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-   -   Flow around cylinder, forcing term (https://www.cfd-online.com/Forums/openfoam/103231-flow-around-cylinder-forcing-term.html)

 krsp June 14, 2012 08:07

Flow around cylinder, forcing term

Hi everyone,

I am a new FOAM user and I am trying to model an oscillating (sinusoidal) flow around a free circular cylinder in 2D. I have some trouble, trying to add a forcing term to the UEqn. (I use a pimpleFoam solver).

The UEqn looks like this:

================================================== ======
dimensionedVector forcing(
"forcing",
dimensionSet(0,1,-2,0,0,0,0),
(Um1*omega*Foam::cos(omega*runTime.value()) + slope)*forceDir
);

fvVectorMatrix UEqn
(
fvm::ddt(U)
+ fvm::div(phi, U)
+ turbulence->divDevReff(U)
- forcing
);
if (ocorr != nOuterCorr-1)
{
UEqn.relax();
}
if (momentumPredictor)
{
solve(UEqn == -fvc::grad(p)); // forcing is included in UEqn
}
================================================== ======

It all runs fine with no errors, however the velcity accelerates. This means that if Um1 is eg. 0.2 m/s, then in the beginning Ux oscillates between (-0.2 ; 0.2). But after some time it increases/decreases to oscillate between eg. (0 ; 0.4), (0.2 ;0.6), (0.4 ; 0.8) etc.
I'm thinking, that a pressure gradient is developing, but I don't know why..

I have zeroGradient conditions for both velocity and pressure at inlet+outlet. The top and bottom boundaries are both slip.

Does anyone have an idea of, what I have done wrong? (I use OpenFoam 1.6-ext).