[lowdo]

Hi everyone,

I've recently started using OpenFOAM. I'm setting up a run using the k-omega SST turbulence model, but am having some difficulty understanding setup of boundary conditions at my inlet for k and omega.

For example, in the rotating fan in a room tutorial (https://www.openfoam.com/documentati...ating-fan.html), the inlet BC at the door is set like this:

door
{
type turbulentIntensityKineticEnergyInlet;
intensity 0.05;
value uniform 0.00341;
}

In the incompressible/SRF/simplefoam tutorial, the inlet is set like this:

inlet
{
type fixedValue;
value uniform 0.375;
}

Similarly, for omega, settings are like this for the fan in the room:

door
{
type turbulentMixingLengthFrequencyInlet;
mixingLength 1.2;
value uniform 0.1;
}


Whereas for the incompressible/SRF/simplefoam tutorial, they are like this:

inlet
{
type fixedValue;
value uniform 3.5;
}

Please could someone explain to me what the 'value' represents for the 'turbulentIntensityKineticEnergyInlet'/'turbulentMixingLengthFrequencyInlet' type settings represents (i.e. 0.00341 and 0.1 in these examples)? I assume from the User Guide that they are calculated values of k and omega respectively, but how do these differ to those in the 'fixedValue' type?

[Yann]

Hello Chris,

turbulentIntensityKineticEnergyInlet and turbulentMixingLengthFrequencyInlet are derived boundary conditions and the "value" parameter is just a placeholder you need to define for initialization purpose.

The value will often have no effect because it will be overwritten by the boundary condition anyway. This parameter has to be defined because the boundary condition might be unable to compute an updated value at the first timestep, for instance if the boundary condition relies on a quantity which has not been computed yet. If this happens, the "value" parameter will be used for initialization and it will be updated at the next time step.

This is related to the way the code is written : derived boundary conditions inherit properties from the basic conditions and the "value" parameter usually has to be defined even if it is not used.

This is something which often baffle people starting OpenFOAM so you should be able to find more information about this on the forum.

I hope this helps.
Yann

[lowdo]

Thanks Yann, that's very helpful!