[Nek5000-users] Open Boundary condition in NEK user Documentation

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

Thank you,


this makes sense to me.


/Johan


________________________________
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 6, 2016 2:22 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Open Boundary condition in NEK user Documentation


Dear Johan,

Actually - I was mistaken.

What we use is:

     d/dn (u_i) = 0

for i=1,...,3

We could however (and sometimes do) require Neumann only on the normal velocity component --

grad (phi) . nhat = 0

where phi = U . nhat

The tangential components can be prescribed.   That is the capability provided by the "on "
BC.

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: Thursday, October 06, 2016 6:58 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Open Boundary condition in NEK user Documentation


Thank you for a good answer!


Just to clarify, is it in LaTeX written as this:


(\hat{n} \cdot \nabla)(u \cdot \hat{n}) = 0 ?


If the wall normal is in the z-direction, then the open boundary condition for the velocity is:


d/dz u_z = 0?


Best regards,


Johan




________________________________
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 6, 2016 12:40 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Open Boundary condition in NEK user Documentation


Dear Johan,

The outflow boundary condition, 'O  ', is indeed the same as 'I  ' for the thermal problem
since they both enforce grad T . nhat = 0.

However, both also admit thermal flux by advection for cases where U . nhat is > 0, and that is
the intent of outflow --- to allow thermal flux to be carried out.

For velocity, the situation is a bit more delicate.  To leading order, however, 'O   ' imposes
d/dn (U.nhat) = 0 and p=0.    That is, Neumann conditions for the viscous fluxes and Dirichlet
for pressure.

hth,

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: Thursday, October 06, 2016 4:59 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Open Boundary condition in NEK user Documentation


Dear Neks,


I am trying to understand what is actually meant by an "open boundary" in Nek.

Un the user documentation, eqn (4.5) sais that, for the temperature, it is


grad(T) dot n = 0, where n is the normal to the boundary.


Since the heat flux is q = - grad(T), I would rather interpret this as a zero flux boundary condition.


And indeed, the insulated boundary described by equation (4.6) is identical to the open boundary condition.


Is equation (4.5) a type-O or is the open boundary for the temperature actually the same as an insulated boundary?


Also: Should not the open boundary condition for the "no-stress forumlation" also be given by the same expression as for the "stress-formulation" eqn (4.2)?


Best Regards,


Johan
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