[Tobi]

Dear OpenFOAMer,

I am trying to apply a very simple BC on a 2D tube test case. This conditions should be able to apply a engine at the outlet. Therefor I have a pressure based velocity.
The pressure profil is shown in the attachment. I applied it with groovyBC which is working very fine. Unfortunately I have problems with the velocity field.

Case setup

• pressure

wall --> zeroGradient
inlet --> fixedValue = 0;
outlet --> groovyBC;


    outlet
    {
        type            groovyBC;
        variables       "f=-15*sin(pi*2/0.03*time());";
        valueExpression "(f-mag(f))/2";
        value           uniform 0;
    }

• velocity

wall --> fixedValue (0 0 0)
inlet --> zeroGradient
outlet --> zeroGradient

If I start this simulation everything is working fine BUT the velocity field is increasing every pressure pulse. After the pressure gradient in the field is zero again, the velocity field still exist or in other words it does not decrease. To solve that problem I can change the viscosity from 1e-5 to 1e-2 and then its okay, but that is not the aim. After the pressure gradient is gone the velocity should stop (maybe with some shock waves).


After that I decided to stop the outflow at the outlet with a groovyBC too.
While pressure gradient is available the outlet is zeroGradient, otherwise it is closed (fixedValue (0 0 0)). The BC for the velocity outlet is :

        type            groovyBC;
        variables       "f=-15*sin(2*pi/0.03*time());";
        valueExpression "vector(0,0,0)";
        gradientExpression "vector(0,0,0)";
        fractionExpression "(f) <= 0 ? 0 : 1";
        value           uniform (0 0 0);

With these settings the outlet is "closed" or better it seems to be closed but the air is still moving forward without stopping (see picture U-1 and U-2).

Can someone give me some hints to build a BC for such a situation?


Thanks in advance and I hope someone has some hints.

Kind regards
Tobi

[Tobi]

Hi all,

topic solved

I used a mixed bc for the pressure at the outlet so that I have gradient 0 when velocity is fixed and fixed pressure when velocity is zero.

[ashishmagar600]

Hello Tobi.

Can you show a short guide on what/how BCs to apply at the inlet/outlet of an engine (expressions for variables)?

Just as a trial case I am using kivaTest tutorial, and I have extracted valve patches. But I don't know what BCs to apply.

Thanks a lot.

[Tobi]

Hi,


depending on your case, you have to apply your appropriate BC. In general, 2T-engines are pressure driven. Thus, I had a pressure based inlet and outlet (groovyBC) while the velocity field was a pressure based inlet-outlet velocity. However, nowadays, I prefer the codedMixed or codedFixed boundary conditions for such purpose.