MeshPhysicsGeometrySimulation of Turbulent Flow in an Asymmetric Diffuser58:160 Intermediate Mechanics of FluidsCFD LAB 34. ExercisesSimulation of Turbulent Flow in an Asymmetric Diffuser 58:160 Intermediate Mechanics of Fluids CFD LAB 3By Tao Xing and Fred SternIIHR-Hydroscience & EngineeringThe University of IowaC. Maxwell Stanley Hydraulics LaboratoryIowa City, IA 52242-15851. Purpose The Purpose of CFD Lab 3 is to simulate turbulent flows inside a diffuser 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 conduct verification and validation for total pressure change, velocity and turbulent kinetic energy, skin friction factor, etc. Effect of turbulent models will be investigated, with/without separations. Students will manually generate meshes, solve the problem and use post-processing tools (contours, velocity vectors, and streamlines) to visualize the flow field. Students will analyze the differences between CFD and EFD and present results in a CFD Lab report.1CoarseMediumFineAutomaticManualStructuredUnstruct-uredGeometryGeometryparametersPhysicsMeshPost-processingSelect GeometryTop wall skin frictionModified VelocityModified TKEContoursVectorsStreamlinesAsymmetricXY plotValidationVerification BoundaryConditionsFlowPropertiesViscousModelsOne Eq.Two Eq.Density and viscosityLaminarTurbulenInviscidSAk-ek-wIncompressibleInitialConditionsSolveIterations/StepsConverge-ntLimitPrecisionsSingleDoubleNumerical Schemes1st orderupwind 2nd order upwindQuickSteady/Unsteady?ReportFour Eq.v2fResidualsBot. wall skin friction Pressure differenceFlow Chart for ISTUE Teaching Module for Diffuser Flow (red color illustrates the options youwill use in CFD Lab 3)2. Simulation DesignThe problem to be solved is that of turbulent flows inside an asymmetric diffuser (2D). Reynoldsnumber is 17,000 based on inlet velocity and inlet dimension (D1). The following figure shows thesketch window you will see in FlowLab with definitions for all geometry parameters. Before thediffuser, a straight channel was used for generating fully developed channel flow at the diffuser inlet.The origin of the coordinates is placed at the inlet of the channel before diffuser. In CFD Lab3, 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) 1. Select Geometry: Asymmetric22. Inlet dimension (D1) (2 m)3. Inlet length L1 (60 m)4. Diffuser half angle (10 or 4, read exercises at the end)5. Outlet dimension (D2) (9.4 m)6. Outlet length (L2) (70 meters).Click <<Create>>, after you see the airfoil geometry created, click <<Next>>.Step 2: (Physics)(1). Incompressible“Incompressible”, which is the default setup.(2). Flow PropertiesUse the values shown in the above figure. Input the values and click <<OK>>(3). Viscous Model3In this lab, the two equation (k-ε) model and the four equation v2f model will be used. “Wall function” with “Enhanced Wall” means that “near wall” models will be used for k-ε.(4). Boundary ConditionsNOTE: for k-e and v2f models, boundary conditions are the same.At “Inlet”, we use constant pressure and fix the velocity to 1.25 m/s. Use default values for “k” and“e”.At “Outlet”, FlowLab uses magnitude for pressure and zero gradients for velocities and turbulentquantities. Input “0” for the Gauge pressure and click <<OK>>.At “Wall” or “Bottom wall”, FlowLab uses no-slip boundary conditions for velocities and zero-gradient for pressure. Turbulent quantities k and ε on the wall are also specified to be zero. Read all thevalues and click <<OK>>.4(5). Initial ConditionUse 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 inlet dimension youspecified. Click <<Next>>. This takes you to the next step, “Mesh”. Step 3: (Mesh)In CFD Lab 3, “Structured” meshes will be generated using “Manual” option5Use the above setup to generate the mesh in this lab. For verification study, this mesh will be used as the “fine” mesh. After you create the mesh, you should zoom in the inlet and outlet of the diffuser, and think about where the mesh was refined and why. Click <<Create>> after you input the parameters for each edge and then click <<Create>> in the mesh step window to generate the whole mesh.Step 4: (Solve)In this Lab, ONLY 2nd order numerical schemes will be used.6D1L1D2DLL2The flow is steady, so turn ON the <<Steady>> option and the <<Unsteady>> button will automatically be turned OFF. Specify the iteration number and convergence limit to be 10000 and 10-5, respectively. 7 axial positions can be specified (use values shown in the above figure). These positions will be used to plot modified velocity and modified TKE. Choose “Double precision” with “2nd order scheme”. Use “New” calculation for this Lab. Then click <<Iterate>> and FlowLab will start calculation, whenever you see the window, “Solution Converged”. Click <<OK>>. The following is an example of XY plot for residuals.7Step 5: (Reports)“Reports” first provide you the information on “Total frictional force on the upper wall”, “wall sharestress”, “pressure difference (between inlet and outlet)”. “XY Plots” provides the following options:8In this Lab, all the four EFD data files can be downloaded from the class website, with the following names:1. EFD data for modified velocity (10u+x) is: Modified_u-10degree.xy2. EFD data for modified Turbulent Kinetic Energy (500k+x) is: Modified_TKE-10degree.xy3. EFD data for bottom wall friction factor distribution: Skin_friction_bot_wall.xy4. EFD data for upper wall friction factor distribution: Skin_friction_top_wall.xyThe following figure shows an example for modified velocity, for both EFD and CFD using v2f model. It is possible to modify the style of the curves by clicking “curves”, selecting a curve with theright button and the clicking on “change style”. For this lab, it is recommended to use lines (without symbols) for CFD and symbols (solid circles) for EFD data. Also, same color for same abscissa of CFD and EFD. NOTE: 1. For
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