Using dependent variable values at specific points on a boundary

[Conjugate Heat Transfer v4.0a]
[2D Stationary study]

I want to use the temperature at two fluid-solid boundaries to generate the boundary condition of heat generation at one boundary. Something like: Flux at boundary 1 = f(T at boundary 1, T at boundary 2). Boundary 1 and 2 have the same x-location, different y-location.

I have tried defining local variables with geometric scope at the boundary (e.g. Tb1 defined as mod1.T, geom scope at Boundary 1) but when it computes for the boundary condition using Tb1 in the expression, it says that it "fails to evaluate variable".

I also want to get a point-by-point value for the boundary condition so I would not prefer integrating/averaging the temperature over each boundary.

In essence, this is a boundary condition dependent on the dependent variable (T) which is also dependent on position. How could I do this in COMSOL?

Thanks!

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Abe