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groovyBC problem with radiation boundary

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Old   April 29, 2013, 06:24
Default groovyBC problem with radiation boundary
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Mohammad Shakil Ahmmed
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Hi,
I am trying to use the following boundary condition with groovyBC:
k*dT/dn = h*(T-Tinf)+(sigma*emissivity)*(T^4-Tinf^4)
where,
k= thermal conductivity=5.91 [W/mK]
h = heat transfer coefficient =12.4 [W/m2K]
Tinf =300K
emissivity=0.775
What happens in my case is that the simulation diverges after some time steps. Does anyone implemented this boundary condition in OpenFOAM? or any idea how to solve this problem???
Thanks in advance.
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Old   April 29, 2013, 11:18
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Quote:
Originally Posted by ahmmedshakil View Post
Hi,
I am trying to use the following boundary condition with groovyBC:
k*dT/dn = h*(T-Tinf)+(sigma*emissivity)*(T^4-Tinf^4)
where,
k= thermal conductivity=5.91 [W/mK]
h = heat transfer coefficient =12.4 [W/m2K]
Tinf =300K
emissivity=0.775
What happens in my case is that the simulation diverges after some time steps. Does anyone implemented this boundary condition in OpenFOAM? or any idea how to solve this problem???
Thanks in advance.
Have you analyzed HOW it diverges? My guess is that it starts oscillating because it over/undershots Tinf. Try smaller timesteps.
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Last edited by gschaider; April 29, 2013 at 14:01. Reason: Missing word
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Old   May 3, 2013, 08:59
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Thanks gschaider,
yes it's because of time steps. When I minimize the time steps the problem solved
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Old   May 4, 2013, 07:12
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Originally Posted by ahmmedshakil View Post
Thanks gschaider,
yes it's because of time steps. When I minimize the time steps the problem solved
OK. You can stabilize it by implementing a mixed boundary with TInf as the Dirichlet-term (that would avoid overshooting). The fractionExpression is to be calculated according to the heat-flux
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Old   August 31, 2013, 07:22
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Originally Posted by gschaider View Post
OK. You can stabilize it by implementing a mixed boundary with TInf as the Dirichlet-term (that would avoid overshooting). The fractionExpression is to be calculated according to the heat-flux
Dear Bernhard,
Can you please explain this a little bit more?

How can I avoid overshooting?
What does fractionExpression has to do with this?

Thank you,
Mojtaba
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Old   September 2, 2013, 18:19
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Dear Bernhard,
Can you please explain this a little bit more?

How can I avoid overshooting?
What does fractionExpression has to do with this?

Thank you,
Mojtaba
fractionExpression (switching between Neuman and Dirichlet is explained on the groovyBC-page)

The thing is that an infinitely fast heat transfer it is basically a Dirichlet condition. For a "very fast" HT it is something in between a Dirichlet and a Neuman. If you implement a pure Neuman you get the overshoots in this case (just sit down with pen an paper, discretize a boundary cell with a big temperatur difference, a big heat transfer coeff and a big timestep and you'll see the overshoot). You can get rid of this by making the timestep very small or by using a mixed conditon
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Old   October 13, 2013, 05:44
Smile how to implement radiation BC...
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Quote:
Originally Posted by ahmmedshakil View Post
Hi,
I am trying to use the following boundary condition with groovyBC:
k*dT/dn = h*(T-Tinf)+(sigma*emissivity)*(T^4-Tinf^4)
where,
k= thermal conductivity=5.91 [W/mK]
h = heat transfer coefficient =12.4 [W/m2K]
Tinf =300K
emissivity=0.775
What happens in my case is that the simulation diverges after some time steps. Does anyone implemented this boundary condition in OpenFOAM? or any idea how to solve this problem???
Thanks in advance.
Hi Mohammad,

Could you, please, help me with the implementation of this boundary condition using groovyBC? I'm new in it and some days ago I was trying to solve a problem where I had to use exactly that BC but I didn't know how to do that using groovyBC. I have been reading and looking for more info about the use of groovyBC but I'm stuck with this problem...

Thanks in advance!
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Old   October 13, 2013, 21:08
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Hi sorry for the late response. For implementing this boundary condition you can do as follows:
type groovyBC;
variables "h=10;k=50;sigma=5.6e-7;E=0.7;Tinf=300;";
gradientExpression "h/k*(T-Tinf)+(sigma/k)*E*(T-Tinf)*(T-Tinf)*(T-Tinf)*(T-Tinf)";
fractionExpression "0";
values are given as dummy. Use your time step smaller. Its works for me.

cheers
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Old   October 14, 2013, 08:28
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Thank you Mohammad!

Now I realised that maybe this is not exactly what I was looking for because in my case the radiation heat flux is known. Therefore, maybe i shoud make some change to adapt your boundary conditions to my case.

Anyway, thank you so much! Your help was very useful to me and it helped me to understand a little how groovyBC works! Although I still need to practise quite a lote to learn the power of groovyBC...

Cheers!
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Old   October 18, 2013, 01:54
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Apropos, you can solve the nonlinear equation k*dT/dn = h*(T-Tinf)+(sigma*emissivity)*(T^4-Tinf^4) concerning T by Newton method (few iterations are enough). And for better stability use a relaxation: Tnew=alpha*T+(1-alpha)*Told where alpha < 1. But it is necessary to rewrite k*dT/dn as k/delta*(T-Tinner). I do not know how to find Tinner exactly but something like that can be helpful: Tinner = Twall-snGrad(T)*mag(delta()).
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