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integral power boundary condition

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can somebody plese tell me how i can impose a boundary condition on the weakly compressible navier stokes equation that keeps the input power over the boundary fixed to a given value.
i know i read about it somewhere but can't find it now.
i defined a boundary integral coupling variable on the boundary but when i entered it into equation system/boundary settings/ weak constraint the solution comes up with the wrong value for the integrated power.

what is the correct way of entering this kind of boundary condition?

thank you

1 Reply Last Post 18 Jun 2009, 13:27 GMT-4
Mina Sierou COMSOL Employee

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Posted: 10 years ago 18 Jun 2009, 13:27 GMT-4
Hi Robert,

The general approach to specify integral constraints, is to set-up an integration coupling variable (that specifies the integral of the quantity of interest) and then also add a global equation (Physics>Global equations) that introduces a new degree of freedom and its equation specifies the constraint (so that the integral =1 say). Then this new degree of freedom is used somewhere in the model (in the boundary conditions, for example).
You can see such an example (for a flow problem) in the Heat Transfer Module Model Library>Process & Manufacturing>Turbulent Heat Exchanger (where the mass flux is used to calculate the inlet pressure).

Mina Sierou, COMSOL Inc.

Hi Robert, The general approach to specify integral constraints, is to set-up an integration coupling variable (that specifies the integral of the quantity of interest) and then also add a global equation (Physics>Global equations) that introduces a new degree of freedom and its equation specifies the constraint (so that the integral =1 say). Then this new degree of freedom is used somewhere in the model (in the boundary conditions, for example). You can see such an example (for a flow problem) in the Heat Transfer Module Model Library>Process & Manufacturing>Turbulent Heat Exchanger (where the mass flux is used to calculate the inlet pressure). Mina Sierou, COMSOL Inc.

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