SolveMeshPhysicsGeometrySimulation of Turbulent Flow over the Ahmed Body58:160 Intermediate Mechanics of FluidsCFD LAB 44. ExercisesSimulation of Turbulent Flow over the Ahmed Body 58:160 Intermediate Mechanics of Fluids CFD LAB 4By Tao Xing and Fred SternIIHR-Hydroscience & EngineeringThe University of IowaC. Maxwell Stanley Hydraulics LaboratoryIowa City, IA 52242-15851. 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 axialvelocity 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.1CoarseMediumFineSelectschemeMeshdensityGeometryGeometryparametersPhysicsMeshPost-processingAhemd bodyContoursVectorsStreamlinesXY plotValidationVerification BoundaryConditionsFlowPropertiesViscousModelsOne Eq.Two Eq.Density and viscosityLaminarTurbulenInviscidSAk-ek-wIncompressibleInitialConditionsSolveTimeStepsConvergentLimitSteady/Unsteady?PrecisionsSingleDoubleNumericalSchemes1st orderupwind 2nd order upwindQuickReportWall Y plusModified VelocityModified TKEResidualsTime-stepsizeIteration/time stepAutosave frequencyx/h positionsViscosity ratioDrag historyLift historyPressure coefficientUnsteady formulationFlow Chart for ISTUE Teaching Module for External flow (red color illustrates the options youwill use in CFD Lab 4)2. Simulation DesignThe problem to be solved is unsteady turbulent flows over the Ahmed body (2D). Reynolds number isaround 768,000 based on inlet velocity and vehicle height (h). The following figure shows the sketchwindow you will see in FlowLab with definitions for all geometry parameters. The origin of thesimulation is located at the rear of the body. θ is the slant angle. L is the length of the body and h is theheight of the body. Uniform velocity specified at inlet and constant pressure specified at outlet. The topboundary of the simulation domain is regarded as “Symmetry” and there is a distance between the carbody and road, GL.In CFD Lab4, all EFD data for turbulent airfoil flow in this Lab will be provided by the TA and savedon the Fluids Lab computers.3. CFD ProcessStep 1: (Geometry)21. 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 PropertiesUse the values shown in the above figure. Input the values and click <<OK>>.3(2). Viscous Model In this lab, the two equation (k-ε) model will be used. (3). Boundary ConditionsAt “Inlet”, we use uniform velocity (40 m/s) and zero gradient for pressure. Use default values forturbulence intensity and viscosity ratio. Here, viscosity ratio is defined as the ratio of turbulenceviscosity and molecular viscosity.At “Outlet”, pressure is fixed to be atmosphere pressure. Zero gradients are applied for all otherquantities. Read the parameters and click <<OK>>.For “Ahmed body” or “Road”, FlowLab uses no-slip boundary conditions for velocities and zero-gradient for pressure. Turbulent quantities (intensity and viscosity ratio) on the Ahmed body and Roadare also specified to be zero. Read all the values and click <<OK>>.4For “Symmetry” boundary, vertical component of velocity is fixed zero and 1 atm specified forpressure. Zero gradients are applied for all other quantities. Read the parameters and click <<OK>>.(4). Initial ConditionsUse the default setup for initial conditions.After specifying all the above parameters, click the <<Compute>> button and FlowLab willautomatically calculate the Reynolds number based on the inlet velocity and the height of the Ahmedbody you specified. Click <<Next>>. This takes you to the next step, “Mesh”. Step 3: (Mesh)5Unstructured 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.6The 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 visualizethe 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>>.7Slant angle:0 Slant angle:25The following is an example of XY plot for residuals (only example)Step 5: (Reports)Time averaged drag force
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