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

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

Good point, Mike -- you are right, vdiff(*,*,*,*,1) is overwritten by udiff in uservp of .usr once you set 

      ifuservp = .true.

Have you tried more moderate conditions when viscosity does not get so large?

Thanks,
Aleks


----- Original Message -----
From: nek5000-users at lists.mcs.anl.gov
To: nek5000-users at lists.mcs.anl.gov
Sent: Thursday, September 8, 2011 11:39:06 AM
Subject: Re: [Nek5000-users] Non-Newtonian flows: nek options

Aleks,

Thanks for the response.

In regard to the torque_calc routine and variable viscosity, vdiff is filled with param(2) for the first calculation, but isn't vdiff then defined by udiff (as defined in uservp) for all subsequent steps?

Yes, my current implementation is closely related to the example in var_vis.

--Mike

On Sep 8, 2011, at 10:25 AM, <nek5000-users at lists.mcs.anl.gov> wrote:

> Hi Mike,
> 
> I have not worked with stress formulation yet but I wanted to point out that torque_calc routine in the current form won't work for variable viscosity since vdiff array for viscosity in drqtrq() is filled by constant param(2) -- navier5.f:3586
> 
>      if (istep.lt.1) call cfill(vdiff,param(2),n)
> ...
>                  call drgtrq(dgtq,xm0,ym0,zm0,sij,pm1,vdiff,ifc,ie)
> ...
>      subroutine drgtrq(dgtq,xm0,ym0,zm0,sij,pm1,visc,f,e)
> 
> Have you also checked the demonstration example nek5_svn/examples/var_vis/st2.* ?
> 
> Best,
> Aleks
> 
> 
> 
> 
> 
> ----- Original Message -----
> From: nek5000-users at lists.mcs.anl.gov
> To: nek5000-users at lists.mcs.anl.gov
> Sent: Tuesday, September 6, 2011 12:13:45 PM
> Subject: [Nek5000-users] Non-Newtonian flows: nek options
> 
> 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
> 
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users

_______________________________________________
Nek5000-users mailing list
Nek5000-users at lists.mcs.anl.gov
https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users