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thermal analysis with buoyantBoussinesqSimpleFoam

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Old   May 27, 2010, 05:29
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Eugene de Villiers
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Dear Valeria,

The solvers are applicable to any case where the Boussinesq approximation can be justified, they just do not work very well on poor meshes. So if you have a perfect block structured mesh, then the solvers work very well (in our experience), but if you have tets, polyhedra or deformed hexes, then the results are completely unphysical. It might just be that we did not use the right combination of differencing schemes, but believe me we tried many.

Also, always use 2nd order or near 2nd order schemes like linearUpwind if you are trying to match experiment - you will be disappointed otherwise.
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Old   May 29, 2010, 01:37
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Andrea Pasquali
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Hi Eugene,
so what do you think, buoyantBoussinesq is good to study cool down or warm up into automotive cockpit?

Thank you

Andrea
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Old   June 1, 2010, 05:54
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Eugene de Villiers
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Hi Andrea,

In general, for the default solver implemented in 1.6.x, no. It is highly unlikely that you will be able to make a mesh for an automotive cockpit such that spurious numerical errors from the solver discretisation errors will not overwhelm the balance of buoyant and hydrostatic forces.

The most viable solution we were able to come up with was to reimplement the Boussinesq solvers to more closely match the Boussinesq approximation, i.e.

rhoPrime = -beta(T-Tref)

and then to solve for pPrime instead of p. This formulation more closely matches the "pd" formulation for the compressible buoyant solvers in 1.5.
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Old   June 7, 2010, 09:29
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Andrea Pasquali
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Hi Eugene,
the problem is just for the Boussinesq approximation, or else for the buoyant solver?
Could I try with buoyantSimpleFoam?

Thanks

Andrea
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Old   June 7, 2010, 10:30
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Eugene de Villiers
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Hi Andrea,

The problem is that grad(x) where x is a linearly varying field does not return a constant value field on distorted meshes. When you have a relatively high gradient, such as that induced by the hydrostatic force (rho*g), then trying to balance it with grad(p) fails and produces large spurious momentum sources on poor mesh elements.

buoyantSimpleFoam in 1.6 uses the same kind of formulation as the Boussinesq solvers (balancing the pressure gradient against the hydrostatic component). As such I would expect it to have the same problems (I haven't tested it for this purpose myself though, so I cant be sure). buoyantSimpleFoam in 1.5 on the other hand uses the old formulation that splits the hydrostatic component of pressure, so does not suffer from this problem.

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Old   June 7, 2010, 11:23
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Andrea Pasquali
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Hi Eugene,

Thank you very much for your time and for you explanation!

I will try a comparison between OF 1.5 and 1.6.

Another question, and if I put g (0 0 0) what will it happen?

Thanks

Andrea
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Old   June 9, 2010, 11:34
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Eugene de Villiers
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If g is zero, the buoyancy force will be zero too. The issue with poor meshes should go away, since there will be no hydrostatic pressure gradient and you will be left with something like simpleFoam + uncoupled temperature solution.

Eugene
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Old   May 15, 2013, 13:15
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Mihai Pruna
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why is buoyantPressure used in the p field and not in p_rgh like the tutorials show?
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Old   May 27, 2013, 11:12
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Stefan Gracik
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Quote:
Originally Posted by mihaipruna View Post
why is buoyantPressure used in the p field and not in p_rgh like the tutorials show?
I'm wondering the same thing, when I try to calculate it that way, I get an error. When I choose p as the "calculated" boundary condition and p_rgh as the buoyant pressure boundary condition, the solver runs, but I haven't been able to get p_rgh to converge.
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Old   May 28, 2013, 10:36
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Mihai Pruna
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I figured it out, p_rgh is actuall p minus rgh so in your BCs p is the buoyant component and p_rgh the rest (mainly dynamic)
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Old   May 1, 2015, 04:46
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Hi All,

I am using buoyantBoussinesqSimpleFoam in order to simulate heat transfer in horizontal pipe which means my g=(0 0 0), but still I am getting minus pressure at outlet,
what does it mean? I have some wrong BC or it is normal?

Thanks in advance!

Kanarya
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