# how to make Robbins condition(additional variable)

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 May 8, 2006, 15:10 how to make Robbins condition(additional variable) #1 steven Guest   Posts: n/a 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

 May 8, 2006, 15:50 Re: how to make Robbins condition(additional varia #2 opaque Guest   Posts: n/a 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, Opaque..

 May 10, 2006, 10:14 Re: how to make Robbins condition(additional varia #3 steven Guest   Posts: n/a 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! regards

 May 10, 2006, 12:15 Re: how to make Robbins condition(additional varia #4 opaque Guest   Posts: n/a 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) or - 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, Opaque..

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