1Simulation of Turbulent Flow in an Asymmetric Diffuser 58:160 Intermediate Mechanics of Fluids CFD LAB 3 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 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 validation of velocity, turbulent kinetic energy, and skin friction factor. 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. Flow Chart for ISTUE Teaching Module for Diffuser Flow (red color illustrates the options you will use in CFD Lab 3) CoarseMediumFineAutomaticManualStructured Unstruct-ured Geometry Geometry parameters Physics Mesh Post-processingSelect Geometry Top wall skin friction Modified Velocity Modified TKE Contours Vectors Streamlines Asymmetric XY plot Validation Verification Boundary ConditionsFlow PropertiesViscous ModelsOne Eq. Two Eq. Density and viscosity Laminar TurbulenInviscid SAk-ek-wIncompressible Initial Conditions SolveIterations/ Steps Converge-nt LimitPrecisions SingleDoubleNumerical Schemes1st order upwind 2nd order upwindQuick Steady/ Unsteady?Report Four Eq.v2fResiduals Bot. wall skin friction Pressure difference22. Simulation Design The problem to be solved is that of turbulent flows inside an asymmetric diffuser (2D). Reynolds number is 17,000 based on inlet velocity and inlet dimension (D1). The following figure shows the sketch window you will see in FlowLab with definitions for all geometry parameters. Before the diffuser, 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 saved on the Fluids lab computers. 3. CFD Process Step 1: (Geometry) 1. Select Geometry: Asymmetric 2. Inlet dimension (D1) (2 m) 3. Inlet length L1 (60 m)34. 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 Properties Use the values shown in the above figure. Input the values and click <<OK>> (3). Viscous Model4 In 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 Conditions NOTE: 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 turbulent quantities. 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 the values and click <<OK>>.5 (5). Initial Condition 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 inlet dimension you specified. Click <<Next>>. This takes you to the next step, “Mesh”. Step 3: (Mesh) In CFD Lab 3, “Structured” meshes will be generated using “Manual” option6 Use 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. D1 L1 D2 L2 DL7 The 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.8 Step 5: (Reports) “Reports” first provide you the information on “Total frictional force on the upper wall”, “wall share stress”, “pressure difference (between inlet and outlet)”. “XY Plots” provides the following options:9In 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.xy 2. EFD data for modified Turbulent Kinetic Energy (500k+x) is: Modified_TKE-10degree.xy 3. EFD data for bottom wall friction factor distribution: Skin_friction_bot_wall.xy 4. EFD data for upper wall friction factor distribution: Skin_friction_top_wall.xy The 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 the right 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 modified velocity and
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