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# Simple CFD problem

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### Simple CFD problem

Posted by Mark Beeman at February 22. 2016

All:

I am new to Salome, but am trying to mesh a simple flow volume for CFD.  Now many times in CFD one wants to look at the flow around a body.  So I am using the simple example of a cylinder.  I can create the cylinder and have used Salome enough to get a great surface mesh.   Then for the flow volume around the cylinder, I am using a large box for this example.  Here is where it gets tricky:   Of course I can define a volume of the large, external box minus the cyllinder within and use Netgen 1D-2D-3D to grid the volume....that is easy.  However, to get a good mesh, one wants a certain surface mesh on the inner body and then wants to extrude the mesh (either structured or unstructured) out to the outer box to create the flow volume.  During this 3D extrusion, a viscous 3D layer is often used, and the surface grid on the outer box of the flow volume is, in general, more coarse than near the body.  So you end up with a fine grid near the body that gets extruded and becomes more coarse until you reach the outer box of the flow volume.

I have seen many threads on this forum "dancing around" this topic, but is there an easy way to do this?  It would be a shame that a program with the potential of Salome lacks this basic feature.  The only way I can see around it is to create multiple sub volumes within the main flow volume...but then there is the problem of "stitching" these together for the final grid.

I know someone out there has to know how to do this kind of extrusion...I have seen some really smart threads on this forum.

Thanks,

Mark

### Re: Simple CFD problem

Posted by Saint Michael at February 24. 2016

Hi Mark

The mesh shown in the picture is built in the attached study. It's actually easy. Is it what you are looking for?

St.Michael

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### Re: Simple CFD problem

Posted by rajat tripathi at June 26. 2018

Hi Michael !

May I  know how did you create this kind of meshing. I am facing the exact same problem. Your guidance will help me a lot

### Re: Simple CFD problem

Posted by Saint Michael at June 26. 2018

Hi

This mesh was created as follows:

In Geometry module:

1. Create a box and a cylinder
2. Cut the cylinder from the box
3. On the cut, create a group of faces of the cylinder

In mesh module, create a mesh on the cut with algorithms:

1. NETGEN 3D + Viscous Layers hypothesis (walls = the group of cylinder faces)
2. NETGEN 2D

Optionally one could create a sub-mesh of the group of cylinder faces in order to define 2D mesh on cylinder different from that on the box.

St.Michael