[Nek5000-users] Problem with conjugate heat transfer + Low Machnumber formulation

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

Dear all,

I still have the same problem running the case, no way to solve it. The mass flow rate at the outlet is almost 50% bigger than the mass flow rate at the inlet. I obtain the same results both running it serially and in parallel, therefore I don't think that the problem is in the stiffness summation executed in qthermal (the stiffness summation does nothing when you run the code serially, isn't it?), but maybe in opgrad or opdiv.
I tried to run a similar case, but I put Dirichlet boundary conditions for the temperature at the interface between solid and fluid, which of course does not make sense for a conjugate heat transfer problem. The strange thing is that this case works fine and the mass flow rate at the inlet is the same as at the outlet.
Do you have any suggestion?

Regards,
Andrea.


-----Original Message-----
From: nek5000-users-bounces at lists.mcs.anl.gov on behalf of nek5000-users at lists.mcs.anl.gov
Sent: Mon 5/16/2011 7:21 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Problem with conjugate heat transfer + Low Machnumber formulation
 
Dear all,

I am running a case with conjugate heat transfer + Low Mach number formulation, the domain is a 2D channel. The boundary conditions for the temperature are:
- fluid: constant inlet temperature equal to 1;
- solid: constant temperature equal to 2 at the outer side, at the inner side there is no boundary condition because the energy equation has to be solved, the other sides are insulated.
What I see is that the flow starts to accelerate almost immediately even though at the beginning the temperature of the fluid domain is almost 1. The outlet velocity becomes more than two times the inlet velocity.
The reason is that at the wall (and especially at the inlet corner) the thermal divergence is quite large, while it is almost zero in the rest of the fluid domain. 
In a conjugate heat transfer problem the derivative of the temperature is discontinuos across the wall because of the different thermal conductivities. Could there be a problem when "opgrad" and "opdssum" are evaluated in "subroutine qthermal"? I know that they are evaluated only on the fluid domain but maybe the discontinuity in the derivative of the temperature could give some problems.

Thanks a lot in advance for your help.

Regards,
Andrea
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