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Boundary condition for free convection

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Old   October 16, 2013, 04:40
Default Boundary condition for free convection
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Hi
What are the suitable boundary condition for free convection in starccm+. I am trying to simulate natural convection of a sphere in open atmosphere. so my thought was that freestream conditon all around with very low velocity and boundaries at a very large distanec would work but it dooesnt seem to give physical results. Any suggestions.I have also already triend with adiabatic walls at a large distance.
any suggestions or your experiences with such problems will be appreciated.
Thanks
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Old   June 13, 2017, 11:49
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I know this answer is coming pretty late, but I think it still may be useful for someone looking for it.
You should build a spherical domain around your object, with a diameter 3 to 5 times the major diagonal of the object.
Boundary condition should be 'Pressure Outlet'. As pressure you should probably set Field Function 'Pressure' (not 100% sure here, but for me it worked most of the times).
When I tried to use a bigger domain (say 20 times the major diagonal) the solution couldn't converge.
I already posted some question in the forum about this specific problem.
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Old   June 15, 2017, 14:44
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Quote:
Originally Posted by Cobra View Post
Boundary condition should be 'Pressure Outlet'. As pressure you should probably set Field Function 'Pressure' (not 100% sure here, but for me it worked most of the times).

Don't do this. The field function 'pressure' is solved for. The whole point of a pressure condition is to specify the pressure. You can make your own field function named pressure and use that, but do not use the internal field function pressure. I don't think it makes any sense to do that.
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Old   June 16, 2017, 05:29
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I agree, I was wrong in my previous reply. In the Adapco website I found a guide to set the boundary conditions for open air natural convection

https://thesteveportal.plm.automatio...S/FAQ/RD-5-112

From what I understood the Field Function should be written as:

-1.18*9.81*$$Position[2]*((300/$Temperature)-1)

where 1.18 is representing the reference density.

Right now I'm waiting for a renewal of my licence so I can't test its validity.

Last edited by Cobra; June 18, 2017 at 10:40.
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Old   June 16, 2017, 09:17
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How do you people get access to StevePortal? - Only via purchasing the license?
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I'm a rare guest on the forum. Got questions? - Skype: cwl.net
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Old   June 18, 2017, 22:57
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Remember that function requires you set your z=0 value to be the altitude where the piezometric pressure is the same as the static pressure.
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Old   July 10, 2017, 14:39
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Update: I tried to run some simulations with the new boundary condition for pressure, i.e. -1.18*9.81*$$Position[1]*((300/$Temperature)-1), which I found on the Steve Portal.
Unfortunately I can't make my simulation work with this BC either.
I still get fluid movements driven by pressure differentials at the atmospheric contour.
I tried the condition with both the Boussinesq and the Ideal Gas models.
I don't know what else I could try.
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