Learning Forum

[sklara]

To whom it may concern,

I have been running some sims for the past month now in supersonic regime, and the 2-D cases have been working exactly as expected, but all of the 3-D cases I have tried have had a problem where any airfoil or slender body I use has an unexpected high pressure point at the trailing edge.  Whether its using a 2-D axi-symmetric case or general 3-D case, it always shows up, please see the attached image for an example.  The static pressure around the leading edge is as it should be, but the static pressure around the trailing edge shouldn't get above ambient pressure, and yet, every time, there's a pressure spike there.  My setup is using FLUENT density based solvers, both inviscid and k-omega SST models, with the energy equation on, and ideal gas for air, with solution steering towards the supersonic case of the second order.  far field and outlet pressure are 101100 Pa, Temperature is 300 K, V in is 686 m/s and Mach far field is 2. Do you have any advice to correct this problem?  Any help would be much appreciated, thank you.

Sincerely,

A student who has tried the same thing and expected different results

[Raef.Kobeissi]

I am interested to see how the mesh was constructed in 3D. Supersonic flow regimes are very sensitive and a good mesh is a requirement. Can you possibly upload the mesh to have a look?

[sklara]

Im having trouble uploading the mesh, are you looking for the .meshdat file?  It says that file type isn't allowed.

[Karthik R]

Hello,

Perhaps, you might want to share a few screenshots and explain the mesh details a bit?

Thank you.

Best Regards,

Karthik

[sklara]

Attached are the screen shots of the model I used, with dimensions, the mesh used, and the results for the 2-D planar case (matching expectations), and 2-D axisymmetric case (likely erroneous, with a high pressure point where a shock wave is expected).  The mesh has three zones of different meshing size, .2 m, .02 m, and .002 m, giving a total of ~455,000 nodes.  Again, setup is far field, outlet, and gauge pressure at 101100 Pa, inlet velocity is 686 m/s, Mach far field is 2, all temperatures are 300 K.  Inviscid model was used.  Hope this helps.

[sklara]

Hi,

So, I have continued trying different meshes, including refining just around the trailing edge with .0000001 m mesh sizing, half of the mean free path, although I am limited to 512000 nodes/elements.  I have found absolutely no success with the axisymmetric case.  I think the axis condition causes the pressure to skyrocket to infinity, because regardless of the mesh I use, I get an extremely high pressure (5e+10) around the trailing edge, where the flow meets up again, even though even a course 3-D run gives a much closer, although still erroneous result.  My question is:  is any kind of 3-D supersonic flow resolvable with the 512k limit, and if so, how?

[sklara]

I figured out my issue with the 2-D axisymmetric runs.  You should use the symmetry boundary condition for the axis, not the axis boundary condition.

[Karthik R]

Hello,

Glad you figured your issue and thank you for updating us.

Best, Karthik