Simulation of Turbulent Flow over the Ahmed Body

Transcription

1 1 Simulation of Turbulent Flow over the Ahmed Body 58:160 Intermediate Mechanics of Fluids CFD LAB 4 By Timur Dogan, Michael Conger, Maysam Mousaviraad, Tao Xing and Fred Stern IIHR-Hydroscience & Engineering The University of Iowa C. Maxwell Stanley Hydraulics Laboratory Iowa City, IA 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 ANSYS. Students will have hands-on experiences using ANSYS to predict drag coefficients and axial velocity for slant angle 25 degrees and compare them 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. Geometry Physics Mesh Solution Results Ahmed body (ANSYS Design Modeler) Turbulent General (ANSYS Fluent - Setup) Model (ANSYS Fluent - Setup) Boundary Conditions (ANSYS Fluent - Setup) Reference Values (ANSYS Fluent - Setup) Unstructured (ANSYS Mesh) Solution Methods (ANSYS Fluent - Solution) Monitors (ANSYS Fluent - Solution) Solution Initialization (ANSYS Fluent - Solution) Plots (ANSYS Fluent- Results) Graphics and Animations (ANSYS Fluent- Results) Flow Chart for CFD Process for ahmed body

2 2 2. 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 ANSYS 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 can be found on the class website

3 3 3. Open ANSYS Workbench Software 3.1. Start > All Programs > ANSYS 15.0 > Workbench Drag and drop three component onto the Project Schematic, name the components and create connections between components as per below. 4. Geometry Creation 4.1. From the Project Schematic right click Geometry and select New Geometry.

4 Make sure that Unit is set to Meter (default value) Select the XYPlane then click the New Sketch button.

5 Enable the auto constraints option to pick the exact point as below 4.5. Use the Rectangle tool under Draw to make a rectangle starting from the origin and ending in the first quadrant. Dimension it using General dimension as per below.

6 Use the Rectangle tool again to draw a rectangle as per below Use the Fillet tool in Modify to put a radius on the front corners of the Ahmed Car as per below. Use the Radius size of 0.1m.

7 Use the Chamfer tool to put a chamfer on the back of the Ahmed Car as per below. Use the Length of 0.25m Put a constraint on the two radii using the Equal Radius tool in Constraints. (Note: in the bottom left corner next to the checkmark in a green circle is the notes on how to use a tool. This works for all tools.)

8 Dimension the sketch as per below using General, Horizontal, Vertical, Radius, and Angle under Dimensions Concept > Surface From Sketches. Select the sketch you just created and click Apply. Click Generate.

9 Select XYPlane and click the New Sketch button. Use the Line tool under Draw to make the lines as per below (Three lines total). Make sure that the C appears when you are on the line and the V/H appears next to the line being created indicating you are both on the existing edge and the line is vertical/horizontal Use the Horizontal and Vertical dimension tool to dimension the lines as per below.

10 Tools > Face Split Select the surface for Target Face and click Apply. For Tool Geometry select two endpoints of a single line while holding Ctrl then click Apply. Select Tool Geometry again and select two more endpoints and click Apply, repeat this process for the last line and click Apply. Click Generate. This splits the surface into six pieces.

11 Tools > Merge Change the Merge Type to Faces and select the top three faces. Click Apply then Generate.

12 All the dimensions needed are shown as per below. H1 H10 H11 V2 H3 R8 H4 H5 A9 V7 V12 V File > Save Project. Close the Design Modeler window. 5. Mesh 5.1. Right click Mesh and from the dropdown menu select Edit

13 Right click Mesh > Insert > Inflation For Geometry, select the surface of the domain which borders the Ahmed Car and click Apply. For the Boundary, select the edges of the Ahmed Car by holding Ctrl and selecting the edges and then click Apply. There should be seven edges selected for the Boundary. Change the parameters in Details of Inflation Inflation as per below.

14 Right click Mesh > Insert > Method. Select the domain for Geometry and click Apply. Change the Method to Triangles Right click Mesh > Insert > Sizing. Select the line as per below and click Apply. Change the parameters of sizing as per below. Repeat this for the following figures below. There should be 22 edge sizings.

15 15

16 16

17 17

18 18

19 19

20 20

21 21

22 22

23 23

24 24

25 5.6. Click on Mesh under the Outline and change the Physics Preference to CFD Click Generate Mesh. 25

26 Select the top edges of the domain by holding Ctrl while selecting, right click, from the dropdown menu select Create Named Selection. Name the top edge symmetry Repeat step 5.8 for the bottom edges and name them road.

27 5.10. Repeat step 5.8 for the left edges and name them inlet. 27

28 Repeat step 5.8 for the right edges and name them outlet Repeat step 5.8 for the filleted corners and the straight segment that connects them and name them nose.

29 Repeat step 5.8 for the sloped edge of the Ahmed Car and name it slope Repeat step 5.8 for the top edge of the Ahmed Car and name it ahmed top Repeat step 5.8 for the bottom edge of the Ahmed Car and name it ahmed bottom.

30 5.16. Repeat step 5.8 for the right vertical edge of the Ahmed Car and name it back. 30

31 File > Save Project. Close Meshing window Update the mesh by right clicking Mesh and from the dropdown menu select Update. 6. Setup 6.1. Right click Setup and select Edit 6.2. Select Double Precision and click OK Solution Setup > General. Change solver to Transient as per below.

32 6.4. Solution Setup > Models > Viscous > Edit Change the turbulent model and nearwall treatment as per below. 32

33 Solution Setup > Materials > Fluid > air > Create/Edit Change the air Density and Viscosity as per below and click Change/Edit then close the window Solution Setup > Boundary Conditions > inlet > Edit Change the inlet boundary conditions as per below and click OK.

34 Solution Setup > Boundary Conditions > Zone > outlet > Edit Change the outlet boundary condition as per below and click OK Solution Setup > Reference Values. Change the reference values as per below.

35 7. Solution 7.1. Solution > Solution Methods. Change solutions methods as per below. 35

36 Solution > Monitors > Residuals Print, Plot > Edit. Change the parameters a s per below and click ok Solution > Monitors > Residuals, Statistics and Force Monitors > Create > Drag. Select parameters as per below. Click Axes and make changes to the x-axis as per below and click apply. Finally click ok and close windows. Note: Do not forget to change window.

37 Solution > Solution Initialization. Change X-Velocity and turbulent parameters as per below Solution > Calculation Activities > Solution Animations > Create/Edit. Change the parameters as per below.

38 7.6. Click Define for the stream animation (see previous image where the button is labeled with blue rectangle). Select PPM image for storage type, change window then select contour to plot stream function. Change parameter for the stream as per below. Click Close for contours window and then click set and ok. Note: In order to see animation without any problem, keep the following instruction. 1. Do not forget to change window and click set button. 2. Click display button before pressing close button and make sure the new window #3 followed by contours of stream function show up. 38

39 7.7. Click Define for the viscosity ratio (see previous image where the button is labeled with blue rectangle). Select PPM image for storage type, change window then select contour to plot turbulent viscosity ratio. Change parameter for turbulent viscosity ratio as per below. Click Close for contours window and then click set and ok. Note: In order to see animation without any problem, keep the following instruction. 3. Do not forget to change window and click set button. 4. Click display button before pressing close button and make sure the new window #3 followed by contours of stream function show up. 39

40 Solution > Run Calculation. Change parameters as per below and click Calculate. If you have the correct setup you should see four windows on the upper left corner as per below (2 nd image). You can change what the window shows by changing the option on the upper left corner as shown on the second image below. Window 1-4 shows the residuals, coefficient of drag history, streamlines and turbulent viscosity ratio. After running for about 0.05 flow time you should see vortices at the back of the ahmed car for windows 3 and 4. NOTE: This simulation will take several hours. Please make sure your setup is correct before running the simulation. If you close Fluent window after running the simulation, you will lose your time history. As a result, the videos will not work and the drag coefficient history will not show on window 2. After the calculation you should see the images below

41 41

42 42 8. Results 8.1. Creating line to plot modified TKE and modified U. Surface > Line/Rake. Create 10 lines at the locations given at the table below.

43 43 Surface Name x0 y0 x1 y1 Position Position Position Position Position Position Position Position Position Position Create custom function Define > Custom Field Functions. Create custom field functions and click Define. You will need to create two custom field functions shown in the table below.

44 44 Function Name Definition y-by-h y / Modified-U (mean-x-velocity / 120) + (x / 0.288) Modified-TKE (turb-kinetic-energy / 500) + (x / 0.288) Operand field function including coordinate (x and y), mean-x-velocity and turb-kinetic-energy are found as in the following table and figures Operand field function From field functions x, y Mesh Mean-x-velocity Unsteady statistics Turb-kinetic-energy Turbulence

45 Plotting values along the lines created. Results > Plots > XY Plot > Set Up. Click Load File and load the experimental data. Select the lines you created (position-1 through position-10) and experimental data then click Plot. (Note: You can download the EFD data from the class website) Note: You change the style and color of the data by clicking Curves button and changing the parameters below then clicking Apply. Click Axes and adjust the Y axis maximum to 2.5 and minimum to -0.5.

46 Plotting drag coefficient Results > Report > Forces > Set Up. Select the region where you want to calculate the drag coefficient under wall zone then click print Plotting contours and vectors Results > Graphics and Animations > Contours. Change parameters as per below and click Display.

47 Results > Graphics and Animations > Vectors > Set Up Change parameters as per below and click Display. 47

48 8.6. Creating videos Results > Graphics and Animations > Solution Animation Playback > Set Up Change the window to streams or viscous ratio then click play button. You can click print screen shot to save images as the video plays. Below few images are shown from the video. 48

49 49

50 50

51 51

52 52 Note: 1. Don t get panic even if you cannot see the animation on the screen. The snapshots of the animation are saved to files in ppm format in your project directory, e.g. if your project name is lab4 then you can find the snapshots in lab4_files\dp0\flu\fluent. The name of snapshots will have stream or viscosity-ratio followed by serial numbers. Converting them into movie file (e.g. *.avi or *.mpg) will be explained later. 2. In case you failed to see the animation on the screen, you can go to the project directory and find ppm files. Holding left shift button and clicking right mouse button, open command window. By using ImageMagick program installed, type in following command into the command window. convert -delay 6 -quality 95 streamline*.ppm streamline.mpg convert -delay 6 -quality 95 viscosity-ratio*.ppm viscosity-ratio.mpg

53 53 9. Trouble shooting for the animation part A script file is provided in case of not producing correct snapshot files although you think computational setup is right (you can download the script file from the class website). The script file assumes that you have followed this instruction up to at least section 8.2. What the script does is to: 1. Redefine custom functions 2. Set calculation activities For you. In order to run the script, it is recommended to save your project and close and reopen Fluent component. You should look for Journal in File-Read menu as shown below. For the files of type, choose All files and find the script file in the folder you saved the script file.

54 54 When you load the script file, the following window will appear. Then press No button instead of Yes. Note: Wait without clicking anything until all the batch commands are finished and popped-up windows are closed. It takes a couple of minutes depending on the system. Run the simulation and see if snapshot images are generated in the folder described in page Exercises You need to complete the following assignments and present results in your lab reports following the lab report instructions. Simulation of Turbulent Flow over the Ahmed Body Simulation of turbulent flows over Ahmed body (slant angle=25 degree):

55 55 a. Fill in the table for the four drag coefficients and compute the relative error between CFD and EFD (Ahmed data), EFD data for C k, C B, and C s can be found from the figure * * * below. Where Ck = Ck, CB = CB, and Cs = Cs. The definitions of the drag coefficients are: C k is the forebody pressure drag coefficient, C B is the vertical based pressure drag coefficient, C R is the friction drag coefficient, C s is the slant surface pressure drag coefficient, and Cw = CD is the total drag coefficient. So, Cw = CD = CS + CB + Ck + CR Ck CB CS CD Ahmed (EFD) k-e Error (%) b. Questions: Do you observe separations in the wake region (use streamlines)? If yes, where is the location of separation point?

56 What is the Strouhal number based on the shedding frequency (CD vs. time), the height of the Ahmed body and the inlet velocity? Note: the shedding frequency f=1/t where T is the typical period of the oscillation of CD that can be evaluated using the peaks between 0.1<time<0.14. Figures to be saved: 1. XY plots for residual history, modified U vs. y-by-h (with EFD), Modified-TKE vs. y-by-h and time history of drag coefficient, 2. Contour of pressure, contour of axial velocity and velocity vectors, 3. 3 or 4 snapshots of animations for turbulent-viscosity-ratio and streamlines (hints: you can use <<Alt+print Screen>> during the play of the animations). Data to be saved: the above table with values. 56