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Extracting Internal Volume for Flow analysis (GTAC Article)

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internal volume.gif


Here's a good post from the GTAC team that Jerry found and I decided to add as a topic to this resource library. 


90% of the time taken to do CFD analysis is in pre-processing mostly because we're always concerned with internal or external volumes around an object... for which we never have the CAD. More than any other type of analyst, CFD analysts must care about CAD and with NX we make it pretty easy to create these flow volumes and here's how!




Following is another great article from the Siemens PLM GTAC team.



NX Flow solver in Advanced Simulation requires all elements used in modeling Flow to be 3D elements.  While the solid mesh for a design part occupies the part itself, the fluid mesh occupies the void between the bodies.  Because the void itself is not a body, one must create a solid body to occupy the void.  This solid body is generated in the idealized part I and then a mesh is applied on this body with 3D fluid elements using the NX meshing tools.  This internal fluid volume can be created by one of the following 3 methods:


  1. Define a sketch representing the 2D shape of a regular fluid volume and then Extrude and/or Revolve the sketch.  Use this technique if one knows the shape and dimensions of the fluid volume.
  2. Use Boolean operations to create the fluid volumes.  Users create a solid representing the enclosed space and select the Unite, Subtract, or Intersect to modify the shape of the volume based on the intersecting solid component geometry.  Use this technique if you want to create this fluid volume for single use in an analysis and if you are not interested in maintaining links for geometry updates.
  3. Create the enclosed fluid volume as a separate part file and component in the assembly file structure using the WAVE Geometry Linker command.  This command copies an instance of the geometry of the inner volume and the components from the assembly part file.  One then modifies the contour to represent the fluid volume by using the Delete Face command (on the Synchronous Modeling toolbar).  Use this technique if you want to maintain a link to the geometry of assembly components to allow for geometry updates.


In this article we will go over the 3rd method specified above to extract the flow volume.  Consider the task of extracting the internal flow volume in the following valve:


Here are the steps to extract the internal volume:

  1. Open this CAD model in NX. Select ‘Start -> Advanced Simulation’ to switch to Advanced Simulation application.
  2. In the Simulation Navigator, select ‘MB3 -> New FEM and Simulation’.  Set the solver to ‘NX Thermal / FLOW’, Analysis Type to ‘Flow’ and OK all the forms.
  3. Double-click on the idealized part to make it the ‘Displayed’ part.
  4. If you do not see ‘All but Selected’ icon in the ‘Selection’ toolbar, select ‘Tools -> Customize’ command.  Here, in the ‘Commands’ tab, select ‘All But Selected’ from the ‘Selection Bar’ category as shown below.  Then, drag and drop this command in the ‘Selection’ toolbar.flow2.png
  5.  Now, select ‘Insert -> Associative Copy -> Wave Geometry Linker’ command. Set the ‘Type’ to Body and select the valve.  In the dialog, toggle ON ‘Associative’ and ‘Hide Original’. OK the dialog.  This creates an associative copy of this valve in the idealized part. Any changes made to this copy will not affect the original CAD valve geometry.
  6. Select ‘Insert -> Synchronous Modeling -> Delete Face’ command.  Set the ‘Face Rule’ to ‘Boss or Pocket Face’ and select any internal face of the valve as shown below:flow3.pngflow4.png
  7. Now, select ‘All But Selected’ iconicon.gif  from the ‘Selection’ toolbar as shown in the previous image.  This will select 66 Faces as shown below.                                                                     flow5.gif
  8. Choose OK from the ‘Delete Face’ dialog.  This will result in the internal volume being extracted as shown below.  This internal volume can then be meshed with 3D tetrahedral mesh in the fem file to represent the flow volume.  



Thanks to SR for the tip.





Simcenter 3D Community Manager
Simulation and Test Solutions