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temperature negative occurred in rhoCentralFoam at high M

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Old   August 29, 2018, 08:06
Cool temperature negative occurred in rhoCentralFoam at high M
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FENG LM
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Hi,guys

When I use rhocentralFoam to solve flow over wedge or other simple shape( using RAS model), negative temperature occurred easily when M>5, even dt and maxco are set to very small value.
I followed the example in tutorial, it works when computing grid is course. I only refined the grid in boundary layer and the problem just occurred.
Is the slover unsteady at high M?
Who knows how to slove the problem? Please Help
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Old   October 22, 2018, 21:19
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Hi Longming,

I am encountering the same problem. Have you found a solution yet?

Jason
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Old   October 22, 2018, 23:37
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Shujaut H. Bader
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Quote:
Originally Posted by longming View Post
Hi,guys

When I use rhocentralFoam to solve flow over wedge or other simple shape( using RAS model), negative temperature occurred easily when M>5, even dt and maxco are set to very small value.
I followed the example in tutorial, it works when computing grid is course. I only refined the grid in boundary layer and the problem just occurred.
Is the slover unsteady at high M?
Who knows how to slove the problem? Please Help

I've had such kind of issues in passive scalar transport in incompressible solvers though. Usual practice is to try to use a bounded scheme (div and grad) for your energy/TEqn. You can start with upwind which is more stable as it incurs numerical dissipation which stablizes the run, but it could be less accurate. Relaxing the equation can also help in some cases. In my case, the culprits were high-aspect ratio cells away from the boundary layer- When I removed those high aspect cells, my problem was solved.

Here's the link to my post:
TEqn (passive scalar) diverges all of a sudden
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Old   October 23, 2018, 15:40
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Andy S.
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Usually setting deltaT to a very small number (1e-15 for the 1st time step) will fix this, the solver will adjust the timestep to a larger value automatically.
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