How to validate udf for shear stress on a fluid
 

Hi everyone!

I am trying to calculate shear stress on a fluid inside a pipe.
Since this can't be computed by Fluent, I made a UDF that computes shear stress by using viscosity*C_STRAIN_RATE_MAG(c,t).

I managed to get results from my UDF but now I want to check if these results are correct.
In order to do this I created some phantom surfaces inside the pipe thinking that I could compare the WSS by Fluent on these surfaces with the SS of my UDF, but I didn't get any results from Fluent (probably because these surfaces as seen as interior and not wall).
So the only way to validate my results was to do it analytically by calculating the velocity derivative on my phantom surfaces and then compare it with the results from my UDF.

I wanted to ask you if all of this makes sense and if it's the right way to proceed.

Thank you in advance.