CFD Online Discussion Forums - Understanding p and p

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Understanding p and p_rgh

I’m having some trouble understanding p and p_rgh. I’ve set up a test case using buoyantPimpleFoam. I have an air inlet blowing cold air into a hot rom. There is also an outlet on the other side. When I look at the results p and p_rgh looks completely opposite to what I would expect. I thought p_rgh would show highest pressure close to the floor with air above. (Just like p is now)

Could anyone help me understand?

P_rgh:
https://ipfs.io/ipfs/QmVw6yjL5rebg9V...GRytDPiBksfjif

p:

https://ipfs.io/ipfs/QmVpiWHu8bxstk1...hYLPtdJoP83JxY

Files:
P:

Code:

  internalField   uniform 101325;

   

  boundaryField

  {

      rom

      {

          type            calculated;

          value           $internalField;

      }

   

      pipes

      {

          type            calculated;

          value           $internalField;

      }

   

      inlet

      {

          type            calculated;

          value           $internalField;

      }

   

      outlet

      {

          type            calculated;

          value           $internalField;

          

      }

  }

P_rgh:

Code:

  nternalField   uniform 101325;

   

  boundaryField

  {

      inlet

      {

          type            fixedFluxPressure;

          gradient        uniform 0;

          value           uniform 101325;

      }

   

      outlet

      {

          type            fixedValue;

          value           uniform 101325;

      }

   

      pipes

      {

          type            fixedFluxPressure;

          gradient        uniform 0;

          value           uniform 101325;

      }

   

      rom

      {

          type            fixedFluxPressure;

          gradient        uniform 0;

          value           uniform 101325;

      }

   

  }

Hi Victor,

You should have a look at Guide: How to ask questions in the forum and How to give enough info to get help, otherwise it is so difficult to find help.

Secondly, the issue of p_rgh and p has been extensively discussed in the forum (e.g. 1, 2). Use Search to navigate through old posts. The answer is out there.

Best,
Petros

Maybe my question was a little thin but you answered perfectly with the two links. Just what I was looking for. Thanks

The following response from Henry gives a (tiny) little more information about his "enhanced buoyancy treatment" and p_rgh field, which is unique to OpenFOAM.

https://bugs.openfoam.org/view.php?id=1170

In a flow with constant density, then p_rgh should remove the vertical variation of pressure with height (like in your test case?) in the background flow. In a compressible gas, the same is approx true for small scales, but is clearly not true for atmospheric dispersion cases where some vertical variation in p_rgh will still occur ... just not as much.