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-   -   p, p_rgh and buoyant pressure (http://www.cfd-online.com/Forums/openfoam/111555-p-p_rgh-buoyant-pressure.html)

Ahmed Khattab January 9, 2013 17:39

p, p_rgh and buoyant pressure
 
Dear foamers,

why we define p, and p_rgh in some cases and don't define only one?

what is the difference in buoyant pressure boundary condition?

thanks in advance.

bioexplore January 11, 2013 10:33

I have the same question, can anyone help to explain these items?

hz283 January 11, 2013 17:05

Hi,

According to my understanding, p is the total pressure, p_rgh is the hydrodyunamic pressure, actually, there should another pressure-therdynamic pressure. For low speed flows or for low ma assumption, p=rho*g*h + thermodynamic pressure. For compressible cases, p=rho*g*h + thermodynamic pressure + p_rgh. In the former case, the pressure appearring in EoS and the pressure appearring in momentum are not the same quantity. If we do not take the boyuance, rho*g*h can be removed.

We can also call the thermodynamic pressure as background pressure.

Ahmed Khattab January 23, 2013 05:16

Hi every body,

we all know that p = p_rgh + rho*g*h. but it this case both p, p_rgh are defined, so

which variable is calculated from the other, we can define only one. the other

question which one of them is used in N.S equations or energy eqn (i noticed that

p_rgh is used). another thing we know that p expresses total pressure and rgh

expresses hydrostatic pressure, so the difference expresses the dynamic pressure

which can be used for calculation velocity which already an input. I'm really confused ??
thanks

Ahmed Khattab January 23, 2013 14:16

Dear Foamers,



please read this abstract from release note of OF v1.7.0

Quote:

Modifications to multiphase and buoyant solvers

  • Multiphase and buoyant flow solvers now solve for http://www.openfoam.org/archive/1.7....img/menu0x.png, rather than the static pressure p. This change is to avoid deficiencies in the handling of the pressure force / buoyant force balance on non-orthogonal and distorted meshes.
  • Improvements to boundary conditions and pressure referencing in closed domains have been developed to avoid the problems encountered in previous attempts to decompose pressure for buoyant flow.



my question is if OPreplaced p by p_rgh why we still define p in input files.

thanks


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