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July 2, 2010, 09:59 
p_rgh in VOF solvers in version 1.7

#1 
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I would like to ask you, if any of you knows, what is the explanation of the use of p_rgh in multiphase solvers that use VOF. One would say that it provides a correction which represents the buoyancy, but this is as far as I see it, minor in terms of VOF applications that involve two different phases (e.g. air and water). I put a few points up for discussion and I would really like to hear your ideas about them:
Cheers 

July 3, 2010, 02:35 

#2  
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Alberto Passalacqua
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Quote:
The set of equations solved in 1.6.x and in 1.7.x are formally identical. What changes is how they are treated to obtain a numerical solution. Quote:
This is really basic stuff! ;) You are solving for a momentum equation for a mixture, but the density of this mixture is not constant, as a consequence the term rho*g in the momentum equation contains the density gradients. Quote:
Quote:
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

July 4, 2010, 11:54 

#3  
Member

Quote:
On my first point, I was comparing to Eulerian multiphase cases (e.g. fluidised beds), hence the question. "Inaccurate" wasn't the right expression. I will phrase my question differently: having a big difference in densities, gives big gradients in a single cell. So, does the p_rgh have any effect on the convergence? But this is a case specific problem. I was wondering how other people understand the convergence effect (if any) of rgh in VOF in large density differences. On my last point, in order to get physically accurate results, does one need to "raise" his domain at the height where the actual application is performed in real life? And finally, having a symmetrical domain at h=0, does it have any effect apart from calculating buoyant terms which almost completely counter each other? Thanks 

July 4, 2010, 12:52 

#4  
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Alberto Passalacqua
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Quote:
Quote:
In a bed which tends to the packed condition, you have a competition between the particle pressure, the drag force and the gravity. Solve this balance incorrectly (solving it correctly is harder due to the presence of the particle pressure!), and your code won't be even close to the concepts of accuracy and stability :) Quote:
Think to how the pressure is specified in your system. :) Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

July 4, 2010, 13:12 

#5 
Member

Thank you Alberto, I am getting the idea better now.


July 4, 2010, 13:14 

#6 
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Alberto Passalacqua
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Maybe share why you do not have to raise your domain. Others might find that useful ;)
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. Last edited by alberto; July 4, 2010 at 13:15. Reason: Grammar 

July 4, 2010, 13:50 

#7 
Member

It really depends on what pressure one wants to present. p_rgh can give negative p_rgh values. Somebody might not want to have it like that. That's how I see it.


July 4, 2010, 15:49 

#8 
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Alberto Passalacqua
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You can plot both (the code computes the actual p too), and this has nothing to do with the independence from the "height".
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

June 10, 2012, 14:13 
Hydrostatic Pressure

#9 
Member

Dear Friends
Hi I want to know how I can obtain hydrostatic pressure. The results in a multiphase flow show that p is less than prgh, why? I couldn't understand which parameter I should use in comparison with experimental data. ( In reality we can have the hydrostatic pressure using pizometer) Also, What is the atmospheric pressure in these cases? Thanks in advance 

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