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-   -   Zigzagging in the pressure (https://www.cfd-online.com/Forums/openfoam-solving/82103-zigzagging-pressure.html)

 fcortes November 16, 2010 19:39

Zigzagging in the pressure

Hello everyone,
I'm using groovyBC to make a pulsatile velocity with a parabolic profile at the inlet of a tube. The solver I'm using is pisoFoam; turbulence model is a RAS kOmega; fluid is Newtonian.

The problem is that when I plot the pressure over time (anywhere, but especially near the inlet), it oscillates heavily from one time step to the next (zigzagging). In spite of this, the pressure does follow a certain tendency related to the velocity. The pressure at the outlet is set on zero and it's zeroGradient at the other patches.

Inlet //a circular patch in the XY plane
{
type groovyBC;
value (0 0 0);
valueExpression "(time()<0.2) ? fourierSeries1*para : fourierSeries2*para";
variables "yp=pts().y;minY=min(yp);maxY=max(yp);para=(-(maxY-pos().y)*(pos().y-minY)-(maxY-pos().x)*(pos().x-minY))/(0.5*pow(maxY-minY,2))*normal();";
}

The function fourierSeries1 stars on cero and is used as a transition to connect to fourierSeries2, which is a periodic pulse.

This phenomenon also occurs when using timeVaryingFixedValue (for a time series equivalent to the one used with groovyBC but a flat profile) but it does not occur when using a fixedValue (for an average value of the time series) nor with its equivalent on timeVaryingFixedValue. In other words, the phenomenon occurs when the velocity is time dependent.

Francisco

 fcortes November 19, 2010 15:46

Finally I solved it. I was using CrankNicholson 1 as time scheme and I changed it for Euler.

 fisch January 25, 2012 11:28

Hello,

is it not possible to calculate with groovyBC and 2nd order time integration?

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