[Nek5000-users] what exactly is thermal flux and how to set it on different sides?

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

Van,

It is a real flux:    q" = k dT/dn

If it is blowing up it's probably because of the nonlinear response of the Boussinesq
equations to the I.C./B.C. combination that you are using --- you might be able to
cure this with sufficiently small timestep.   For inhomogeneous Neumann conditions
there is no stability issue of the type that's encountered with Robin (i.e., Newton 
law of cooling) BC's, where a sign change can indeed lead to an instability.

2.  I like things like:

if (y.lt.0.5) flux = 1
if (y.gt.0.5) flux=2

as discriminators for different surfaces.

3. The BC types are assigned in the .rea file.   The functionality for each type
is prescribed in the .usr file.   Having extra definitions in the .usr file doesn't matter
if there is no reference to them.

Paul

________________________________________
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: Monday, June 16, 2014 4:55 PM
To: Nek5000 Users
Subject: [Nek5000-users] what exactly is thermal flux and how to set it on      different sides?

1.
if n is a normal direction,
is it that
flux = dTEMP/dn (what I supposed)
or is it a  real thermal flux
flux = k dTEMP/dn with k as conductivity?

A am asking because I dont understand the source of instability in solution when I simulate a stratified flow
between two boundaries with
TEMP=1-z as initial condition
When I set flux on upper and lower boundaries to 0  the program works,
but when I set it to -1 the program blows up in few steps.

2. The next question is how to set differen user defined BC on different boundaries

if you set in .rea files lowcase "f"
for two boundaries it means that the BC is determined by "flux" in userbc subroutine,
but I have different expressions for fluxes for different sides.
I have not found this question described in docs.

3. And sorry for another qiestion:
In benard example there are files like ray_nn.rea ray_dn.rea ray_dd.rea for different combinations of BCs.
But only one .usr file - ray_cr.usr.
I suppose that these BCs must be defined in userbc subroutine of .usr file.
So how does it work?

Van
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