[tschenkel]

Hello,


I am modelling laminar non-Newtonian flow where the viscosity is dependent on strain and transported scalars. The model works fine, and I can validate it with scalar ratios and velocity profiles.

The purpose of the model is to look into the WSS and that's where my problem lies. The usual wallShearStress objects only work for Newtonian (laminar or turbulent), but not for non-Newtonian flow.

I guess I could set the flow to Newtonian/laminar with viscosity of 1 for the wallShearStress postprocessing tool and then multiply with the local viscosity? How is the velocity gradient at the wall calculated?

I guess what I would need to do is create a surface field for WSS vector (although the postprocessor seems to use a volume vector field?), calculate dV/dn and multiply that with local viscosity.

I looked at the WSS postprocessor code, but that seems to be quite intimately linked with the turbulence models.


Anybody has already done WSS for non-Newtonian?

Thanks a lot.

Best
Torsten

P.S. Using the laminar flow properties to calculate the WSS (simpleFoam -postprocess -func wallShearStress) and multiplying with the local viscosity in paraview seems to give me reasonable results, but it is a bit clunky, so I'd rather work on a "proper" solution.

[tschenkel]

Played around with it a bit and solved the problem.

Modified wallGradU to read the local viscosity field to calculate the wall shear stress.

This is a standalone utility to be run after the simulation is finished.

As the regular wallShearStress utility it calculates the kinematic WSS, in line with OpenFOAM's standard of using kinematic pressure and stress.

EDIT: I found that the normal wallShearStress utility does calculate the WSS for the "standard" nonNewtonian models, but not for models where viscosity depend on transported scalars.


Error message for missing volScalarField:


--> FOAM FATAL ERROR:

request for volScalarField C from objectRegistry region0 failed
available objects of type volScalarField are

0
(
)


From function objectRegistry::lookupObject<Type>(const word&) const
in file /home/torsten/foam/foam-extend-4.0/src/foam/lnInclude/objectRegistryTemplates.C at line 139.

FOAM aborting

[Maveryck]

What do you mean by "but not for models where viscosity depends on transported scalars"? What are those transported scalars you are referring to?

[tschenkel]

Originally Posted by Maveryck

What do you mean by "but not for models where viscosity depends on transported scalars"? What are those transported scalars you are referring to?

I model the migration of particles in shear gradients. So viscosity becomes a function of
strain rate and local particle concentration. The particle concentration is described as a scalar with a non-linear scalar transport equation.


T