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Instability of SimpleFoam

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Old   September 27, 2016, 13:23
Default Instability of SimpleFoam
  #1
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Hey everybody,
I'm new to Openfoam and I'm just about to set up my first simulation. However I'm struggleing with my simplefoam case as it's instable for low viscosity and high pressuredifferences. (For other values I made it runing)

When I run the simulation with these values for p and a viscosity of 153e-7 it is unstable. I'm using k-epsilon-model.
Can someone tell me what the problem is? I'm using timestep 1 sec.

P
dimensions [0 2 -2 0 0 0 0];

internalField uniform 84437.5;

boundaryField
{
wall
{
type zeroGradient;
}

inlet
{
type fixedValue;
value uniform 833333; //10bar
}

outlet
{
type fixedValue;
value uniform 84437.5; //1bar
}

}


FoamFile
{
version 2.0;
format ascii;
class volScalarField;
location "0";
object epsilon; // EPSILON = Turbulent Dissipation Rate recalculate
// for each new case. e=Cu*((k^(3/2))/l)
// Cu=Turbulent Constant=0.09
// k=Turbulent Kinetic Energy l=Turbulent Length Scale
// l can be estimated. l=0.038*dh dh=pipe diameter
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 2 -3 0 0 0 0];

internalField uniform 2.67; // was 0.000765

boundaryField
{
wall
{
type epsilonWallFunction; // think this is correct type BC
value uniform 2.67; //check this value
}
inlet
{
type fixedValue; // think this is correct type BC
value uniform 2.67; //check this value

}
outlet
{
type zeroGradient; // think this is correct type BC
}
}

FoamFile
{
version 2.0;
format ascii;
class volScalarField;
location "0";
object k; //k is the Turbulent Kinetic Energy
// for each new case. k=(3/2)*((U/I)^2) U=Freestream fluid velocity
// I is Turbulence Intensity and can be estimated
// I=0.16*Re Re=Pipe Diameter Reynolds #
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 2 -2 0 0 0 0];

internalField uniform 0.0503; //// was 0.00325

boundaryField
{
wall
{
type kqRWallFunction; // think this is correct
value uniform 0.0503; //check this value
}
inlet
{
type fixedValue; // think this is correct
value uniform 0.0503; //check this value
}
outlet
{
type zeroGradient; // think this is correct
}
}

FoamFile
{
version 2.0;
format ascii;
class volScalarField;
location "0";
object nut; // This is "Turbulent Viscosity"
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 2 -1 0 0 0 0];

internalField uniform 0;

boundaryField
{
wall
{
type nutkWallFunction;
value uniform 0;
}
inlet
{
type calculated; // This is a guess
value uniform 0;
}
outlet
{
type calculated; // This is a guess
value uniform 0;
}
}
FoamFile
{
version 2.0;
format ascii;
class volVectorField;
object U; // This is the fluid velocity at the various boundaries
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 1 -1 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{
outlet
{
type inletOutlet;
inletValue uniform (0 0 0);
value uniform (0 0 0);
}
inlet
{
type zeroGradient;
}
wall
{
type noSlip;
}
}


The timestep conitnuity error is increasing during simulation.

Thanks a lot!
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Old   September 27, 2016, 14:25
Default
  #2
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Mikko
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Quick answer:
Change the pressure inlet boundary condition to
P
inlet
{
type zeroGradient;
}
and define velocity for inlet
U
inlet
{
type fixedValue;
value uniform (10 0 0);
}
and adjust the velocity so that you get the desired pressure difference.
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Old   September 27, 2016, 15:17
Default Thanks
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Thanks for the quick reply. However I would like to calculate the massflow at different pressure-differences! Doing it the other way round would mean a lot of calulationtime. There must be a way to do it my way. Do you recommend another turbelence model?
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Old   September 27, 2016, 16:58
Default
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Then a quick search
link1
link2
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Old   October 2, 2016, 14:01
Default View the residuals when executing ./Allrun
  #5
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Another simple question: How can I view the residuals, timesteps, etc when executing the Allrun script in the OpenFoam tutorials?
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Old   October 15, 2016, 15:47
Default i still didn't manage to solve the problem
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Hey,
I still didn't manage to run the pressure-driven flow. It's not a mesh problem. I ran checkMesh. The simulations is running, however the results are shit. Ridiculous high values. And the time step continuity error is increasing as well.

I ran potentialFoam at first. I added the boundarys for pressuredriven-flow. What else can I do? This is for a cfd programm actually a simple task. I expected more from Openfoam.
I've been trying to run the simulation since 2 weeks.
Please give me some advise.

Thanks!
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