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-   -   High negative pressure with interPhaseChangeFoam (http://www.cfd-online.com/Forums/openfoam-solving/112134-high-negative-pressure-interphasechangefoam.html)

 legione January 22, 2013 15:17

High negative pressure with interPhaseChangeFoam

Hi Foamers,

I'm new in the OpenFoam world and I'm trying to study the cavitation phenomenon in a 3D simple, and very small, cylindrical duct (diameter 0.24 mm) that links two cubic capacities (one cube edge is long 6-7 mm more or less) .
The fluid is the same of Throttle Tutorial in CavitatingFoam Solver and the geometry system is similar (the substantial difference is that my case is 3D).
I'm running with interPhaseChangeFoam, laminar, because this is more accurate that CavitatingFoam, thanks to the implementation of VOF method . I've set pressure inlet (160 bar) and pressure outlet (1.5 bar) BC and wall on the other surfaces. The simulation seems to work fine, however when I try to visualize pressure p_rgh in paraFoam I find very high negative pressure (-60 bar more or less) in the initial circular section of the duct (where begin cavitation) , in the other regions the pressure values are positive. Are these negative values correct? When I achieve these negative pressure values the simulation goes ahead without problem, so I could think that this behavior is normal in a incompressible calculation....hope someone could help me in understand this. Thanks.

 alterfritz January 24, 2013 10:43

High negative pressure with interPhaseChangeFoam

I'm new in the OpenFoam world, too.

But raise up the nOuterCorrectors in fvSolution for more implicit calculations.

 legione January 25, 2013 07:46

Thank you so much alterfritz for your reply. Can you explain me how an increase of outer loop iterations can decrease the negative pressures? I have another theory. In my results I have noticed that the flow separates with a vortex in the negative pressure location. For this reason I could think that what I'm visualizing is not absolute pressure but a pressure gradient deltaP that becames negative because of the flow separation. But, in this case, my question is: what is the reference pressure for the pressure calculations? in the bottom I insert an image that explain better the situation of pressure (I have cut off the positive pressures so you can see only the negative pressures regions.). The flow goes from left to right.
http://www.mediafire.com/view/?6ima64q5597ezcj

Also I attach the p_rgh boundary condition in the 0 folder:

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
location "0";
object p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [1 -1 -2 0 0];

internalField uniform 160e5;

boundaryField
{
inlet
{
type fixedValue;
value uniform 160e5;
U U;
rho rho;
psi none;
gamma 1;

}

outlet
{
type fixedValue;
value uniform 1.5e5;
}

wall
{
}

cyl
{
}

frontBack
{
type empty;
}
}

// ************************************************** *********************** //

 legione January 25, 2013 07:48

sorry but the image seems don't work...try this link http://www.mediafire.com/view/?6ima64q5597ezcj

 abe April 8, 2013 07:24

Hi,

I am using InterPhaseChange solver to simulate cavitation in OF2.1x
Although by old versions (i.e. OF1.5) I can get good results, with this new version results are not good at all. Dose anybody else get similar conflicts?

ABE

 sandy April 11, 2013 02:51

Hi legione, I think, it is wrong if the pressure is negative in the internal field, because staturation pressure is higher so that it and water evaporate early.

 sfigato July 4, 2013 02:35

Hi Foamers,

I am dealing with the same problem! I have a strong negative pressure where cavitation occurs! Legione have you found a solution?
Anyone can help me?

Regards
Marco

 alvariten July 5, 2015 10:28

Hi all,

I know this thread is long time ago.. but, I have the same question and I've not found an explanation to negative pressure in the vortex zone (referred to interPhaseChangeFoam).
Somebody finally got a reasonable explanation?

Moreover, in my case I found that the velocity in the nozzle walls where cavitation occurs is not completely zero (It is of the order of 5-10 m/s). I have taken into consideration the mesh refinement in the wall region, but still not zero velocity. Anyone else has obtained these results?

Thanks.
Regards.

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