[mostanad]

Hi all,
I am using the meanVelocityForce option through fvOptions in order to constrain the volumetric flow rate in a three-dimensional periodic box. So I am using the following setting my fvOptions file:

momentumSource
{
    type            meanVelocityForce;
    active          yes;
 
    meanVelocityForceCoeffs
    {
        selectionMode   all;
 
        fields          (U);
        Ubar            (0.000001 0.000001 0.000001);
        relaxation      1.0;
    }
}

As you see, I have used a very small value for the desired Ubar since the meanVelocityForce.C cannot detect the flow direction vector by setting zero value. Now I have two questions in this regard:
1- Is it possible to implement the desired Ubar by fvOptions three times instead of just one time? I mean I want to split my Ubar=(0.000001 0.000001 0.000001) to (0.000001 0 0), (0 0.000001 0), and (0 0 0.000001). I want to do so since when I confine the z velocity for a box with PBC in z direction, the result is reasonable. It seems that the
meanVelocityForce has some problem with the velocity directions other than the principal Cartesian coordinates.

2- When I am using meanVelocityForce with "selectionMode all", the mean velocity in the entire domain would be equal to the the desired value? Am I right?
Thank you in advance.
Mo

[HPE]

Hi,

>> meanVelocityForce.C cannot detect the flow direction vector by setting zero value.

- Could you please elaborate what you meant here? I couldn't follow you well.
- And also the first item please?
- Second item, with `selectionMode=all` you select the entire computational domain, and the settings should lead to the bulk velocity to the input value for the whole domain.

[mostanad]

Hi,

>> meanVelocityForce.C cannot detect the flow direction vector by setting zero value.

- Could you please elaborate what you meant here? I couldn't follow you well.

Hi,
Thank you HPE for your response. As you can see in meanVelocityForce.C and its line 92:

flowDir_(Ubar_/mag(Ubar_)),

if I put Ubar equal to a zero vector in fvOptions, it fails to find the Ubar velocity direction. Indeed, the code is trying to correct the velocity and to put a source term into the NSE by this direction.

- And also the first item please?

I have two cases which aim to have zero volumetric flow rate. The first one is a box with PBC just in z-direction and wall BC in the other ones, which I don't have any problem when I put Ubar=(0 0 0.000001) in the fvOptions file. I put a small value since I am not able to put the zero value (due to the flow direction) and also imposing the pressure gradient in this direction for obtaining a very small volumetric flow rate.
On the other hand, I have another case with PBC in all directions. So when I set Ubar=(0.000001 0.000001 0.000001), the result is not what I expect. However, Ubar=(0 0 0.000001) can work very well for this case.
So do you think Ubar=(0 0 0.000001) is still suitable for the case with PBC in all directions and zero volumetric flow rate in the box? I think you should be familiar enough with meanVelocityForce.C to understand my problem.

- Second item, with `selectionMode=all` you select the entire computational domain, and the settings should lead to the bulk velocity to the input value for the whole domain.

Do you mean using all option for selectionMode applies the force term in the entire domain?
Thank you for your help.

[mostanad]

No reply???

[HPE]

Hi,

- The issue I didn't understand is that why do you want to have zero volumetric flow rate for the bulk velocity? This should indicate to be no flow by using the meanVelocityForce. Then what is the driving force of the flow? I am just confused.
- `selection=all` applies the relevant fvOptions to the entire computational domain.

Hope it helps.

[mostanad]

Originally Posted by HPE

Hi,

- The issue I didn't understand is that why do you want to have zero volumetric flow rate for the bulk velocity? This should indicate to be no flow by using the meanVelocityForce. Then what is the driving force of the flow? I am just confused.

Hope it helps.

The problem is related to the settling of a particle in an unbounded box. The driving force is gravity, so the particle is falling down. Due to the unbounded box, the net volumetric flow rate should be zero to mimic that condition.