
[Sponsors] 
Fluid flow through wall U %3d 0 ignored when set pressure gradient for buoyancy 

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February 29, 2008, 17:40 
Hi there,
I added buoyant f

#1 
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Kārlis Repsons
Join Date: Mar 2009
Location: Latvia
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Hi there,
I added buoyant force (1 + alpha*T)*g to an extended icoFoam solver, g = (0 9.81 0), then set BCs for p as pressure gradient such that eq grad(p) = g is satisfied. And what happens is  fluid goes through a boundary where BC U=(0 0 0) is set! Maybe you have an idea why? K. 

March 1, 2008, 01:27 
Karlis,
I'm guessing you'r

#2 
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Michael Jaworski
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Karlis,
I'm guessing you're setting this BC at the bottom of the domain. Have you tried setting it to a fixedValue boundary with a value of rho*g*h where h is your fluid depth? That strikes me as a more direct approach. Regards, Mike J. 

March 1, 2008, 07:33 
the flow you are seeing is bec

#3 
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Hrvoje Jasak
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the flow you are seeing is because of your PRESSURE boundary condition. A nonzero pressure gradient means there must be flow through the boundary because of the pressure Laplacian. You did not account for this in the pressure equation and hence you are getting nonzero flux.
Remember, boundary conditions on p and U are not independent  as I am sure you already know. Enjoy, Hrv
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March 1, 2008, 12:34 
Harvoje,
I wouldn't ask and

#4 
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Kārlis Repsons
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Harvoje,
I wouldn't ask and make such useless experiments if I knew about BC nonindependence etc. Decided to implement gravitational force and temperature dependent density in icoFoam. Looking at the buoyantFoam solver I see some dynamic pressure introduced and only that is used for laplace( p ). Don't understand that stuff yet... > Where to read more about how to build buoyant solver for incompressible fluid? I read your thesis, Harvoje, but I would appreciate if someone point me in the right direction for buoyancy stuff.. > When I finish that solver, I would be happy to share it. Also I haven't found a BC patch type for convective + radiation heat transfer; BC is this: grad(T) =  surfaceNormalVector/k * ( h*(T  Treference) + sigma*epsilon(T^4  Treference^4) ) For sake of good usage constants h, Treference, epsilon must be read from 0/T file. I already begin a thread about this, but there is no reply so far. I wasn't able to access T field defined in solver from separate *.C file which defined class temperatureOpeningFvPatchScalarField : public fixedGradientFvPatchScalarField. Best regards, Kārlis 

March 1, 2008, 13:25 
Harvoje,
I found in an old

#5 
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Kārlis Repsons
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Harvoje,
I found in an old thread ( http://www.cfdonline.com/OpenFOAM_D...ges/1/815.html ), there was boussinesqBuoyantFoam by you, but link doesn't work... Maybe you could post it in again if you consider it useful? K. 

March 1, 2008, 23:53 
Karlis,
I do not believe t

#6 
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Michael Jaworski
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Karlis,
I do not believe the pressure gradient term is correct as a boundary condition in the way you have used it. Namely, the gradient of the pressure is necessary *within* the fluid because of momentum balance. At a physical wall, however, pressure has a specific value. If you write out the equations for a momentum balance of the wall itself, the wallfluid force is the (pressure)*(area), and has nothing to do with the gradient. There already is a buoyant solver in OpenFOAM if I recall correctly. What does it specify for pressures? Regards, Mike J. 

March 2, 2008, 06:42 
Michael,
first about BC: ph

#7 
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Kārlis Repsons
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Michael,
first about BC: physically there is gradient. Is there any reason to say it cannot be implemented in OpenFOAM? Harvoje said: "You did not account for this in the pressure equation and hence you are getting nonzero flux." So, my problem (one of..) appears to be writing correct pressure eq! Second  I don't like specifying pressure because it is an integral quantity. There is static pressure and dynamic, but in case when density changes and flow is present, I'm not sure if it's still that simple. So I decided to use gradient as BC! About buoyantFoam  it is for compressible flow and tries to separate p = pd + rho*gh + pRef; But I currently cannot accept that. How to show that it solves the same NS equation with same conditions as for full pressure p? K. 

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