Simulation 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 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. Mesh Physics Geometry CoarseMediumPost-processingContours Vectors Viscosity ratio Streamlines XY plot Validation Verification Solve Time Steps Convergent Limit Steady/ Unsteady?Precisions SingleDoubleNumerical Schemes1st order upwind 2nd order upwindUnsteady formulationx/h positionsAutosave frequency Iteration/ time step Time-step size Lift history Pressure coefficient Drag history Modified TKE Modified Velocity Wall Y plus Residuals Report Quick FineMesh density Initial Conditions Boundary ConditionsIncompressible Flow PropertiesOne Eq. k-wk-eSATwo Eq. Density and viscosity Laminar TurbulenInviscid Viscous ModelsSelect scheme Geometry parameters Ahemd body Flow Chart for ISTUE Teaching Module for External flow (red color illustrates the options you will use in CFD Lab 4) 12. 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. θ 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) 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 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-ε) 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 zero-gradient 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>>. 4For “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) 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. 6Slant 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>>. 7The following
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