Residuals not converging, but staying stable and fluctuating

ashf9 · February 26, 2021, 05:19

Hi,
I am doing a hypersonic flow simulation on re-entry spacecraft, and for some reason, my residuals don't converge but rather stay stable for a while and then fluctuate (attached is a picture of it). I understand that the fluctuations could be decreased using relaxation factors, but could someone explain to me why the continuity and other variables aren't converging, as I would expect some sort of convergence after Would it be something to do with the mesh or initial BCS
Thanks
Residual 1.JPG

Residual 2.JPG

Residual 3.JPG

aerosayan · February 26, 2021, 07:26

Do you have a fluctuating von-karman vortex street in the wake region?
Transient phenomenons cause fluctuations in residuals.

You can run the simulations more 10000 iterations *here*, and see if the residuals converge or keep fluctuating. If they keep fluctuating but don't diverge, the solution might be okay.

ashf9 · March 3, 2021, 03:48

Ah I see thank you. I do notice Von-Karman Vortex sheets, and the results do look quite similar to the expected values. I have noticed though that my meshing is quite inconsistent, on of the blocks in a multi block grid. Do you think thatwould also have an effect?

aero_head · March 3, 2021, 12:40

Quote:

Originally Posted by ashf9

Ah I see thank you. I do notice Von-Karman Vortex sheets, and the results do look quite similar to the expected values. I have noticed though that my meshing is quite inconsistent, on of the blocks in a multi block grid. Do you think thatwould also have an effect?

Hello,

Could you post a photo of the 'inconsistent' meshing?

ashf9 · March 3, 2021, 16:13

Sure, I've attached some photos of the mesh. For some reason, Ansys won't let me use the edge sizing for one block of my grid, and I can't seem to find out why.

Quote:

Originally Posted by aero_head

Hello,

Could you post a photo of the 'inconsistent' meshing?

February 26, 2021, 05:19	Residuals not converging, but staying stable and fluctuating	#1
ashf9 New Member Ash Join Date: Feb 2021 Posts: 20 Rep Power: 5	Hi, I am doing a hypersonic flow simulation on re-entry spacecraft, and for some reason, my residuals don't converge but rather stay stable for a while and then fluctuate (attached is a picture of it). I understand that the fluctuations could be decreased using relaxation factors, but could someone explain to me why the continuity and other variables aren't converging, as I would expect some sort of convergence after Would it be something to do with the mesh or initial BCS Thanks Residual 1.JPG Residual 2.JPG Residual 3.JPG

February 26, 2021, 07:26		#2
aerosayan Senior Member Sayan Bhattacharjee Join Date: Mar 2020 Posts: 495 Rep Power: 8	Do you have a fluctuating von-karman vortex street in the wake region? Transient phenomenons cause fluctuations in residuals. You can run the simulations more 10000 iterations here, and see if the residuals converge or keep fluctuating. If they keep fluctuating but don't diverge, the solution might be okay. momahadi30 likes this.

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March 3, 2021, 03:48		#3
ashf9 New Member Ash Join Date: Feb 2021 Posts: 20 Rep Power: 5	Ah I see thank you. I do notice Von-Karman Vortex sheets, and the results do look quite similar to the expected values. I have noticed though that my meshing is quite inconsistent, on of the blocks in a multi block grid. Do you think thatwould also have an effect?