[AGP]

I am trying to calculate the shear stress on the fluid on a user defined surface in the interior of the fluid. How can I do this? What is the formula to define a custom field function for this?

Thanks,
AGP

[Entwistle]

Hi;

the shear stress, by its own definition, only can be computed on solid walls.

Regards.

[AGP]

This user-defined surface is a wall. I am using a conformal mesh due to which all interfaces/walls are changed to interior regions in FLUENT.

[swtbkim]

Why don't you use Reports>Surface Integrals ?

[Entwistle]

Originally Posted by AGP

This user-defined surface is a wall. I am using a conformal mesh due to which all interfaces/walls are changed to interior regions in FLUENT.

If these walls are changed into interior type, you will not be able to obtain the wall shear stress. In that case, you need to maintain these walls as walls (you can delete the mesh interface in the proper menu), so fluent will be able to show you the wall shear stress in Contours - Wall Fluxes - Wall Shear Stress.

Regards.

[AGP]

But if I maintain them as walls, there will be no flow through them. This particular wall is the boundary between the fluid and a porous zone and I need to find the shear stresses on it. Can I write a custom field function to calculated the WSS for it?

[swtbkim]

I didn't try it, but how about this?
Make plane there by using sweep surface or Surface>Plane
and calculate wall shear stress about this plane

[Entwistle]

The problem is that the wall shear stress only can be computed on walls. One possible solution is to write an UDF that applies the definition of the wall shear stress, even if your surface is not a wall: You will need the velocity derivatives and the area vector of your surface. From the definition of the viscous stress tensor you can build it and compute the force vector on your surface and its magnitude. It would be something like that:


You can implement it on a DEFINE_AT_END if your problem is transient, or in a DEFINE_ON_DEMAND if it is stationary. You will also need use the Lookup_Thread in order to specify the ID of your surface and one UDM position.

Regards.

[AGP]

I tried doing this in CFD post. I created a new expression to calculate the shear stresses. But when I compared the contours on a real wall - the results were qualitatively the same. But the numbers were way off. I am not sure how to proceed now.

[wxn]

I also want to know how to calculate the shear stress on interior surface of the fluid. Did you find the solution ?

[Mwbadawy]

I have the same issue. Did you find the answer?