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-   -   CFX converge problem caused by shock waves (http://www.cfd-online.com/Forums/cfx/67263-cfx-converge-problem-caused-shock-waves.html)

littlelz August 10, 2009 05:41

CFX converge problem caused by shock waves
 
dear all,



I have got a serious convergence problem in CFX12 transition simulation.

I am doing the transition simulation on a isolated nacelle using CFX12. The mesh and the CFD model setup are ok. We can get successful results for all the test cases with various operating conditions except the cases with shock waves (in our cases, freestream mach number reaches 0.88). I have done refining the mesh on both nacelle surface and normal to it, but no success. I also follow the suggestion as in this forum (http://www.cfd-online.com/Forums/cfx...r-3d-wing.html), set max continuity loops = 2 with high res solver, however, no success at all. The general trend is no matter what I did, the simulation will converge slowly first, then diverge totally. (I monitor some critical parameters, like Cp, Cf, they canít be converged enough).

I guess it is a converge problem caused by shock waves. I really appreciate any of your help and advice.



regards,


littlelz

ghorrocks August 10, 2009 06:22

Hi,

Is it steady state? Assuming it is:

There are some general tips here http://www.cfd-online.com/Wiki/Ansys...gence_criteria

But in this case I would recommend concentrating on using local timescale factor. Also have you tried a solution which is subsonic, converging the solution, then slowly increasing the mach number to the desired figure? This can also help.

If that does not work then I would consider doing a full transient simulation. They tend to be the last resort when nothing else works. Slow, but the most reliable convergence.

Glenn Horrocks

littlelz August 10, 2009 17:47

many thanks, Glenn

always get your help and advice, thank you very much.

the tips in your link I have tried before because there is separation flow in our nacelle case, the flow can't be converged very well in steady state simulation. however, i am still using steady state simulation because we found the convergence problem caused by separation is just a local probloem, which doesn't influence the overall result. as I monitor the Cl and Cd, they converge well.

however, this time the converge problem is caused by shock wave. we have tried M=0.8, M=0.82 they are converging very well. as M reaches 0.88, it can't converge at all. so I am wondering if there is any special tips for shock wave converge problem

anyway, I will try your advice one by one.

many thanks again


littlelz

Timon August 17, 2009 09:35

Besides using local timestepping, try enabling "high speed numerics" in compressibility control.


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