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Volume Force

Posted 10 mars 2012, 10:26 UTC−5 Fluid & Heat, Heat Transfer & Phase Change Version 5.0 4 Replies

Jean-Pierre Bedecarrats
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Hello everyone,

I am trying to model free convection in air and water using the Boussinesq approximation.
I use a volume force but I hesitate in the right value of this term.
I have seen a lot of example with two cases : -g_const*(spf.rho-rho_ref) ou only -g_const*spf.rho.
What is the good value ?

Thank you for your advice.

Regards
4 Replies Last Post 9 sept. 2015, 08:04 UTC−4
Ahmed Eissa
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Posted: 1 decade ago 11 mars 2012, 21:17 UTC−4
Hi Jean
I believe it is the second one, (-g_const*spf.rho).
The rho_ref is cancelled with the pressure term in N-S equation. Please see a brief discussion in Bird Transport Phenomena book (Section 10.9)
Good luck
Ahmed
Hi Jean I believe it is the second one, (-g_const*spf.rho). The rho_ref is cancelled with the pressure term in N-S equation. Please see a brief discussion in Bird Transport Phenomena book (Section 10.9) Good luck Ahmed
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 12 mars 2012, 13:47 UTC−4
For most cases both should work, and the second one will also generate a pressure gradient in the direction of gravity as Ahmed mentioned. Also if you don’t have density as a function of temperature, you can use the following more approximate (and sometimes sufficient) force term: g_const*rho*beta*(T-T_ref) where beta is the volumetric thermal expansion coefficient and T is the temperature.

Nagi Elabbasi
Veryst Engineering
For most cases both should work, and the second one will also generate a pressure gradient in the direction of gravity as Ahmed mentioned. Also if you don’t have density as a function of temperature, you can use the following more approximate (and sometimes sufficient) force term: g_const*rho*beta*(T-T_ref) where beta is the volumetric thermal expansion coefficient and T is the temperature. Nagi Elabbasi Veryst Engineering
Jason Lin
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Posted: 9 years ago 7 sept. 2015, 23:08 UTC−4

For most cases both should work, and the second one will also generate a pressure gradient in the direction of gravity as Ahmed mentioned. Also if you don’t have density as a function of temperature, you can use the following more approximate (and sometimes sufficient) force term: g_const*rho*beta*(T-T_ref) where beta is the volumetric thermal expansion coefficient and T is the temperature.

Nagi Elabbasi
Veryst Engineering

Hi, Mr. Elabbasi. I have noticed that you have mentioned something about "density as a function of temperature'. I am interested in this topic. Hence, i have a model which can derive the temperature distribution in a closed boundary condition (no flow in or out, all gas molecules are restricted inside and the amount of molecules are constant). However, since the temperature gradient will result in the motion of molecules and change the density or concentration distribution. If i use the conjugate heat transfer module, how can i model to find out the density or concentration distribution? do i need other module? Thanks
[QUOTE] For most cases both should work, and the second one will also generate a pressure gradient in the direction of gravity as Ahmed mentioned. Also if you don’t have density as a function of temperature, you can use the following more approximate (and sometimes sufficient) force term: g_const*rho*beta*(T-T_ref) where beta is the volumetric thermal expansion coefficient and T is the temperature. Nagi Elabbasi Veryst Engineering [/QUOTE] Hi, Mr. Elabbasi. I have noticed that you have mentioned something about "density as a function of temperature'. I am interested in this topic. Hence, i have a model which can derive the temperature distribution in a closed boundary condition (no flow in or out, all gas molecules are restricted inside and the amount of molecules are constant). However, since the temperature gradient will result in the motion of molecules and change the density or concentration distribution. If i use the conjugate heat transfer module, how can i model to find out the density or concentration distribution? do i need other module? Thanks
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 9 years ago 9 sept. 2015, 08:04 UTC−4
Hi

read carefully the doc for V5 as COMSOL has improved the way of considereing volume changes due to thermal expansion, particulalrly for Thermal models, now you have the different frames to cope for geoemtry change.

Do not forget that the conservation equation is on MASS and NOT on Density =mass/Volume and Volume is formally a "field" V(x,y,z,t) :)

--
Good luck
Ivar
Hi read carefully the doc for V5 as COMSOL has improved the way of considereing volume changes due to thermal expansion, particulalrly for Thermal models, now you have the different frames to cope for geoemtry change. Do not forget that the conservation equation is on MASS and NOT on Density =mass/Volume and Volume is formally a "field" V(x,y,z,t) :) -- Good luck Ivar

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