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Zigzagging in the pressure

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Old   November 16, 2010, 19:39
Default Zigzagging in the pressure
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Francisco Cortés
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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.

If someone knows what's wrong (or needs more info to do so) please let me know.

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Old   November 19, 2010, 15:46
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Francisco Cortés
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Finally I solved it. I was using CrankNicholson 1 as time scheme and I changed it for Euler.
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Old   January 25, 2012, 11:28
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is it not possible to calculate with groovyBC and 2nd order time integration?
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parabolic inlet, time dependent velocity, zigzagging

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