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d(P,x) -- The d-operator is not well defined (?)
Posted 27 ago 2024, 14:15 GMT-4 General, Equation-Based Modeling Version 6.2 4 Replies
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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
Attachments:
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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.
-------------------Henrik Sönnerlind
COMSOL
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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
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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'.
-------------------Henrik Sönnerlind
COMSOL
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Thank you very much Henrik.
Best, Andres