d(P,x) -- The d-operator is not well defined (?)

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Dear,

I have performed a calculation for a convection problem using COMSOL's General Form Equation.

The problem arises when I want to introduce a Flux/Source type condition. The software gives me this error:

The d-operator is not well defined in the current context, use dtang or mean and d operators instead

I make the change to the indicated function but it keeps giving me an error and I have to change to a Dirichlet type condition, which is not the most correct for this problem. With this condition I try to calculate d(P,x) at a point at the end of the domain and I get the same error. And I can't solve it with the function change that COMSOL indicates.

It should be something easy to solve, in the end it is just a matter of calculating a derivative or imposing a derivative. I attach the file in case you want to take a look at the model.

Could you help me with this?

Thank you all very much.

Best, Andres



4 Replies Last Post Aug 29, 2024, 4:27 p.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 2 weeks ago Aug 28, 2024, 4:45 a.m. EDT
Updated: 2 weeks ago Aug 28, 2024, 8:50 a.m. EDT

Define a variable, say PX as d(P,X). You must define it in a Variable node which has domain selection.

The problem is that you cannot define a gradient in a point. You can define it in a domain, and then evaluate it in a point in that domain.

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Henrik Sönnerlind
COMSOL
Define a variable, say PX as d(P,X). You must define it in a *Variable* node which has domain selection. The problem is that you cannot define a gradient in a point. You can define it in a domain, and then evaluate it in a point in that domain.

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Posted: 2 weeks ago Aug 28, 2024, 4:19 p.m. EDT
Updated: 1 week ago Aug 29, 2024, 3:51 a.m. EDT

Thank you very much for your help Henrik

The gradient issue solved. Anyway, how could I impose a flow-type boundary condition on this problem? A condition of the type -n*Flow=c*u. With this condition the software gives me the same error as when trying to evaluate the function.

Best, Andres

Thank you very much for your help Henrik The gradient issue solved. Anyway, how could I impose a flow-type boundary condition on this problem? A condition of the type -n\*Flow=c\*u. With this condition the software gives me the same error as when trying to evaluate the function. Best, Andres

Henrik Sönnerlind COMSOL Employee

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Posted: 1 week ago Aug 29, 2024, 4:00 a.m. EDT

As I understand it, you want a Robin boundary condition https://en.wikipedia.org/wiki/Robin_boundary_condition.

Such BC:s are common, for example convection in heat transfer, and spring foundation in structural mechanics. The implementation is usually 'Add a Neumann condition where the flux is proportional to the value of the dependent variable'.

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Henrik Sönnerlind
COMSOL
As I understand it, you want a Robin boundary condition . Such BC:s are common, for example convection in heat transfer, and spring foundation in structural mechanics. The implementation is usually 'Add a Neumann condition where the flux is proportional to the value of the dependent variable'.

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Posted: 1 week ago Aug 29, 2024, 4:27 p.m. EDT

Thank you very much Henrik.

Best, Andres

Thank you very much Henrik. Best, Andres

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