NVH-1000: Acoustic Cavity

The model file used in this exercise can be found in the es.zip file. Copy the file(s) from this directory to your working directory.

Load the HyperMesh-Nastran, HyperMesh-OptiStruct or NVH-OptiStruct User Profile

  1. Click the Load User Profile icon, , on the Standard toolbar.
  2. Click HyperMesh > OptiStruct, HyperMesh > Nastran or Engineering Solutions > NVH > OptiStruct from the User Profiles dialog.
  3. Click OK.

Open the Model

  1. From the menu bar, click File > Open or click the Open Model icon, , on the Standard toolbar.
  2. Select the acoustic.hm file.
  3. Click Open to load the model.


    Figure 1.

Hide the Seat Cushion Components

  1. Expand the Component folder in the Model Browser.
  2. Select Rear_Seat_Cushion, IN-driver seat back cushion, IN-drv seat head rest, IN-driver seat lower cushion, IN-pass seat lower cushion, and IN-pas seat head rest from the list of components.


  3. Right-click and select Hide.

Set the Options for Creating Acoustic Cavity Mesh

The imported model has been restricted to just those parts that will be used to create the cavity. The seat cavities have already been built in this model.

  1. Click Mesh > Create > Acoustic Cavity Mesh to open the Acoustic Cavity Mesh Generation panel.
  2. Click the Options icon, , in the Acoustic Cavity toolbar.
    The Options dialog opens.
  3. Specify options for: Mesh type: Hexa-tetrahedra, Minimum hexa corner jacobian: 0.2, Minimum tet-collapse: 0.1, Display structural cavities Largest: 10, No. of elements per wavelength: 6.
  4. Enter property values for acoustic cavity meshes.
  5. Activate the checkbox for Create material and property cards.

Preview the Mesh

  1. Click the icon in the Acoustic Cavity toolbar.
  2. Click the comps selector for structure to open the Component Selection panel.
  3. Click comps > displayed to select all components.
  4. Click select to complete the selection.
  5. Click the comps selector for seats.
  6. Click Last Page to go to the last page of components.
  7. Select IN – pass seat back cushion, IN – pas seat head rest and Rear_Seat_Cushion on this page.
  8. Click Previous Page and select IN – driver seat back cushion, IN – drv seat head rest, IN – driver seat lower cushion, and IN – pass seat lower cushion.
  9. Click select to complete the selection of the seats.
  10. Make sure the seat coupling toggle is set to node to node remesh.
  11. Enter 40 for target element size=.
  12. Enter 10 for min element size=.
  13. Enter 40 for max element size=.
    The max frequency value will be automatically calculated and populated based on max element size and no. of elements per wavelength from the Options dialog. It is also possible to specify the max frequency value, and then max element size will be calculated accordingly.
  14. Enter 50 for gap patch size =.
  15. Enter 250 for hole patch size =.
  16. Enter 45 for feature angle=.
  17. Click the create hole elements checkbox to activate it.
  18. Click preview to see a preview of the mesh.
    The mesh appears in the graphics area and the Acoustic Cavity tab opens in the tab area. Note that the Front Door volumes are separate but trunk volume is a part of the overall cavity.


    Figure 2. Preview of Mesh

Review the Mesh

  1. Right-click AC_Structural.1 in the Acoustic Cavity tab and click Isolate.


    Figure 3. AC_Structural.1 Isolated
  2. Right-click the other components in the list and click Show.
  3. Right-click AC_Structural.1 and click Hide.
  4. Click the icon next to each of the seat cavities to see the mesh.


    Figure 4. Seats Added to the Visible Components

Delete Two of the Patched Holes

  1. In the Model Browser, right-click the ^patched_holes component and click Show.
  2. Zoom into the area, as shown in the image below.


  3. If necessary, right-click AC_Structural.1 and click Hide.
  4. Delete the highlighted elements shown in the picture below.
  5. Delete the similar type of element patch on the other side.


    Figure 5.
  6. Click return to exit the panel.

Preview the Mesh

With the patched holes deleted, the acoustic cavity mesh will be previewed again.

  1. Click Reject on the Acoustic Cavity tab.
  2. Click the icon on the Standard Views toolbar to reset the view of the model.
  3. Click Mesh > Create > Acoustic Cavity Mesh to open the panel.
  4. Click the comps selector for structure to open the Component Selection panel.
  5. Click comps > displayed to select all components.
  6. Click select to complete the selection.
  7. Click the comps selector for seats.
  8. Click Last Page to go to the last page of components.
  9. Select IN – pass seat back cushion, IN – pas seat head rest and Rear_Seat_Cushion on this page.
  10. Click Previous Page and select IN – driver seat back cushion, IN – drv seat head rest, IN – driver seat lower cushion, and IN – pass seat lower cushion.
  11. Click select to complete the selection of the seats.
  12. Enter 40 for target element size=.
  13. Enter 10 for min element size =.
  14. Enter 40 for max element size =.
  15. Enter 50 for gap patch size =.
  16. Enter 0 for hole patch size =.
  17. Enter 45 for feature angle =.
  18. Uncheck create hole elements.
  19. Click preview to preview the mesh.
    The front door volumes are now a part of the overall volumes.


    Figure 6.

Create a Patch Manually for Separate Volumes

  1. Click Reject.
  2. Click Mesh > Hole/Gap Fill.
  3. Select Patch Fill for the Fill Type.
  4. Create a manual patch separating the trunk from the main volume.
    A new component with the name Patch is created.


    Figure 7. Connection Between Trunk and Main Volumes


    Figure 8. Patch Separating Trunk and Main Volume
  5. Click Mesh > Create > Acoustic Cavity Mesh to open the panel.
  6. Click the comps selector for structure to open the Component Selection panel.
  7. Click comps > displayed to select all components.
  8. Click select to complete the selection.
  9. Click the comps selector for seats.
  10. Click Last Page to go to the last page of components.
  11. Select IN – pass seat back cushion, IN – pas seat head rest and Rear_Seat_Cushion on this page.
  12. Click Previous Page and select IN – driver seat back cushion, IN – drv seat head rest, IN – driver seat lower cushion, and IN – pass seat lower cushion.
  13. Click select to complete the selection of the seats.
  14. Enter 40 for target element size=.
  15. Enter 10 for min element size =.
  16. Enter 40 for max element size =.
  17. Enter 50 for gap patch size =.
  18. Enter 0 for hole patch size =.
  19. Enter 45 for feature angle =.
  20. Uncheck create hole elements.
  21. Click preview to preview the mesh.
    The main cavity and the trunk cavity is separate. This way it is possible to create separate cavities.


    Figure 9.
  22. Click Reject.
  23. Delete the Patch component and repeat steps 5-21.

Create the Mesh

  1. Select AC_Structural.2 through AC_Structural.10, right-click and select Hide.


    Figure 10.
  2. Set the Response points to Read from file.
  3. Click … to browse for the file.
  4. Select ACOUSTIC_RESPONSE_PTS.csv and click Open.
  5. Click Mesh to create the mesh.


    Figure 11. Completed Mesh
  6. Click return to exit the panel.

Rename the Components Created by the Acoustic Cavity Mesh

  1. Expand the Components folder in the Model Browser.
  2. Right-click AC_Structural.1 and click Rename.
  3. Name the component BODY_CAVITY and press Enter.
  4. Right-click AC_Seat.1 and click Rename.
  5. Name the component DRV_SEAT and press Enter.
  6. Right-click AC_Seat.2 and click Rename.
  7. Name the component REAR_SEAT and press Enter.
  8. Right-click AC_Seat.3 and click Rename.
  9. Name the component PASS_SEAT and press Enter.


    Figure 12.

Assign the Properties to the Components

  1. In the Model Browser, Component folder, select REAR_SEAT, DRV_SEAT and PASS_SEAT.
    The Entity Editor opens.
  2. For Property, click Unspecified > Property.


    Figure 13.
  3. In the Select Property dialog, select _seat_prop and click OK.
  4. In the Model Browser, Component folder, select BODY_CAVITY.
    The Entity Editor opens.


    Figure 14.
  5. For Property, click Unspecified > Property.
  6. In the Select Property dialog, select _int_prop and click OK.


    Figure 15.

Review Interface Between Structure and Cavity

  1. Click the icon in the Acoustic Cavity toolbar.
  2. Select All components.
  3. Enter 3 for NORMAL:.
  4. Enter 0.5 for INTOL:.
  5. Enter 2 for SKNEPS:.
  6. Activate the checkbox to Create ACMODL card, in case ACMODL card with the above defined parameters is to be created.
  7. Select the different types to Review interface and click Show to review it.


    Figure 16.

Create MPCs

This step creates additional MPCs between different acoustic and structure components, if needed.

  1. Click the icon in the Acoustic Cavity toolbar.
    The Create MPCs dialog opens.
  2. Select the relevant Fluid and Structure components.
  3. Set them as Main and Secondary.
  4. Select Options.
  5. Specify tolerance = 50.
  6. Click Create.

Renumber the Nodes, Elements, Properties and Materials

  1. Click Geometry > Renumber > Nodes to open the Renumber panel.
  2. Click nodes > displayed to select the displayed nodes.
  3. Enter 9000000 in the start with field.
  4. Click renumber to renumber the nodes.
  5. Change the selector to elems.
  6. Click elems > displayed to select the displayed elements.
  7. Click renumber to renumber the elements.
  8. Change the selector to props.
  9. Click props and select BODY_CAVITY and SEAT_CAVITY.
  10. Click renumber to renumber the properties.
  11. Change the selector to mats.
  12. Click mats and select BODY_CAVITY and SEAT_CAVITY.
  13. Click renumber to renumber the materials.
  14. Click return to exit the panel.

Export the Model

  1. Click File > Export > Solver Deck.
    The Export tab opens.
  2. For File type select OptiStruct.
  3. For Template select Standard format.
  4. Browse to a location in the File field and enter acoustic.fem as a name for the model.
  5. Expand the Export options.
  6. For Export select Displayed from the menu.
  7. Activate Write HM comments.