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-   -   Pressure field not changing (https://www.cfd-online.com/Forums/openfoam-solving/194310-pressure-field-not-changing.html)

Dreoasteh October 13, 2017 05:10
Pressure field not changing
 
1 Attachment(s)
Hi,

I'm currently running a chtMultiRegionSimpleFoam case which I have uploaded in this dropbox link.

The geometry of the problem can be seen in the first image attached. The red region corresponds to the fluid and the blue region is the solid. The solid is hot and cold fluid flows through it, heating up in the process.

My problem is that, although the temperature and velocity fields seem to be ok, the pressure fields (p and p_rgh) do not change and remain at their initial value (i.e. 100 000 Pa).

In case you do not want to download my case (although it only takes 5 minutes) , as the most probable source of error (I think) is the boundary conditions for p and p_rgh in the fluid side, I'll post them here:

p:
Code:
internalField  uniform 100000;

boundaryField
{
    inlet
    {
        type            calculated;
        value          uniform 100000;
    }
    outlet
    {
        type            calculated;
        meanValue      100000;
        value          uniform 100000;
    }
    top
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    bottom
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    right
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    left
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    fluid_to_solid
    {
        type            calculated;
        value          uniform 100000;
    }
}
p_rgh:
Code:
internalField  uniform 100000;

boundaryField
{
    inlet
    {
        type            fixedMean;
        meanValue      100000;
        value          uniform 100000;
    }
    outlet
    {
        type            fixedFluxPressure;
        value          uniform 100000;
    }
    top
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    bottom
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    right
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    left
    {
        type            symmetryPlane;
        value          uniform 100000;
    }
    fluid_to_solid
    {
        type            fixedFluxPressure;
        value          uniform 100000;
    }
}
I have also tried changing the boundary contions above as well as fvSchemes and fvSolution based of the OpenFoam tutorials, the planeWall2D case and the oneFluidOneSolid2D case but I have not managed to solve it.

I would greatly appreciate any help on this topic

Tobi October 13, 2017 05:21
Remarks:

- pressure + velocity at one patch for Ma<0.3 is not recommended
- deltaT 5 does not influence your solution (I hope you know that). You only change the how fast you end at tEnd e.g. you have set 200 iterations with dT = 5, so you actually do only 40 iterations.

If you activate the gravity, you will get the influence. However, the simulation crashes after 2 iterations. I have no time to investigate into that. By the way, a nice geometry. Reminds me to some similar structure I had a few years ago.

Good luck.

Dreoasteh October 13, 2017 05:33
Quote:
pressure + velocity at one patch for Ma<0.3 is not recommended
I do not understand what you mean by this statement.

Quote:
deltaT 5 does not influence your solution (I hope you know that). You only change the how fast you end at tEnd e.g. you have set 200 iterations with dT = 5, so you actually do only 40 iterations.
Yes, I know, I used the controlDict from an unsteady case but was too lazy to change it.

I will look into activating gravity, I hope that puts me on the right path. However, why should it affect the pressure?

Oh and thanks for the props to the geometry :)

Tobi October 13, 2017 05:51
The gravity will affect the pressure because of the equations. We are using the p_rgh quantity to calculate the pressure and recalculate p with that one and the hydrostatic part. The pressure used in the code is (as Ferziger et al. stats) working pressure:

Code:
p_rgh = p - rho*gh
So actually if you set the gravity to zero both are equal. Unfortunately I did not have the time to go through all the stuff in order to know what everything means. I had already some problems with the p_rgh pressure but is related to my limited knowledge about that. However, I would expect the same as you. If you unset the gravity, both pressures get equal but the pressure should change (as a feeling) - e.g. dynamic pressure contribution etc. I guess there are experts in that field who can tell us why this happens.


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