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Buoyancy effects for turbulence model

Posted May 9, 2013, 6:46 a.m. EDT Heat Transfer & Phase Change Version 4.3a 5 Replies

Jhon Wei
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In nitf (non-isothermal flow), the way for setting the Buoyancy force is only can set the volume force(-rho*g)?
However in somecase it is not accuracy for compare to the experiment result (the buoyancy effect is low).

I know some CFD software has own defluat Buoyancy model in turbulence model or in laminar model , how about in comsol?

Any comparsion for comsol and experiment for buoyancy has done to know the accuracy?
5 Replies Last Post Aug 1, 2016, 4:15 a.m. EDT
Mattia Montanari
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Posted: 1 decade ago May 9, 2013, 7:29 a.m. EDT
Hello,

what all Finite Element CFD software do is exactly what Comsol does... add a volume force to take into account buoyancy effect. I don't know what you mean with " it is not accuracy for compare to the experiment result", but the idea of adding a volume force term simply comes from FEM mathematical formulation. This is done without any further simplification!

Hope it helps,
Mattia
Hello, what all Finite Element CFD software do is exactly what Comsol does... add a volume force to take into account buoyancy effect. I don't know what you mean with " it is not accuracy for compare to the experiment result", but the idea of adding a volume force term simply comes from FEM mathematical formulation. This is done without any further simplification! Hope it helps, Mattia
Jhon Wei
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Posted: 1 decade ago May 10, 2013, 4:21 a.m. EDT
The question is come from when I see another CFD like Fluent slove the k-epsilon turbulence model,
When I see the transport equation for two variables k and epsilon.(these two equation is same as I saw in Comsol for k and epsilon without buoyancy term).

However when consider buoyancy effect , the transport equation for two variables k and epsilon will have buoyancy term (and it need input one more parameter), the weight for buoyany effect in k transport equation and epsilon transport equation is difference, but I can not found this term in Comsol.
The question is come from when I see another CFD like Fluent slove the k-epsilon turbulence model, When I see the transport equation for two variables k and epsilon.(these two equation is same as I saw in Comsol for k and epsilon without buoyancy term). However when consider buoyancy effect , the transport equation for two variables k and epsilon will have buoyancy term (and it need input one more parameter), the weight for buoyany effect in k transport equation and epsilon transport equation is difference, but I can not found this term in Comsol.
Jhon Wei
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Posted: 1 decade ago May 13, 2013, 9:27 a.m. EDT
anyone can answer my question?
anyone can answer my question?
Mattia Montanari
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Posted: 1 decade ago May 13, 2013, 1:39 p.m. EDT
Turbulence models are needed to close the Navier Stokes equations, in which, once you decompose the velocity, the quadratic term of fluctuating velocity needs to be modeled by means of k and epsilon.

Adding buoyancy effect, as far as I know, has nothing about turbulence theory.. actually "buoyancy" is just a term added to the momentum equation, and this term is (usually) linear in the temperature and it doesn't even include any velocity components!

My suggestion is to read through Fluid Mechanics references ;)

Mattia
Turbulence models are needed to close the Navier Stokes equations, in which, once you decompose the velocity, the quadratic term of fluctuating velocity needs to be modeled by means of k and epsilon. Adding buoyancy effect, as far as I know, has nothing about turbulence theory.. actually "buoyancy" is just a term added to the momentum equation, and this term is (usually) linear in the temperature and it doesn't even include any velocity components! My suggestion is to read through Fluid Mechanics references ;) Mattia
Parth Swaroop
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Posted: 8 years ago Aug 1, 2016, 4:15 a.m. EDT

anyone can answer my question?


Dear John, I'm also facing the same problem. I modelled my volume force using the formula : rho*beta*Delta T * g , where rho is the density , beta is the expansion coefficient , delta T is the temperature difference, g is acceleration due to gravity.

But this seems to work for algebraic yplus ( prandtl mixing length ) model , k-w is unable to model any volume change due to temperature.
I'm attaching the simulation result for lid driven cavity with stratification . The moving wall is kept at higher temperature and the bottom wall is kept at lower temperature. You can find more information here : arc.aiaa.org/doi/pdf/10.2514/6.1992-713 and here ascelibrary.org/doi/abs/10.1061/%28ASCE%290733-9429%281985%29111:2%28334%29

Did you find a solution?


[QUOTE] anyone can answer my question? [/QUOTE] Dear John, I'm also facing the same problem. I modelled my volume force using the formula : rho*beta*Delta T * g , where rho is the density , beta is the expansion coefficient , delta T is the temperature difference, g is acceleration due to gravity. But this seems to work for algebraic yplus ( prandtl mixing length ) model , k-w is unable to model any volume change due to temperature. I'm attaching the simulation result for lid driven cavity with stratification . The moving wall is kept at higher temperature and the bottom wall is kept at lower temperature. You can find more information here : arc.aiaa.org/doi/pdf/10.2514/6.1992-713 and here http://ascelibrary.org/doi/abs/10.1061/%28ASCE%290733-9429%281985%29111:2%28334%29 Did you find a solution?

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