"wall normal velocity" on moving surfaces (FSI)
 

Dear all,

recently, I've discovered some problems calculating the energy transport rate of a moving boundary wall in an FSI-analysis (ANSYS/CFX). The equation I've tried to use is

dE/dt = int_S( v ( -p + 2 mu )( -n) dS

with
v: boundary velocity
p: pressure
mu: shear force
n: normal vector
S: moving surface

My problem is the normal velocity of the surface, which should be calculatable by

normalvelo = areaInt(Wall Normal Velocity)@FSIN

Although I do not get any error messages during preprocessing nor solution, I get an "ERROR. The following unrecognised name was referenced: Wall Normal Velocity" in postprocessing.

The help files are not too helpful this time. Has anybody have a hint for me? Or even a suggestion how I could obtain these values from the structural (ANSYS) side?

Thanks a lot in advance,
best regards,
Andreas

First, I hope you mean areaAve rather than areaInt.
Try creating a monitor point for your expression. If that works, then it just means that variable is not written to the res file by default, hence is not available for post-processing. Under output control you can select extra variables to write to the res file.

Hi Stumpy,

thanks a lot for the fast reply! Do you think that "areaAve" is correct? Since finally I need an integrated value over the surface.

I already put the variables in the Output Control ("Wall Normal Velocity", "Wall Normal Velocity X", "Wall Normal Velocity Y", and "Wall Normal Velocity Z"). Nevertheless for some reason they cannot be taken for post processing.

I've also tried to set a Monitor Point for the WNV but - according to the help files and my try outs - WNV is not available for monitoring. Nevertheless, the help files say that post processing would be a available.

Do you maybe have another idea?
Thanks again and best regards,
Andreas

Try using "Mesh Velocity" instead.
Sorry, I didn't read the equation when suggesting areaAve. It just seemed like areaInt(velocity) was a strange thing to calculate. Still, is the wall velocity the same everywhere? In other words can you move v outside of the integral in your equation? If not, then I think you need to compute v ( -p + 2 mu )( -n) before performing the integral, so you'd need an Additional Variable equal to this part.

Hi Stumpy,

thanks for the hint. "Mesh Velocity" should work, but in this case I loose the normal direction information. I actually do not have a too precise idea how i can calculate ( -p + 2 mu ) in normal direction. The way I now try to go is to write a little script in ADPL to get the normal wall velocity from the ANSYS side.

Best regards,
Andreas

There's a normal vector variable. I think it's Normal X, Normal Y and Normal Z. You could dot the Mesh Velocity vector with the Normal vector.

Great! Thanks a lot!