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olhe0002 April 15, 2010 12:12

flowRateInletVelocity compressible
Hello world!

I have a question about the unit of the inlet boundary flowRateInletVelocity.

When using an incompressible solver like simpleFoam, the flowRate is given in [ m/s ].

Which unit is given when using a compressible solver, for example buoyantPisoFoam?
Is is it still [m/s] or then [kg/s] ?

Best Regards

CedricVH April 15, 2010 12:44

Your question is answered in the following part of flowRateInletVelocityFvPatchVectorField.C:


00129    if (phi.dimensions() == dimVelocity*dimArea)
00130    {
00131        // volumetric flow-rate
00132        operator==(n*avgU);
00133    }
00134    else if (phi.dimensions() == dimDensity*dimVelocity*dimArea)
00135    {
00136        const fvPatchField<scalar>& rhop =
00137            patch().lookupPatchField<volScalarField, scalar>(rhoName_);
00139        // mass flow-rate
00140        operator==(n*avgU/rhop);
00141    }

If the dimension of phi is in m^3/s, the value of flowRateInletVelocity will be a volumetric flow rate in m^3/s. If the dimension of phi is in kg/s, the value of flowRateInletVelocity will be a mass flow rate in kg/s. I think this is indeed the difference between an incompressible and a compressible solver, but I'm not sure.

CedricVH April 16, 2010 03:56

I was indeed correct:
Incompressible solver: phi is in m^3/s --> flowRateInletVelocity is in m^3/s
Compressible solver: phi is in kg/s --> flowRateInletVelocity is in kg/s

nileshjrane August 27, 2010 05:31

I didnt get this concept. How i suppose to give the mass flow rate value, for the same case, when i am solving with incompressible solver and with compresible solver??? Say i have a mass flow rate of 5kg/s through an inlet. can anyone elaborate please???

CedricVH August 27, 2010 05:42

Assuming that you use a fluid with the density of water (rho = 1000 kg/m) and you want to achieve a mass flow rate of 5kg/s, you will have to fill in this value for the flowRate:
  1. Incompressible solver: flowRate 0.005 (= massflow/rho = (kg/s)/(kg/m) = m/s)
  2. Compressible solver: flowRate 5 (= massflow = kg/s)

nileshjrane August 27, 2010 05:56


Thanks Cedric for the quick reply..Have a good day..:)

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