[sungmin130]

Hi foamers. I am new to openFoam and I am currently trying to illustrate pressure driven flow. The point being is I would like to obtain velocity values based on pressure change between inlet and outlet. I am using simpleFoam solver because I want do it under steady-state, incompressible, laminar flow condition. I have been trying to modify BCs and simulate pressure driven flow but I haven't had any luck. I would really appreciate if someone pointed out what I have done wrong. I attached my boundary condition below. Thank you so much for your time and consideration. Hope I can hear some feedback soon!




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

internalField uniform 0;

boundaryField
{
inlet
{
type fixedValue;
value uniform 102598;
}
outlet
{
type fixedValue;
value uniform 0;
}

wall
{
type zeroGradient;
}
}


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

internalField uniform (0 0 0);

boundaryField
{
inlet
{
type zeroGradient;
}
outlet
{
type zeroGradient;
}
wall
{
type noSlip;
}

}

[t.teschner]

The point being is I would like to obtain velocity values based on pressure change

And where do you want to get those velocity values? I presume Paraview would be the simplest choice here to extract any information around the velocity.

If you want to sample data directly during your simulation, you may want to put some probes into the flow. Maybe this document will get you started: http://www.wolfdynamics.com/training...g_sampling.pdf

[sungmin130]

Originally Posted by t.teschner

And where do you want to get those velocity values? I presume Paraview would be the simplest choice here to extract any information around the velocity.

If you want to sample data directly during your simulation, you may want to put some probes into the flow. Maybe this document will get you started: http://www.wolfdynamics.com/training...g_sampling.pdf

Thanks for the information. I just quite dont understand why my BC does not work. I gotta look into it more while I am waiting for other people point out what I did wrong.

[shock77]

Hi,


I think in your case you need to use the total pressure BC, because otherwise you have an infinit number of possible solutions for problem. I havent run something like that before so the provided informations are without guarantee.

[sungmin130]

Originally Posted by shock77

Hi,


I think in your case you need to use the total pressure BC, because otherwise you have an infinit number of possible solutions for problem. I havent run something like that before so the provided informations are without guarantee.

Thank you for your reply. How would you change my BC differently then? Thank you in advance

[shock77]

Depends on your problem and which physical quantities you know in your problem.



Use totalPressure at one end and fixedValue at the other end. For U pressureInletOutletVelocity with totalPressure and zeroGradient with fixed value. I would try that and see whether it works or not.


Look at your p and u fields to see what is the problem, e. g. the velocity increases without convergence.

[sungmin130]

Originally Posted by shock77

Depends on your problem and which physical quantities you know in your problem.



Use totalPressure at one end and fixedValue at the other end. For U pressureInletOutletVelocity with totalPressure and zeroGradient with fixed value. I would try that and see whether it works or not.


Look at your p and u fields to see what is the problem, e. g. the velocity increases without convergence.

Thanks for the reply again. So, the only thing I know is inlet pressure and outlet pressure. How would you set up the pressureInletOutletVelocity value? Is it just going to be uniform (0,0,0)? The whole point of this simulation is that I would like to see the velocity profile based on pressure change between inlet and outlet.

[shock77]

The velocity value in pressureInletOutletVelocity is just meant for initialization. You can start with 0 0 0. You should try to initialize your entire field with the velocity you are expecting for faster convergence.

[sungmin130]

Originally Posted by shock77

The velocity value in pressureInletOutletVelocity is just meant for initialization. You can start with 0 0 0. You should try to initialize your entire field with the velocity you are expecting for faster convergence.

So I changed to what you said and it did not converge unfortunately. I attached my BC below. Your comments and inputs are much appreciated.


FoamFile
{
version 2.0;
format ascii;
class volVectorField;
object U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField uniform (0 0 0);

boundaryField
{
inlet
{
type pressureInletVelocity;
value uniform (0 0 0);
}

outlet
{
type zeroGradient;
}

wall
{
type fixedValue;
value uniform (0 0 0);
}

}




FoamFile
{
version 2.0;
format ascii;
class volScalarField;
object p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField uniform 0;

boundaryField
{
inlet
{
type totalPressure;
p0 uniform 102010;
}
outlet
{
type fixedValue;
value uniform 101325;
}

wall
{
type zeroGradient;
}
}

[shock77]

To more comments on that:


1. Look at the dimensions of your pressure. Its not Pascal for incompressible problems in openfoam. It is called kinematic pressure and calculated as follows: https://www.openfoam.com/documentati...-pressure.html


So I guess 101325 Pa is wrong and you have to calculate the kinematic pressure by using the density.


2. I suppose you are not using a turbulence model. Therefore I would use the slp BC at the wall since you would need a fine grid to resolve the boundary layer.