# Mixed boundary condition

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 December 19, 2010, 06:05 Mixed boundary condition #1 New Member   Join Date: Aug 2010 Posts: 14 Rep Power: 8 Hello, For a scalar field T, I want to implement a boundary condition which ensures following: A*T + B*(dT/dn) = C I'm having difficulties understanding how to translate this equation to expressions for valueInternalCoeffs, valueBoundaryCoeffs, gradientInternalCoeffs and gradientBoundaryCoeffs. Any help would be appreciated.

 January 2, 2011, 14:35 #2 New Member   Join Date: Aug 2010 Posts: 14 Rep Power: 8 I can easily understand the fixedValue where: VIC = 0; VBC = value; GIC = -1/delta; GBC = value/delta; with delta being the distance from centrum to face and value being the boundary value. The fixedGradient I can also understand: VIC = 1; VBC = gradient * delta; GIC = 0; GBC = gradient; but I can not see how to derive expressions for an equation where both the gradient and value occur, such as: A*T + B*(dT/dn) = C Can anyone help?

January 11, 2011, 10:36
#3
Senior Member

Ben K
Join Date: Feb 2010
Posts: 140
Rep Power: 11
Are you trying to understand the implementation or are you just trying to implement it?

If you just need to implement it, I think the easiest way is to install groovyBC and then:

Quote:
 rightWall { type groovyBC; variables "Tinf=298;"; valueExpression "Tinf"; fractionExpression "1.0/(1.0 + K/(mag(delta())*h))"; }

or you can take a look at the wallHeatTransfer boundary condition which does the same thing.

This is for a convective heat transfer boundary condition which looks like:

Code:
```K*dT/dx = h(T-Tinf) or
h/K*T - dT/dx = h/K*Tinf```
I really don't understand this implementation though and I'm hoping somebody can take the time to explain this to me too!

 February 17, 2011, 11:35 #4 New Member   Raimonds Vilums Join Date: Oct 2010 Posts: 17 Rep Power: 8 I tried to explain this implementation here. It took me several days of thinking and browsing the forum to understand this implementation. Hope this will help for others to find answers quicker.

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