Simulation of Turbulent Flow over the Ahmed Body 58 160 Intermediate Mechanics of Fluids CFD LAB 4 By Tao Xing and Fred Stern IIHR Hydroscience Engineering The University of Iowa C Maxwell Stanley Hydraulics Laboratory Iowa City IA 52242 1585 1 Purpose The Purpose of CFD Lab 4 is to simulate unsteady turbulent flows over the Ahmed body following the CFD process by an interactive step by step approach and conduct verifications using CFD Educational Interface FlowLab 1 2 Students will have hands on experiences using FlowLab to investigate effect of slant angles 25 and 0 degrees in which predicted drag coefficients and axial velocity for 25 degrees will be compared with EFD data Students will use post processing tools streamlines velocity vectors contours animations to visualize the mean and instantaneous flow fields and compute the non dimensional shedding frequency Strouhal number Students will analyze the differences between CFD and EFD and present results in a CFD Lab report Physics Geometry Mesh Solve Select scheme Incompres sible Flow Properties Viscous Models Boundary Conditions Initial Conditions Mesh density Coarse Steady Unsteady Medium Time Steps Fine Density and viscosity Laminar Turbulen Inviscid One Eq Two Eq Time step size SA Iteration time step k e Autosave frequency k w Convergent Limit Single Ahemd body Geometry parameters Post processing Report Viscosity ratio Verification Streamlines Validation Residuals x h positions Modified Velocity Precisions 1st order upwind Numerical Schemes Quick Vectors XY plot Wall Y plus Double 2nd order upwind Contours Unsteady formulation Modified TKE Drag history Lift history Pressure coefficient Flow Chart for ISTUE Teaching Module for External flow red color illustrates the options you will use in CFD Lab 4 1 2 Simulation Design The problem to be solved is unsteady turbulent flows over the Ahmed body 2D Reynolds number is around 768 000 based on inlet velocity and vehicle height h The following figure shows the sketch window you will see in FlowLab with definitions for all geometry parameters The origin of the simulation is located at the rear of the body is the slant angle L is the length of the body and h is the height of the body Uniform velocity specified at inlet and constant pressure specified at outlet The top boundary of the simulation domain is regarded as Symmetry and there is a distance between the car body and road GL In CFD Lab4 all EFD data for turbulent airfoil flow in this Lab will be provided by the TA and saved on the Fluids Lab computers 3 CFD Process Step 1 Geometry 2 1 Ahmed body slant angle 25 or 0 001 degrees read exercises at the end 2 Flow domain upstream length UL 1 m 3 Down stream length DL 6 m 4 Domain height DH 3 m 5 Gap GL 0 05 m hard coded Click Create after you see the Ahmed car body with domain created click Next Step 2 Physics 1 Material Properties Use the values shown in the above figure Input the values and click OK 3 2 Viscous Model In this lab the two equation k model will be used 3 Boundary Conditions At Inlet we use uniform velocity 40 m s and zero gradient for pressure Use default values for turbulence intensity and viscosity ratio Here viscosity ratio is defined as the ratio of turbulence viscosity and molecular viscosity At Outlet pressure is fixed to be atmosphere pressure Zero gradients are applied for all other quantities Read the parameters and click OK For Ahmed body or Road FlowLab uses no slip boundary conditions for velocities and zerogradient for pressure Turbulent quantities intensity and viscosity ratio on the Ahmed body and Road are also specified to be zero Read all the values and click OK 4 For Symmetry boundary vertical component of velocity is fixed zero and 1 atm specified for pressure Zero gradients are applied for all other quantities Read the parameters and click OK 4 Initial Conditions Use the default setup for initial conditions After specifying all the above parameters click the Compute button and FlowLab will automatically calculate the Reynolds number based on the inlet velocity and the height of the Ahmed body you specified Click Next This takes you to the next step Mesh Step 3 Mesh 5 Unstructured Tri Coarse Meshes will be used in this lab No manual meshing is available for this lab due to the complexity of the geometries After you create the mesh you should zoom in regions close to the Ahmed car body and in the wake of the Ahmed car body and think about where the mesh is refined and why Click Create to generate the whole mesh as per below an example Step 4 Solve In this Lab only QUICK scheme 2nd order upwind biased will be used 6 Slant angle 0 Slant angle 25 The flow is unsteady so turn on the Unsteady option default setup Specify the time steps to be 3000 for slant angle 0 degree and 2000 for slant angle 25 degrees Time step size is 10 4 Iterations timestep is the maximum iterations number for each time step Specify that to be 50 note iterations for each time step will be stopped when either the maximum iterations number or the convergent limit is reached Autosave frequency is to save solutions by skipping certain time steps so finally the saved solutions at different time steps can be used to generating animations to visualize the development flow field Convergent limit is set to be 0 001 considering a lower value could cause tremendous increase of computational costs 10 axial positions x h can be specified use the following values 0 910 0 389 0 215 0 042 0 132 0 306 0 653 1 000 1 521 2 215 These positions will be used to plot time averaged axial velocity and Turbulent Kinetic Energy and be compared with EFD data Choose Double precision Quick scheme for spatial derivatives and 1st order for unsteady time integration Here Quick scheme is a 2nd order upwind biased scheme Use New calculation for this Lab Then click Iterate and FlowLab will begin the calculation whenever you see the window Solution Converged Click OK 7 The following is an example of XY plot for residuals only example Step 5 Reports Time averaged drag force FD is found by integrating surface pressure and the shear stress the corresponding drag coefficient is computed by FD CD 1 U 2 Ax 2 Where is the fluid air density U is the upstream velocity Ax is the projected area of the Ahmed body in x direction Ck CB CS and C D represent the drag coefficient at the nose back the rear slop and the total respectively read reference 1 for Ahmed car on class website for more details XY plots provide you the
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