# chtMultiRegionFoam boundary condition

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March 14, 2014, 05:28
chtMultiRegionFoam boundary condition
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I wonder, why the boundary condition for chtMultiRegionFoam is implemented as mixed and not as fixedValue.

How the boundary "turbulentTemperatureCoupledBaffleMixed" works, is explained here chtmultiregionFoam Boundary condition

For explanation see picture. Temperatur (T) has to be equal at the boundary as well as the normal heat flow (q). Here, they combine the two boundary conditions into one. Tw is wall Temperature, T1 internal field of phase one T2 of phase two. Delta is the distance cell center to face center, n normal vector.

So basically we have a wall temperature given, fullfilling both boundary conditions. I would base this on fixedValue, but it does not work (does not converge).
In cht they base it on mixed with
Tw = refValue*valueFraction + (1-valueFraction)*(Tinternal + refGradient*delta).
If you use the values provided in turbulentTemperatureCoupledBaffleMixed, you get exactly the formula, but why so complicated? Any hints?

Code:
```    // Both sides agree on
// - temperature : (myKDelta*fld + nbrKDelta*nbrFld)/(myKDelta+nbrKDelta)
// - gradient    : (temperature-fld)*delta
// We've got a degree of freedom in how to implement this in a mixed bc.
// (what gradient, what fixedValue and mixing coefficient)
// Two reasonable choices:
// 1. specify above temperature on one side (preferentially the high side)
//    and above gradient on the other. So this will switch between pure
//    fixedvalue and pure fixedgradient
// 2. specify gradient and temperature such that the equations are the
//    same on both sides. This leads to the choice of
//    - refGradient = zero gradient
//    - refValue = neighbour value
//    - mixFraction = nbrKDelta / (nbrKDelta + myKDelta())```
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