Hypersonic flow k-omega SST diverging after 5000+ time steps

quarkz · July 24, 2018, 04:55

Hi,

I am simulating hypersonic double cone external flow k-omega SST which diverges after 5000+ time steps to ~ 1e-6.

It worked when my y+ = 50. Now y+ = 1.

May I know what's the likely problem?

I tried a few things:

1. change from vanleer to upwind in the interpolation scheme - stability greatly improves but still diverges around 0.005. Not sure why since from the contour plots it seems to have reached steady state. However, being a 1st order mtd, the wall heat transfer does not compare well with expt.

2. changing from bounded Gauss limitedLinear to bounded Gauss upwind - slight improvement.

3. using wall function for k, nut - slight improvement but I shouldn't be using any wall function, is that so? Since y+ = 1.

4. changing from smoothSolver to PCG or BICCG - slight improvement

I also have another qn. For omega, should I use omegaWallFunction or fixedValue? I saw somewhere in the forum that I must always use omegaWallFunction. Is that so?

I hope to use a 2nd order interpolation scheme. I'm thinking of Gamma. Is there other recommendations?

Thanks

hyFoam · July 29, 2018, 05:44

Hi,

- Is your temperature field bounded between [T_min; T_max]?

- If you run the exact same simulation but with a laminar setup, does it crash as well?

Thanks

quarkz · July 31, 2018, 03:00

Hi hyFoam,

If I use the standard rhoCentralFoam, I can only reach 1.74279e-05, with initial dt = 1.e-10. Now I managed to get it to run longer, maybe up to 2.e-3 by changing the initial internal velocity to half of the inlet velocity, and also doubling the grid in the y direction.

For hy2Foam, I turned off most setting because the enthaply is not that high. I do get temperature out of range:

^[[1;31mAttempt to use rho2ReactionThermo out of temperature range 838 times during this iteration.
^[[1;31m Thigh: 10000 < 6e+06^[[0m

but I managed to run to 2e-4. Similarly, doing the 2 changes mentioned above I managed to run longer, similar to the above.

I am not sure why it still diverges after 2e-3, because contour plots show that it seems to have stablized.

I didn't run the laminar case because previously it gave very unsteady and incorrect results.

Thanks anyway.

July 24, 2018, 04:55	Hypersonic flow k-omega SST diverging after 5000+ time steps	#1
quarkz Senior Member TWB Join Date: Mar 2009 Posts: 400 Rep Power: 19	Hi, I am simulating hypersonic double cone external flow k-omega SST which diverges after 5000+ time steps to ~ 1e-6. It worked when my y+ = 50. Now y+ = 1. May I know what's the likely problem? I tried a few things: 1. change from vanleer to upwind in the interpolation scheme - stability greatly improves but still diverges around 0.005. Not sure why since from the contour plots it seems to have reached steady state. However, being a 1st order mtd, the wall heat transfer does not compare well with expt. 2. changing from bounded Gauss limitedLinear to bounded Gauss upwind - slight improvement. 3. using wall function for k, nut - slight improvement but I shouldn't be using any wall function, is that so? Since y+ = 1. 4. changing from smoothSolver to PCG or BICCG - slight improvement I also have another qn. For omega, should I use omegaWallFunction or fixedValue? I saw somewhere in the forum that I must always use omegaWallFunction. Is that so? I hope to use a 2nd order interpolation scheme. I'm thinking of Gamma. Is there other recommendations? Thanks

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July 29, 2018, 05:44		#2
hyFoam Member Vince Join Date: Mar 2017 Posts: 45 Rep Power: 9	Hi, - Is your temperature field bounded between [T_min; T_max]? - If you run the exact same simulation but with a laminar setup, does it crash as well? Thanks

July 31, 2018, 03:00		#3
quarkz Senior Member TWB Join Date: Mar 2009 Posts: 400 Rep Power: 19	Hi hyFoam, If I use the standard rhoCentralFoam, I can only reach 1.74279e-05, with initial dt = 1.e-10. Now I managed to get it to run longer, maybe up to 2.e-3 by changing the initial internal velocity to half of the inlet velocity, and also doubling the grid in the y direction. For hy2Foam, I turned off most setting because the enthaply is not that high. I do get temperature out of range: ^[[1;31mAttempt to use rho2ReactionThermo out of temperature range 838 times during this iteration. ^[[1;31m Thigh: 10000 < 6e+06^[[0m but I managed to run to 2e-4. Similarly, doing the 2 changes mentioned above I managed to run longer, similar to the above. I am not sure why it still diverges after 2e-3, because contour plots show that it seems to have stablized. I didn't run the laminar case because previously it gave very unsteady and incorrect results. Thanks anyway.