
[Sponsors] 
April 29, 2013, 06:24 
groovyBC problem with radiation boundary

#1 
Member
Mohammad Shakil Ahmmed
Join Date: Oct 2012
Location: AU
Posts: 97
Rep Power: 3 
Hi,
I am trying to use the following boundary condition with groovyBC: k*dT/dn = h*(TTinf)+(sigma*emissivity)*(T^4Tinf^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. 

April 29, 2013, 11:18 

#2  
Assistant Moderator
Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,755
Rep Power: 36 
Quote:
__________________
Note: I don't use "Friend"feature on this forum out of principle. Ah. And by the way: I'm not on Facebook either. So don't be offended if I don't accept your invitation/friend request Last edited by gschaider; April 29, 2013 at 14:01. Reason: Missing word 

May 3, 2013, 08:59 

#3 
Member
Mohammad Shakil Ahmmed
Join Date: Oct 2012
Location: AU
Posts: 97
Rep Power: 3 
Thanks gschaider,
yes it's because of time steps. When I minimize the time steps the problem solved 

May 4, 2013, 07:12 

#4 
Assistant Moderator
Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,755
Rep Power: 36 
OK. You can stabilize it by implementing a mixed boundary with TInf as the Dirichletterm (that would avoid overshooting). The fractionExpression is to be calculated according to the heatflux
__________________
Note: I don't use "Friend"feature on this forum out of principle. Ah. And by the way: I'm not on Facebook either. So don't be offended if I don't accept your invitation/friend request 

August 31, 2013, 07:22 

#5  
Senior Member

Quote:
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
__________________
Complex Heat & Flow Simulation Research Group If you can't explain it simply, you don't understand it well enough. "Richard Feynman" 

September 2, 2013, 18:19 

#6  
Assistant Moderator
Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,755
Rep Power: 36 
Quote:
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
__________________
Note: I don't use "Friend"feature on this forum out of principle. Ah. And by the way: I'm not on Facebook either. So don't be offended if I don't accept your invitation/friend request 

October 13, 2013, 05:44 
how to implement radiation BC...

#7  
Member
Alex
Join Date: Oct 2013
Posts: 54
Rep Power: 2 
Quote:
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!
__________________
I'm newbie in OpenFOAM's world and not an Englishspeaking, so if I make any mistake a correction will be welcome! 

October 13, 2013, 21:08 

#8 
Member
Mohammad Shakil Ahmmed
Join Date: Oct 2012
Location: AU
Posts: 97
Rep Power: 3 
Hi sorry for the late response. For implementing this boundary condition you can do as follows:
type groovyBC; variables "h=10;k=50;sigma=5.6e7;E=0.7;Tinf=300;"; gradientExpression "h/k*(TTinf)+(sigma/k)*E*(TTinf)*(TTinf)*(TTinf)*(TTinf)"; fractionExpression "0"; values are given as dummy. Use your time step smaller. Its works for me. cheers 

October 14, 2013, 08:28 

#9 
Member
Alex
Join Date: Oct 2013
Posts: 54
Rep Power: 2 
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!
__________________
I'm newbie in OpenFOAM's world and not an Englishspeaking, so if I make any mistake a correction will be welcome! 

October 18, 2013, 01:54 

#10 
Member
Alexander
Join Date: Mar 2009
Posts: 36
Rep Power: 7 
Apropos, you can solve the nonlinear equation k*dT/dn = h*(TTinf)+(sigma*emissivity)*(T^4Tinf^4) concerning T by Newton method (few iterations are enough). And for better stability use a relaxation: Tnew=alpha*T+(1alpha)*Told where alpha < 1. But it is necessary to rewrite k*dT/dn as k/delta*(TTinner). I do not know how to find Tinner exactly but something like that can be helpful: Tinner = TwallsnGrad(T)*mag(delta()).


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
natural convection  mehrdadeng  CFX  10  February 25, 2011 05:25 
problem about periodic boundary condition in Fluent  winnawinna  FLUENT  0  December 28, 2010 23:32 
External Radiation Boundary Condition (Two sided wall), Grid Interface  CFD XUE  FLUENT  0  July 8, 2010 06:49 
Radiation problem  mesumon203  FLUENT  0  June 21, 2010 04:31 
Convective Heat Transfer  Heat Exchanger  Mark  CFX  6  November 15, 2004 15:55 