Simulation 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 Physics Geometry Mesh Solve Unstructured Automatic Coarse Structured Manual Medium Incompress ible Flow Properties Viscous Models Boundary Conditions Fine Density and viscosity Laminar Turbulen Inviscid Initial Conditions Select Geometry One Eq Two Eq SA Post processing Steady Unsteady Iterations Steps Contours Vectors XY plot Convergent Limit Verification Precisions Validation Streamlines Double Numerical Schemes k e k w 1st order upwind v2f 2nd order upwind Bot wall skin friction Quick Top wall skin friction Four Eq Asymmetric Single Report Residuals Modified Velocity Geometry parameters Modified TKE Pressure difference Flow Chart for ISTUE Teaching Module for Diffuser Flow red color illustrates the options you will use in CFD Lab 3 1 2 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 2 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 Properties Use the values shown in the above figure Input the values and click OK 3 Viscous Model 3 In this lab the two equation k 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 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 zerogradient for pressure Turbulent quantities k and on the wall are also specified to be zero Read all the values and click OK 4 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 option 5 L1 D2 L2 D1 DL 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 6 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 7 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 8 In 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 modified TKE use the default setup for x and y ranges 2 For bottom wall friction factor distribution use the following range x 0 120 y 0 001 0 004 You need click Axes in XY plot and turn off Auto Range 3 For upper wall friction factor distribution use the following range x 0 120 y 0 0 004 4 For more detailed information on modified velocity TKE and skin friction factor please read reference 1 for diffuser on class website http css engineering uiowa edu me 160 Lab CFDdiffuser pdf 9 Step 6 Post processing Use the contour vector buttons
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