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Old   October 15, 2008, 19:11
Default buoyancy
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tom
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Hello all,

I am simulating the ventilation of empty room which has a heated floor, and I am funding this extremely hard to converge to steady-state with Star-CCM.

In this simulation I have one small velocity inlet and pressure outlet, the floor is heated to 335K and I am treating air as an ideal gas. I am sure that steady-state exists for this simple flow regime, and I have tried reducing Rayleigh number, and in steady mode, running in unsteady mode but I still cannot achieve it, even with different mesh models and densities. Has anyone any experience with using Star-ccm for this type of flow? I can give further description of the model if necessary

Thanks for any help
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Old   October 16, 2008, 07:11
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steve
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Hi Tom

Perhaps you can try with the coupled solver, but start off with a very small CFL number (~1e-6) so that the solution doesnt kick too much after initialisation. After some iterations ramp the CFL by an an order or two of magnitude and repeat until you reach a sensible CFL. The trick here is to have a plot of the velocity vector field to make sure the solution is not going out of control.

Steve
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Old   October 16, 2008, 08:04
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Tom
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Thanks Steve,

I'll try that out and let you know how I get on. However, I don't know what you mean by "going out of control"; Do you mean diverging, because the solution doesn't diverge it just stalls. Thats why I went to transient mode, and initialised from the half-baked steady solution. Never could get steady-state this way, even though I have kept my calculations as dispersive as possible [first order convection, std. k-e etc. high y+ (y+ valu's are reasonably good)].

Although maybe you mean something else that I should be looking out for in the initial stages.

The reason that I believe that steady-state exists is that I have read a paper in which fluent was used to solve this configuration to steady-state. Besides, there is no possibility for vortex shedding etc., as it is just an empty room with simple inlet and outlets on seperate walls. I have ensured that mesh is dense, and that my turbulence inlet conditions are accurate, so could it just be the Star-ccm+ code?

Tom
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Old   October 19, 2008, 09:27
Default Re: buoyancy
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Tom
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Thanks Steve,

I got it to converge with your suggestion; only using first order convection though and with a courant number of 30. I can't get the same result for 2nd order convection, and lower courant numbers doesn't seem to help. Do you think that this could be due to the mesh? I am using polyhedrals, with no volumetric sources as mesh quality is poor in them. Also, I can only refine the surface mesh up to a certain base size, as over this the mesh gets very patchy. Of course, I am starting from the 1st order solution. If you have any other advice I would be grateful. Thanks again, Tom
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