pressure drop, laminar, use of prism?

galap · November 11, 2009, 13:09

Hi community,

I'm confused by some results calculated with Ansys CFX. I meshed a 3D structure (air flows through) with tetras and calculate the pressure drop at several inlet normal speeds. So far, results fits quite well to practical measurement data. Now I repeat these calculations with an additional prism mesh (4 layers, tetra settings remaining equal) for a better velocity resolution on surfaces. Though I get pressure drop values, increasing for instance from 600mbar/m to 900mbar/m. Quite a difference and none relation to practical measurements!

I don't know to handle that. The flow is laminar, help manual says prism mesh captures boundary layer physics. Maybe prism should only be used when I have turbulent flows..? Im confused, better solution on surfaces should give me more realistic results, isn't it..

ghorrocks · November 11, 2009, 22:16

Laminar flows have boundary layers too. If you have thin laminar boundary layers then a prism mesh is still a good idea.

If when you change the mesh your results change a lot then it sounds like you are a long way from a mesh converged solution. You probably need a much finer grid.

galap · November 12, 2009, 04:50

Thanks, thats a good point. My residuals were set to 10-4 and prism quality aren't good.. I refine my mesh and check results.

November 11, 2009, 13:09	pressure drop, laminar, use of prism?	#1
galap Member Fabian E. Join Date: Nov 2009 Posts: 38 Rep Power: 17	Hi community, I'm confused by some results calculated with Ansys CFX. I meshed a 3D structure (air flows through) with tetras and calculate the pressure drop at several inlet normal speeds. So far, results fits quite well to practical measurement data. Now I repeat these calculations with an additional prism mesh (4 layers, tetra settings remaining equal) for a better velocity resolution on surfaces. Though I get pressure drop values, increasing for instance from 600mbar/m to 900mbar/m. Quite a difference and none relation to practical measurements! I don't know to handle that. The flow is laminar, help manual says prism mesh captures boundary layer physics. Maybe prism should only be used when I have turbulent flows..? Im confused, better solution on surfaces should give me more realistic results, isn't it..

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November 11, 2009, 22:16		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Laminar flows have boundary layers too. If you have thin laminar boundary layers then a prism mesh is still a good idea. If when you change the mesh your results change a lot then it sounds like you are a long way from a mesh converged solution. You probably need a much finer grid.

November 12, 2009, 04:50		#3
galap Member Fabian E. Join Date: Nov 2009 Posts: 38 Rep Power: 17	Thanks, thats a good point. My residuals were set to 10-4 and prism quality aren't good.. I refine my mesh and check results.