Drag Force, Contours of Static Pressure, Contours of Density, Contours of Velocity and Turbulence are shown in the figures respectively. Lift Force From the above diagram we can see the relation of lift force with respect to Flow time. The lift force is shown in exponential manner. As we can see that the relation

4 56 Mohd Zubair Nizami and Mohammed Ahmed Khan between Flow time and Lift force is linear up to 3500 N after 1 unit of time. But after 1 unit of time the Lift force tends to decrease to-3000n. But again it increases after 6 units of time. In this way the Lift force is uniformly distributed over the submarine with respect to time. Where; P = static pressure. 1 2 ρ 2 V = dynamic pressure, usually denoted by q. P 0 is total pressure which is constant along any streamline. Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own static pressure P, dynamic pressure q, and total pressure P 0. Static pressure and dynamic pressure are likely to vary significantly throughout the fluid but total pressure is constant along each streamline. In irrotational flow, total pressure is the same on all streamlines and is therefore constant throughout the flow. Drag Force The diagram shown above is the relation of Drag force with respect to Flow time. These Drag Force and Lift Force look to be quite opposite to each other. The Flow time is taken on x-axis and Drag Force is taken on y-axis. The y-axis is written with an interval of 1000 and x-axis with an interval of 2 units. It is clear to be seen that the Drag Force gets linearly decreased with the increase of time. But after a short time the Drag Force changes with respect to Flow time. Contours of Density The density of a material is defined as its mass per unit volume. The symbol of density is ρ Mathematically: ρ = m V Contours of Static Pressure In fluid dynamics, static pressure is the pressure at a nominated point in a fluid. Bernoulli s equation for incompressible flow can be expressed as P 0 = P ρv 2 where: ρ is the density, m is the mass, V is the volume. Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging. Several of us got into a heated discussion about how and why a submarine submerges and surfaces. We all agree that it is doing so because the density of the submarine is being changed. However, some argue that it is because the water allowed into the ballast tanks to make the sub sink increases the sub s mass and therefore the density increases and the sub sinks. Hence the variation of the density in sub-

5 Evaluation of Parametric Study on Submarine using CFD 57 marine is necessary to be studied. The change of density over the entire surface of the submarine is shown in figure above. As we observed from the result that the density is high at the nose of the submarine which leads to sinking of the submarine under the sea. back of the submarine. Our results satisfies that the turbulence should occur at the back side of submarine to drive in a meaningful direction. Surface: Contours of Modified Turbulent Viscosity Sweep Turbulence is defined as an eddy-like state of fluid motion where the inertial vortex forces of the eddies are larger than any of the other forces that tend to damp the eddies out As the speed increases, at some point the transition is made to turbulent flow. In turbulent flow, unsteady vortices appear on many scales and interact with each other. Drag due to boundary layer skin friction increases. The structure and location of boundary layer separation often changes, sometimes resulting in a reduction of overall drag. As we observed from the three figures of Turbulence, the turbulence is very high at the Sweep Surface: Contours of Radial Velocity Radial velocity is the velocity of an object in the direction of the line of sight (i.e. its speed straight towards you, or away from you). The light of an object with a substantial radial velocity will be subject to Doppler Effect. The figures of Radial Velocity states that the velocity at the back of Submarine is high which lead the Submarine to move forward by the help of propulsion. As we observed the Velocity of water at the front of the nose is average and its get incremented as the Submarine passes over it.

6 58 Mohd Zubair Nizami and Mohammed Ahmed Khan Fluid Flow Perpendicular at the Entrance of the Submarine The three different positions of the Submarine in water are shown in Figures****which states the fluid behavior on the Submarine at various positions. That is, fluid flow acts perpendicularly to the direction of the Submarine. As observed the magnitude of Velocity is higher at the top surface of the submarine whereas it goes decreasing to the bottom of the Submarine. Mass Imbalance v/s Position in x-y Plane Modified Turbulent Viscosity v/s Position in x-y Plane Scale Residuals The fluid doesn t have a specific direction. Every fluid particles move with its own velocity to different direction. So each velocity cannot be studied individually at different angles. As the fluid my not strike the nose of submarine parallel. It makes some angles while striking to the surface of the nose. We assumed three velocities named x, y, z. These velocities are marked on the y-axis and the numbers of iterations are marked on the x-axis. When the speed of each velocity direction is given, the behavior of those particles is seen in the above graph. Each color represents the different parameters. Black is for continuity, Red is for x-velocity, Green is for y-velocity, Blue is for z-velocity and Light blue is the Net velocity. The number of iterations determines the accuracy of the flow parameters. Comparing Static Pressure v/s Position in x-y Plane 5. CONCLUSION Mathematical Modeling of the fluid over the object (Submarine) could be solved by Numerical methods. Numerical methods and the contours and other flow data approximately can be compared with the actual object (Submarine) moment in the fluid. Commercial CFD code fluent is able to approximately give the patterns of the fluid flow with sufficient accuracy to substantiate the problem taken in the object (Submarine).

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