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steven May 8, 2006 15:10

how to make Robbins condition(additional variable)
hi, there

I am running some simulations with additional variable (AV). Apparently, we can set flux and value boundary conditions for AV. But how can I set Robbins condition, i.e.,

A*dC/dY+B*C=0 where A and B are constant, C is the AV concentration, and Y is the direction normal to the boundary. So Robbins condition is like a mixture of value and flux.

Also, what is the "transfer coefficient" for under AV boundary condition selection? I couldn't find explaination in manual.

Thanks a lot

opaque May 8, 2006 15:50

Re: how to make Robbins condition(additional varia
Dear Steven,

First, rewrite the general Robbins condition as

A*dC/dY + B*C = D

Then, write as CFX does

- dC/dY = B/A * C - D/A

one more step

- dC/dY = B/A * (C - D/B)

Now you got the Flux Transfer Coefficient form, where

Flux Transfer Coefficient = B/A

Additional Variable Value = D/B

You should be done, assuming I did not make any mistake with the signs..

Good luck,


steven May 10, 2006 10:14

Re: how to make Robbins condition(additional varia
thank you very much. That is what I need. But would you please tell me where in CFX manul I can find the detail theory and description about this transfer coefficient? or refer me related papers?

I want to know the theory from the starting point.

Thanks a lot!


opaque May 10, 2006 12:15

Re: how to make Robbins condition(additional varia
Dear Steven,

The boundary conditions for Additional Variables are very similar in nature to the heat transfer case.. See the sections

ANSYS CFX-Solver, Release 10.0: Theory | Basic Solver Capability Theory | Boundary Conditions

What kind of theory are looking for? The concept of the Transfer coefficient comes from the idea of a heat transfer coefficient boundary condition

Heat Flux = H * ( T_wall - T_infinity)


- k_wall * dT/dy = H * (T_wall - T_infinity)

That is a Robbins condition.. I just regrouped yours to show that they are the same..

Good luck,


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