[Nek5000-users] Non-Newtonian flows: nek options

[nek5000-users at lists.mcs.anl.gov]

Hi Neks.

I'm trying to get Nek to work for a non-Newtonian incompressible fluid. I'm considering a Herschel-Bulkley fluid, whose yield-stress behavior I model with a regularized representation (defined as udiff in subroutine uservp).

In this approach, the viscosity gets HUGE (e.g. 1e7) as the strain rate goes to zero.

The two main challenges that I face are (i) an excessive number of Helmholtz-solve iterations and (ii) torque-magnitude calculations that are several orders of magnitude greater than I was expecting.

Below is my understanding of what I can and should do with nek.  Please verify that this is all correct, and please clarify if I have any misunderstanding:

1) I must use the stress formulation:  IFSTRS = T

2) The stress formulation only works with PN/PN-2, not PN-PN, e.g.

 ABORT: Stress formulation in Pn-Pn is not supported

3) The viscous term in the stress formulation is treated implicitly; there's no nu_star option for the stress formulation, where one can treat a portion of the viscous term explicitly.

4) The torque_calc routine should work with this variable viscosity setup.

A few more notes: 

-- I found my velocity system to be difficult to solve, and I had to increase the maximum iterations for the solve:  I increased NMXH from 100 to 1000 (or greater) in drive2.f

-- Per Paul's advice, I've set
  0.000000E-05 p021 DIVERGENCE      <--- set these to zero
  0.000000E-08 p022 HELMHOLTZ       <--- set these to zero
   0.00000     p023 NPSCAL
  0.100000E-01 p024 TOLREL          <--- this is the important one
  0.100000E-01 p025 TOLABS

Does anyone have any additional tips or insights that might be helpful in solving such a system?

Thanks.
--Mike