Negative elements due to mesh deformation in transient run

beyonder1 · September 10, 2015, 04:52

Hi,
I have a cascade of airfoils in which i am providing some airfoil oscillation of around 2 degrees. I am running a transient case for this in cfx. I have meshed very fine close to boundary layer for resolution according to Y+(~1).
The problem is after few transient steps, mesh is deforming so much that it creates negative volume elements. I would like to ask if i can increase CFL no. so that i could have less fine mesh near BL since it affects numerical results.

ghorrocks · September 10, 2015, 06:13

First of all, I would do a mesh sensitivity study to see what mesh you require for the accuracy you are happy with. You may be able to use a mesh coarser than y+=1 and that makes EVERYTHING easier. You can do this study with a fixed mesh to make it easier.

Once you know what near wall mesh you need, you might find it works OK with mesh movement. But if not, here are some ideas:
* Look at the mesh smoothing options. If you weight the mesh smoothing appropriately for your motion you can avoid mesh folding.
* As your airfoil is oscillating you might be able to put it in a region of mesh which rotates or translates, and is connected to the rest of the mesh by GGIs. Then you do not need to deform the mesh at all, you just move it rigidly to a new location and avoid the mesh folding problem entirely.

beyonder1 · September 10, 2015, 08:30

Thank you. I will try these and then discuss.

September 10, 2015, 04:52	Negative elements due to mesh deformation in transient run	#1
beyonder1 Member beyonder Join Date: May 2015 Posts: 67 Rep Power: 10	Hi, I have a cascade of airfoils in which i am providing some airfoil oscillation of around 2 degrees. I am running a transient case for this in cfx. I have meshed very fine close to boundary layer for resolution according to Y+(~1). The problem is after few transient steps, mesh is deforming so much that it creates negative volume elements. I would like to ask if i can increase CFL no. so that i could have less fine mesh near BL since it affects numerical results.

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September 10, 2015, 06:13		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	First of all, I would do a mesh sensitivity study to see what mesh you require for the accuracy you are happy with. You may be able to use a mesh coarser than y+=1 and that makes EVERYTHING easier. You can do this study with a fixed mesh to make it easier. Once you know what near wall mesh you need, you might find it works OK with mesh movement. But if not, here are some ideas: * Look at the mesh smoothing options. If you weight the mesh smoothing appropriately for your motion you can avoid mesh folding. * As your airfoil is oscillating you might be able to put it in a region of mesh which rotates or translates, and is connected to the rest of the mesh by GGIs. Then you do not need to deform the mesh at all, you just move it rigidly to a new location and avoid the mesh folding problem entirely.

September 10, 2015, 08:30		#3
beyonder1 Member beyonder Join Date: May 2015 Posts: 67 Rep Power: 10	Thank you. I will try these and then discuss.