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Old   September 20, 2012, 19:22
Default convective bc based on mixed
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mahdiiowa is an unknown quantity at this point
Thanks for the comment.

Is the implementation of convective bc by mixed type in the following way?

refValue = Tinf
refGradient = 0
valueFraction = 1/(1+k/(h*mag(delta())))
value = ???
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Old   September 20, 2012, 19:25
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You can find the description here, just read more careful
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Old   September 21, 2012, 19:03
Default Problem when formulating the energy equation with enthalpy
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mahdiiowa is an unknown quantity at this point
Hi Foamers
Following the discussions above, I implemented the groovyBC for simple 2D heat conduction problem. The problem is the cooling of plate by imposing a convective bc from one side. My results seem to be reasonable when I formulate the energy equation with temperature, I mean
Density*Cp*(dT/dt) = k(d2T/dx2)
But when I am formulating it vs enthalpy as
Density* (dh/dt) = k(d2T/dx2),
the temperature reach values less than Tinf, which is wrong. Any ideas?
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Old   September 21, 2012, 19:31
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Do you make correction loops for each time step?
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Christian Doppler Laboratory for "Advanced Process Simulation of
Solidification and Melting"

Simulation and Modelling of Metallurgical Processes
Department of Metallurgy
University of Leoben

Franz-Josef-Str. 18
A - 8700 Leoben
Österreich / Austria
Tel.: +43 3842 - 402 - 3125
http://smmp.unileoben.ac.at
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Old   September 21, 2012, 20:06
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makaveli_lcf thanks fro reply

Well this is the main part of the code
fvScalarMatrix TEqn
(
Density*fvm::ddt(h)
- Cond*(fvc::laplacian(T))
);
TEqn.solve();
forAll( T, celli)
{
T[celli] = (h[celli]-(gL.value()*Latent.value()))/(Cp.value());
}
is this what you ask for?
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Old   September 21, 2012, 20:13
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Ok then you should put a coupling loop (like pimple e.g.) around it, because temperature values which you use in the energy equation for the enthalpy are from the previous iteration and do not meat the energy equation after you calculate them from enthalpy.
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Christian Doppler Laboratory for "Advanced Process Simulation of
Solidification and Melting"

Simulation and Modelling of Metallurgical Processes
Department of Metallurgy
University of Leoben

Franz-Josef-Str. 18
A - 8700 Leoben
Österreich / Austria
Tel.: +43 3842 - 402 - 3125
http://smmp.unileoben.ac.at
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Old   September 21, 2012, 20:18
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Do you mean previous time step? My formulation is explicit and I am not concerned about using the previous time step values for T in energy equation. By the way, with the same formulation, I get reasonable results when I use constant temperatures as the boundary condition. So I am almost sure that the problem is in the way I have applied the boundary conditions.
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Old   September 21, 2012, 20:44
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No I mean previus ITERATION:

Step #1: you take T from old time step (at least OpenFOAM does it for you in any case) and use them to solve dh(@_new_time_step)/dt=k*laplacian(T(@_old_time_step))

Step #2: update T from new calculated h values

Step #3: since T does not fullfil dh/dt = k*laplacian(T) calculate h using T from Step #2

a) If you pretend, that your solver is transient, you have to make coupling loops Step #1-#2 so many times, that T will correspond to the energy equation. Low temperature values can be explained (if you speak about some transient point in your calculation) by the fact that your system h-T is not converged for the current time step.

b) If you use transient way to get a steady state solution it is not necessary. But do not try to explain intermediate T values then
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Dr. Alexander VAKHRUSHEV

Christian Doppler Laboratory for "Advanced Process Simulation of
Solidification and Melting"

Simulation and Modelling of Metallurgical Processes
Department of Metallurgy
University of Leoben

Franz-Josef-Str. 18
A - 8700 Leoben
Österreich / Austria
Tel.: +43 3842 - 402 - 3125
http://smmp.unileoben.ac.at

Last edited by makaveli_lcf; September 21, 2012 at 20:48. Reason: Rearanged
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Old   September 21, 2012, 20:48
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mahdiiowa is an unknown quantity at this point
Thanks for your comments, but I still think that there is a problem in implementing boundary conditions and not in the solver, because the solver gives good results with fixed value bc for temperature.
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Old   September 21, 2012, 20:56
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You are trying to compare Dirichlet and Robin boundary conditions. First one is fixing your solution to some values, the second contains gradients and nonlinearity, so can couse oscilations around final solution and needs some iterative process if you treat it explicit way and not implicitly... But Ok, it is your choise how check it...
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Christian Doppler Laboratory for "Advanced Process Simulation of
Solidification and Melting"

Simulation and Modelling of Metallurgical Processes
Department of Metallurgy
University of Leoben

Franz-Josef-Str. 18
A - 8700 Leoben
Österreich / Austria
Tel.: +43 3842 - 402 - 3125
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Old   October 9, 2012, 10:35
Default limit of applicability of the 1st order approx.
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Dear all,

I'm now interested in such a boundary condition (applied to other phenomena). I first must thank you all for your work! I'm sure it will be very useful

I just want to mention that the 1D approach is valid for small enough values of Biot number. And NOTE THAT BIOT=1/C!!!!!

For planar surfaces, typically Bi<0.1 is enough. This means that wall tangential temperature gradients does not alter the heat flux (what in fact just states that the boundary and internal nodes are close enough to consider 1D conduction)

For 1D conduction in planar geometries, with no thermal production and constant thermal conductivity, the temperature difference that appears in Fourier's Law is, definitely, first order.
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Old   January 27, 2013, 21:26
Default changing htot value did not affect the results
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Dear All,
Thanks for the good information provided here. I am solving the heat equation (enthalpy) for a food sphere with phase change. I implemented the groovyBC for my problem and in the first glance it seemed to work fine. But when I tried to change the convective heat transfer coefficient (htot) no change in the results occurred and I got the same results. I have tried all the suggestions in this post except the wallHeatTransfer which did not work for me. Any ideas?
best
SA
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Old   July 4, 2013, 13:17
Default Difference between parameter in groovyBC
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Dear all, Thanks for the useful discussion, but I still did not understand the groovyBC completely.

1. What is the difference between the parameters? valueExpression to set Dirichlet-condition, gradientExpression to set Neumann and what is then fractionExpression? The explanation from http://openfoamwiki.net/index.php/Contrib_groovyBC doesn't help me...

2. Still I did not understand the derivation of:
type groovyBC; variables "htot=1000.0;Tinf=20.0;rho=800.0;cp=385.0;k=DT*rho *cp;"; valueExpression "Tinf"; fractionExpression "1.0/(1.0 + k/(mag(delta())*htot))";

, but it works in my case. So I want to know why


GroovyBC is such a powerful tool, so I finally want to understand the tool in detail. Any hints?

Thanks a lot in advance
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Old   October 6, 2013, 18:14
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This is a life saver!!!
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Old   March 28, 2014, 15:17
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Thank you for the explanation. But, how can I use the 1/deltaCoeff() in the BC file T ??

Many thanks.

Alfonso C
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Old   May 4, 2014, 21:02
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I'm new in openfoam. I'm willing to implement this bc:

k (Tf - Tc)/x + h (Tf - Tinf) = 0

k, Tc, x, Tinf are given values.

I want to find Tf, but h is not constant, h should be calculated in each step based in the temperature gradient (-k dT/dy) / (Tf-Tinf).

Is it possible to be done? How could I implement it?

ps. I would give initial values for Tf and h.
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