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Boussinesq model gives constant density

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Old   February 6, 2008, 07:51
Default Boussinesq model gives constant density
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daol
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Hello,

I'm using the Boussinesq model for density for a natural convection problem with water. I've set the operating temperature and thermal expansion coefficient for water.

The temperature difference is a maximum of 25K but still the density contours are constant throughout the entire domain. However I do get a velocity field and the only driving force I have is buoyancy so there must be some density difference. Why can't I see it in reports or contours?

Regards Ola
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Old   February 13, 2008, 03:30
Default Re: Boussinesq model gives constant density
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S. Gatzka
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The Boussinesq-Approximation is applyed just to the buoyancy-expression in the navier-stokes-equations and not to the general density, which is treated as constant.

I experienced the same problem with a natural convection problem. The density was constant all over the area. But I got the right solution for the velocity-field. Therefore I suppose there is an other problem with your simulation.
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Old   February 21, 2008, 07:51
Default Re: Boussinesq model gives constant density
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Necmi Cevheri
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If you did not use a boundary layer in the vicinity of a wall, you may not observe those density gradients. Try meshing with a boundary layer in gambit
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Old   June 11, 2018, 16:54
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Quote:
Originally Posted by Necmi Cevheri
;148668
If you did not use a boundary layer in the vicinity of a wall, you may not observe those density gradients. Try meshing with a boundary layer in gambit
I have a same problem in natural convection.i've made boundary layer beside the wall as u said but that doesn't help.
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Old   June 12, 2018, 09:28
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Andrea
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The entire point of the Boussinesq approximation is to avoid the complications arising from considering the density as a temperature-dependent property.

In the approximation the density of the fluid is taken as a constant and the buoyancy effects are accounted for in the N-S with the extra term -rho_ref*g*beta*(T-Tref) where rho_ref is the (constant) density of the fluid at the reference temperature, g is the gravitational acceleration, beta is the thermal expansion coefficient and (T-T_ref) is the difference between the local and the reference temperature.

Basically it is assumed that g*(rho(T)-rho(T_ref)) ~ g*beta*rho_ref*(T-T_ref), which allows to drop the dependency of rho on the temperature. This is only reasonable if beta*(T-T_ref) << 1. If this is not the case, than you should drop the Boussinesq approximation and go with a varying density instead.

Therefore I would be surprised to see any density gradient in this case, no matter how refined is your mesh.

Andrea
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