[Nek5000-users] Elsasser subroutines in the MHD solver

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

Hi Jan,

Yes, kinematic dynamo is achieved in the examples/mhd/ test which demonstrates it for Galloway-Proctor flow.

Elsasser formulation is used only for the non-linear (time-explicit) terms and after the time-dicretization, two decoupled Stokes problems are solved for u (ifield=1) and B (ifield=ifldmhd) separately so you should use ifield in forcing of userf accordingly (as in the mhd example)

As far the pseudovacuum BC is concerned, I think it requires the modification of stress formulation to handle the BC coupling in B components which is not yet finished -- let's discuss it off-line.

Best.
Aleks

 


________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Thursday, October 29, 2015 9:23 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Elsasser subroutines in the MHD solver

Hi,
I am simulating a flow in a rotating spheroidal shell (made from a cubed
sphere grid) with a periodical forcing and no-slip boundaries for the
velocity.  Without a magnetic field, this is working great. I recently
tried my hand on dynamo simulations and tried to adapt quasi vaccum (n x
B = 0) boundary conditions for the magnetic field. I have a local
orthogonal coordinate system on the surface, where I set the two
tangential components of B to 0 and use div B = 0 to find a value for
the normal component . This vector is then transformed back to cartesian
coordinates and used in userbc. This is of course a bit ad hoc and needs
further testing.
However, I am not quite sure in how the MHD implementation in Nek5000
actually works and wether it can be used for my problem. As far as I
understand, the Elsässer formulation is mainly used for MHD turbulence.
My main questions for further testing are
1) Is a kinematic dynamo possible with Nek5000? (we already have data
from a finite volume code of kinematic dynamos for comparison)
2) Since the Elsasser variables combine u and B, are the forcings in
userf actually seperate? I used "if (ifield .eq. 1)"  in userf, so that
the forcing only adds to the velocity field, but the B field starts to
grow before u.
As mentioned in the earlier mail, I already looked in the source code,
but couldn't get any clear answers.

Cheers, Jan
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