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					F4 Car                                                                               Page 1 of 19




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    Carnegie Mellon


     Mechanical Engineering
                                    FEM/ANSYS
                                              F4 Car




  Fluid #4: 3D Flow Over a Car USING FLOTRAN

  Introduction: In this example you will model a car inside a wind tunnel.
  Physical Problem: Compute and plot the velocity distribution in the wind tunnel shown in
  the figure.
  Problem Description:
     ·         The shape of the car within the wind tunnel is shown in the figure.
            Objective:
                    To plot the velocity profile within the wind tunnel.  
            You are required to hand in print outs for the above.  
            Figure:  




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F4 Car                                                                                     Page 2 of 19




         Important Dimensions:
                      Keypoints to form the Car: (all dimensions are in meters)

                            Pt        X           Y          Z 
                            1         1.4        0.4         4.5 
                            2         1.4        0           4.5 
                            3         2.1        0.4         4.5 
                            4         2.1        0           4.5 
                            5         1.2        0           3.5 
                            6         1.2        0.8         3.5 
                            7         2.3        0           3.5 
                            8         2.3        0.8         3.5 
                            9         1.2        0           2.5 
                            10        1.2        0.8         2.5 
                            11        2.3        0           2.5 
                            12        2.3        0.8         2.5 
                            13        1.3        0           1.75 
                            14        1.3        0.4         1.75 
                            15        2.2        0           1.75 
                            16        2.2        0.4         1.75 

                      Dimensions of the Block defining the Wind Tunnel:

                             The X and Y position of the corner of the block is (0,-1.8)
                             Width = 3.6 m
                             Height = 4 m
                             Depth = 10.25 m

                      The wind traveling over the car is going 24.6m/s. (or 55mph)




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F4 Car                                                                                        Page 3 of 19




  STARTING ANSYS

     ·         Click on ANSYS in the programs menu.
     ·         Select Interactive.
     ·         The following menu that comes up. Enter the working directory. All your files will be
               stored in this directory. Also enter 64 for Total Workspace and 32 for Database.
     ·         Click on Run.




  MODELING THE STRUCTURE

         Go to the ANSYS Utility Menu  
         Click Workplane>WP Settings  
         The following window comes up  




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F4 Car                                                                                   Page 4 of 19




     ·         Check the Cartesian and Grid Only buttons
     ·         Enter the values shown in the figure above.

  In this problem we will model the car, then model the wind tunnel around it, then subtract the
  volume of the car from the wind tunnel. At this point we will then apply fluid flow to the wind
  tunnel and see how its flow is impeded due to the car.

     ·         Now, we will create the model.
     ·         Click Preprocessor>-Modeling-> and create the keypoints to define the car.
     ·         NOTE: It makes the creation of the car MUCH easier to go to the ANSYS Main Menu (the
               top bar) and select PlotCntrls>Pan Zoom Rotate and select the Isometric view (ISO).
               This simply allows you a better view of the 3 dimensional keypoints forming the car.
     ·         Once the keypoints are finished, the model should look like the figure below.




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F4 Car                                                                                    Page 5 of 19




     ·         Now that the keypoints have been created, connect them with lines to form the body of
               the car.
     ·         If any lines are created incorrectly, proceed to Preprocessor>Modeling>Delete>Lines
               Only to delete the incorrect line without deleting the keypoints forming it.
     ·         The car should now look like this:




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F4 Car                                                                                        Page 6 of 19




     ·         Once the lines have been created, create Arbitrary Areas defined by Lines to form the
               outer shell of the car.
     ·         NOTE: DO NOT FORM THE 2 VERTICAL AREAS WITHIN THE CAR. This will only make
               creating the volume of the car much more difficult.
     ·         Once the areas defining the shape of the car are all created, define a volume by areas.
               Since only the areas on the outside of the car have been created you may choose “PICK
               ALL” to select all the areas that make up the car and form them into a single volume.
     ·         If an error appears it is most likely because either an area was not selected (if you
               chose to do the selection by hand) OR an area was created within the volume
               unintentionally. If this is the case, you need to go to
               Preprocessor>Modeling>Delete>Areas Only and delete the incorrect area.
     ·         Once the car volume is finished, create the Block surrounding it that will form the wind
               tunnel.
     ·         Now go to Preprocessor>Modeling>Operate>Booleans>Subtract>Volumes and select
               the wind tunnel block, then the car and click OK. This now deletes the car volume and
               leaves a hollow space within the wind tunnel in the shape of the car. The reason for
               this step is that the wind tunnel block will be defined as nothing but Air, so the volume
               that was removed (in the shape of the car) acts as an impedance for air (as it would in
               real life) and causes the deflection we desire to plot.
     ·         If you have done everything correctly the model should look like this: (in an isometric
               view)




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F4 Car                                                                                       Page 7 of 19




  The modeling of the problem is done.

  ELEMENT PROPERTIES


  SELECTING ELEMENT TYPE:
  ·         Click Preprocessor>Element Type>Add/Edit/Delete... In the 'Element Types' window that
            opens click on Add... The following window opens:




  ·         Type 1 in the Element type reference number.
  ·         Click on Flotran CFD and select 3D Flotran 142. Click OK. Close the 'Element types' window.
  ·         So now we have selected Element type 1 to be a Flotran element. The component will now
            be modeled using the principles of fluid dynamics. This finishes the selection of element
            type.




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F4 Car                                                                                       Page 8 of 19




  DEFINE THE FLUID PROPERTIES:
  ·         Go to Preprocessor>Flotran Set Up>Fluid Properties.
  ·         On the box, shown below, make sure the first two input fields read AIR-SI, and then click
            on OK. Another box will appear. Click OK to accept the default values.




  ·         Now we’re ready to define the Material Properties


  MATERIAL PROPERTIES

  ·         Go to the ANSYS Main Menu
  ·         Click Preprocessor>Material Props>Material Models. The following window will appear




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F4 Car                                                                        Page 9 of 19




  ·         As displayed, choose CFD>Density. The following window appears.




  ·         Fill in 1.23 to set the density of Air. Click OK.
  ·         Now choose CFD>Viscosity. The following window appears:




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F4 Car                                                                                      Page 10 of 19




  ·         Fill in 1.79e-5 to set the viscosity of Air. Click OK
  ·         Now the Material 1 has the properties defined in the above table so the Material Models
            window may be closed.


  MESHING:

  DIVIDING THE CHANNEL INTO ELEMENTS:
  ·         Go to Preprocessor>Meshing>Size Cntrls>ManualSize>Lines>Picked lines. Now select all
            the lines that form the car and Click OK. (NOTE: It will make selection much easier if you
            go to the ANSYS Main Menu (Top Bar) and select Plot>Lines. This will allow you to view
            the lines that form the volume.) In the window that comes up type 0.05 in the field for
            'Element edge length'.




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F4 Car                                                                                     Page 11 of 19




  ·         Click on OK. Now return to Preprocessor>Meshing>Size Cntrls>ManualSize>Lines>Picked
            lines and select the edges of the block forming the wind tunnel around the car.
  ·         Set the ‘element edge length’ to 0.5.
  ·         Now when you mesh the figure ANSYS will automatically create a mesh, whose elements
            have a edge length of 0.05 m at the car, and slowly lengthen until they are approximately
            0.5 m at the edge of the wind tunnel. This is because the mesh should be finer at the car
            because that’s where we want a more precise analysis.
  ·         Now go to Preprocessor>Meshing>Mesh>Areas>Free. Click Pick All. The mesh will look
            like the following.




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F4 Car                                                                                      Page 12 of 19




  NOTE: The car is meshed safely inside the block. Do not be alarmed that you can not see it.



  BOUNDARY CONDITIONS AND CONSTRAINTS

  ·         Go to Preprocessor>Loads>Define Loads>Apply>Fluid CFD>Velocity>On Areas. Pick the
            area of the wind tunnel block facing the front of the car and Click OK. The following window
            comes up.




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F4 Car                                                                                       Page 13 of 19




  ·         Enter -24.6 in the VZ value field and click OK. The -24.6 corresponds to the velocity of 24.6
            meters per second of air flowing over the car.
  ·         Then, set the Velocity to ZERO along all of the axial sides of the block enclosing the car.
             This is because of the “No Slip Condition” acting on the walls of the wind tunnel.
            (VX=VY=0 for all sides)
  ·         Go to Main Menu>Preprocessor>Loads>Define Loads>Apply>Fluid CFD>Pressure DOF>On
            Areas. Pick the area behind the car and click OK.
  ·         Enter 0 as the pressure value. (This sets the pressure as atmospheric allowing the air to
            pass over the car)
  ·         Once all the Boundary Conditions have been applied, we can move on to solving the
            problem.

  SOLUTION




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F4 Car                                                                                    Page 14 of 19




     ·         Go to ANSYS Main Menu>Solution>Flotran Set Up>Execution Ctrl.
     ·         The following window appears. Change the first input field value to 25, as shown. No
               other changes are needed. Click OK.




     ·         Go to Solution>Run FLOTRAN.
     ·         Wait for ANSYS to solve the problem.
     ·         Click on OK and close the 'Information' window.

  POST-PROCESSING

     ·         Plotting the velocity distribution…
     ·         Go to General Postproc>Read Results>Last Set.




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F4 Car                                                                                  Page 15 of 19



     ·         Then go to General Postproc>Plot Results>Contour Plot>Nodal Solution. The following
               window appears:




     ·                     Select DOF Solution and Velocity VSUM and Click OK.
     ·                     This is what the solution should look like:




     Despite what you may think this is the correct solution. Now, in order to view the effects of




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F4 Car                                                                                  Page 16 of 19



     the air flow on the car within the wind tunnel we must move the working plane so that it’s
     positioned along the longest axis of the car and tell ANSYS to show a cut away view. This is
     how you accomplish that:

         First, go to the ANSYS Main Menu>WorkPlane and select Display Working Plane. Now
         that the working plane is selected, go to ANSYS Main Menu>WorkPlane >Offset WP by
         Increments and adjust the working plane such that it sits along the long axis of the car.
         When you are finished moving the plane it should look like this:  




     (NOTE: you can make sure it is properly positioned by selecting ANSYS Main
     Menu>PlotCntrls>Pan Zoom Rotate and changing the views to verify).

     ·         Once the plane is in line, select ANSYS Main Menu>PlotCntrls>Style…>Hidden Line
     Options.. a pop up window will now appear:




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F4 Car                                                                                Page 17 of 19




         In this window change “Type of Plot” to Q-Slice Z-buffer, and “Cutting Plane is” to
         Working Plane and click OK. ANSYS will now display the results of the analysis with the
         working plane as the cutting plane.  
         The final solution now looks like this:  




     ·         Next, go to Main Menu>General Postproc>Plot Results>Vector Plot>Predefined. The




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F4 Car                                                                                         Page 18 of 19



     following window will appear:




     ·         Select OK to accept the defaults. This will display the vector plot of the velocity
     gradient.




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F4 Car                                                                                              Page 19 of 19




  Now that the solution is finished, the Workplane can be moved and different cut-away images
  of the velocity gradient can be plotted using the same method of moving the Workplane and
  setting the Hidden Line Options such that the cutting plane is the Workplane.
   

                  Send mail to the Teaching Staff with questions or comments about this web site.




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Description: smoke flow visualization wind tunnel. tunnel parts and smoke generations system. ansys model of wind tunnel