CFD Analysis of an Internal Flow - Essay Example

CFD ANALYSIS OF AN INTERNAL FLOW YOUR DEPT. YOUR SECTION YOUR ROLL NO. DATE: CFD ANALYSIS OF AN INTERNAL FLOW Introduction: Computational Fluid Dynamics also known as CFD is the set of methodologies that enable a computer to produce a numerical stimulation of fluid flows. The word 'stimulation' indicates the ability of a computer to solve numerically the laws that govern the movement of fluids, in or around a material system, where its geometry is also modeled on the computer. So the whole system has turned in to a 'virtual environment' or virtual product. It also indicates that it is possible for us to visualize the whole system and its behavior with an extraordinary level of realism through computer visualization tools. So we can see the complete system, such as car, airplane, a block of buildings etc. It is opposite to an experimental investigation by a material model or prototype of the system, such as an aircraft or car model in a wind tunnel. The Role of CFD: The role of CFD in this contemporary technological world is crucial. It assists towards the path of global world of Computer Assisted Engineering or CAE that refers to ensemble of simulation tools that support the work of engineer between the initial design phase and the final definition of manufacturing process. The industrial production process uses various software tools for computerization of whole production cycle. Among those tools, the most important are Computer-Assisted Design (CAD), Computer-Assisted Engineering (CAE), and Computer- Assisted Manufacturing (CAM) software. The virtual prototyping environment is made by these CAD/CAE/CAM software systems. (shown in the following figure). In the chart, we can see the different components of a computer-oriented environment used in industry to create or modify the properties of a product. This product also can be a single component such as a cooling jacket in a car engine. In all steps, the related software tools are used in similar ways. Let us see the phases of virtual protype environment for analysis of internal flow of CFD analysis. 1) Definition Phase: The definition phase is the first towards the creation of a product which is based on CAD software. This phase includes creating and defining the geometry of the system in details. It is the day to day routine responsibility of the designers to build the geometrical model on the computer screen. For CFD simulation task, the CAD definition of the geometry is the unavoidable input. 2) The Simulation & Analysis Phase: This is the next phase which uses the software tools to calculate the physical behavior of the system on the computer. It is known as virtual prototyping which is based on CAE software with several sub branches related to the different physical effects that have to be modeled and simulated during the design process. Among these, the most important are: i)Computational solid mechanics (CSM) :- This software evaluates the mechanical stresses, deformations, vibrations of the solid part of the system including fatigue and life estimation. It also has modules for conducting thermal analysis of the materials including heat conduction, thermal stresses, and thermal dilation effects. ii) Computational Fluid Dynamics (CFD):- It refers to the software tools that analyzes the fluid flow including the thermal heat transfer and heat conduction effects in the flow as well as the solid boundaries of the flow domain. For example, CFD software is used in an aircraft engine to analyze the flow in the multistage combination of rotating and fixed blade rows of the compressor and turbine. Then it predicts their performance, analyzes the combustor behavior and the thermal parts to optimize the cooling passages, cavities, labyrinths, seals and similar sub - components. Other simulation area indicates the specialized physical phenomena such as Computational Aero- Acoustics (CAA) and Computational Electromagnetic (CEM). 3) The Manufacturing Cycle Phase: This phase starts when the analysis has been successful and the objectives of the design are reached. It plays role in simulating the fabrication processes and verifying the degree of accuracy of the specific shapes. This phase uses CAM software. A growing number of design and fabrication processes are simulated on computer prior to any prototype construction .Virtual prototyping of a virtual product is the result of the advanced technological progress. The Components of CFD Simulation System: The components of CFD are described in the following steps :- Step 1: It uses the mathematical model which defines the level of approximation to reality that will be simulated. Step 2: It includes the discretization phase which have two components- dicretization of space and discretization of equations. Step 3: It analyses the numerical schemes including its property of stability and accuracy. Step 4: By selecting the most appropriate time integration methods, the solution of the numerical scheme. Step 5: To understand and interpret the physical properties of the obtained simulation results by the existence of powerful visualization software. A Chart of CFD Analysis Process Formulate the flow problem To formulate the flow problem by seeking the answers like what is the objective, the easiest way to obtain the objective, operating conditions, geometry, flow domain, temporal modeling, nature of the viscous flow etc. Model the Geometry & Flow Domain Modeling the geometry with a CAD software package including the structure and topology of the grid generation. Establish the boundary and initial conditions Starting from an initial solution to use of an interactive method to reach a final flow solution. Generate the Grid Defining the structure & topology and generating a grid on that topology. Establish the Simulation Strategy The use of space marching or time-marching, the choice of chemistry model and algorithms. Establish the input parameters & files Creating an input data file with listing the values of parameters. Perform the Simulation The simulation for various options including interactive or batch processing and distributed processing. Monitor the Simulation for Completion Monitoring the solution to determine Iterative Convergence. Post- Process the Simulation to get the Results Extracting the desired flow properties from the computed flow field. Make Comparison of the Results Comparison of results from analytic, computational, or experimental studies to establish the validity of the computed results. Repeat the Process to Examine the Sensitivities Understanding the possible differences in the accuracy of the results. Document Documenting the findings of an analysis. General Form of a Conservation Law: The conservation is the core concept behind the laws of fluid mechanics. Conservation means the variation of an intensive flow quantity within a given volume due to the net effect of some internal sources and the amount of the quantity which is crossing the surface of boundary. This amount is known as flux and its expression is the result of mechanical and thermodynamic properties of fluid. Generally the fluxes and sources depend on the space-time co-ordinates and the fluid motion. The following logical consistency rule states the conservation rule for a quantity U: The variation of the total amount of a quantity U inside a given domain is equal to the balance between the amount of that quantity that enters and leaves the considered domain, plus the contributions from eventual sources generating that quantity. Hence, we can observe the rate of change of quantity U during the movement and changing system of flow evolution. It is also noted that note all flow quantities obey a conservation law. The laws describing the evolution of fluid flows are totally defined by the conservation of the following three quantities: 1. Mass 2. Momentum 3. Energy In an internal flow calculation, mass conservation must be ensured for the consistency of the mass flow in all sections for any grid resolution. The mass flow rates are not constant because eventual numerical sources create or destroy mass. Conservation Law for a Moving Control Volume: The above conservation law is appropriate for a fixed control success. In the case of many flow situations, moving grids are required for the simultaneous presence of moving and fixed parts. A control volume is attached to a moving grid or a moving body having the account of displacement of control surfaces. Some examples are shown below: flow between two crossing trains in the opposite directions or around a train entering a tunnel. flow between an aircraft and a separating store. Flow around an oscillating wing where oscillation can be forced. Flow of gases in internal combustion engines. Hydrodynamic of moving ships. In all of these examples, fixed or deformable grids need to be attached to the moving system. The effects of relative motion between the two systems have to be accounted in the conservation laws of mass, momentum and energy. The fundamental form of a conservation law is applied to obtain the basic equations of fluid mechanics which are known as the system of Navier- Stokes equations. These equations express the conservation of three fundamental quantities: mass, momentum and energy, which can be expressed in the following simplified equations: The first equation represents the conservation of momentum-basically, the Newton's second law-in terms of fluid velocity vector u and pressure p. The second equation represents the conservation of mass. In the above equations, the body force f is frequently absent and the remaining parameters-fluid density and viscosity-are often constant. These simplified Navier-Stokes equations are capable of producing an astounding variety of fluid flows like ocean waves, wingtip vortices, whistling of telephone wires in the wind in those two lines! According to Dill, Richard A & Whitesides, R. Harold (Eleventh Workshop for Computational Fluid Dynamic Applications in Rocket Propulsion, Part 1 p 793-835): Navier-Stokes CFD analysis is very important to predict the internal flow field environment in a solid rocket motor during the design phase of a motor development program. This analysis discovers a variety of potential problems associated with motor performance as well as suggesting solutions to these problems. It is proved that CFD codes have great benefit in explaining problems associated with operational motors such as in the case of the pressure spike problem with the STS-54B flight motor. Although CFD code has drawbacks and conversion (about 20%), still it is very powerful technique to demonstrate simulation and illustrate study of all kinds of fluid flow. Source: 1. Numerical Computation of Internal and External Flows: Fundamentals of Computational Fluid Dynamics. 2nd Edition. Hirsch, Charles (Author) Read More