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-   -   potentialFOAM with non-zero pressure @outlet (http://www.cfd-online.com/Forums/openfoam/70068-potentialfoam-non-zero-pressure-outlet.html)

muellea November 13, 2009 09:23

potentialFOAM with non-zero pressure @outlet
 
Hi everyone,

recently i tried to use potentialfoam for getting a better initial-velocity field for further simulations. But i made the a strange, totally unphysical (or at least i think so:)) observation. Setting the value of the pressure at the outlet to let's say 100000 m^2/s^2 (so 100kPa for a fluid with 1 kg/m^3) leads to a incredibly high max. velocity:eek:. I only get good results with p=0 @outlet.

First i thought that sth is wrong with my settings. But then i observed the same effect in the tutorial case pitzDaily. So with:

outlet
{
type fixedValue;
value uniform 0;
}

i obtain a max. velocity of about 27m/s :).

But with:
outlet
{
type fixedValue;
value uniform 100000;
}

the max velocity is 1e6 m/s :mad:.

As far as i know the velocity in a potential flow shouldn't be affected by the reference value of the pressure.

Has sb an idea what's wrong? Is it eventually a bug in potentialFoam (i briefly looked in the code, but everything look nice to me) ? If sb experienced the same effect, is there a better work around than just shifting the pressure to zero (and back..)?

thanks for your answers

darioth September 8, 2010 00:27

Hello. I've tried something similar but I was looking for the U field. Have you used into an airfoil too?
Regards

guy_smiling September 12, 2010 23:34

What are your inlet boundary conditions? From the tutorials I think they're zeroGradient with an outlet boundary condition of 0. If I understand the problem, it's trying to have an initial pressure of 1e6 Pa at one end and 0 Pa at the other (the whole pressure is uniform 0 at t=0), which is understandably hard to solve. Do you know your inlet/outlet boundary conditions well? Can you try putting in those pressures and seeing how that works out? How about trying to have the whole pressure at something closer to 1e6?


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