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Can Global Equations (ODEs) effect boundary conditions?

Posted 2025年4月17日 GMT+8 04:36 Modeling Tools & Definitions Version 6.3 3 Replies

Sebastian Müller
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Hello!

Can Global Equations (ODEs) effect boundary conditions??

In 1D (time-dependent): I have a (pointwise constraint) boundary condition, which sets the dependent variable V to zero. (It's grounding or short-circuiting, because its an electric context.) But this boundary condition doesn't work anymore. V takes values unequal to 0.

I have to add a second equation in ODE, where the value is unequal to zero or the change dt or dt² must be zero, to make the boundary condition work again. This second equation is not used somewhere, but helps. Kind of workaround.

Is there a more complex relationship between ODEs and boundary conditions (or constraints or constraint forces) in the background, even if the ODE equation and the boundary condition is not direct or not handedly (by hand) coupled?? Has such a background relationship maybe something to do with "Apply reactions terms on: All physics"??

For now it seems like a bug.

Nice Greetings

3 Replies Last Post 2025年4月24日 GMT+8 19:39
Jeff Hiller COMSOL Employee

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Posted: 6 months ago 2025年4月17日 GMT+8 05:22
Updated: 6 months ago 2025年4月17日 GMT+8 05:27

Hello Sebastian,

Consider uploading your .mph files. As always, it's hard to understand what you did without seeing what you did.

Best,

Jeff

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Jeff Hiller
Hello Sebastian, Consider uploading your .mph files. As always, it's hard to understand what you did without seeing what you did. Best, Jeff
Sebastian Müller
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Posted: 6 months ago 2025年4月17日 GMT+8 19:30

I have put the ODE in his own segregated step to avoid any unwanted background coupling. This feels less like a workaround.

Nice greetings

I have put the ODE in his own segregated step to avoid any unwanted background coupling. This feels less like a workaround. Nice greetings
Sebastian Müller
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Posted: 6 months ago 2025年4月24日 GMT+8 19:39
Updated: 6 months ago 2025年4月24日 GMT+8 19:43

Alternatively* helps:

(or-logic)

  • changing constraint force expression from test(V) to test(V)*1.00000001 (of V=0 pointwise boundary)

  • setting Null-space function(nullfun) to (Explicit-)Sparse

  • Using Constant constraint Jacobian

*for getting V=0 pointwise boundary work again

Nice Greetings

Alternatively* helps: (or-logic) - changing constraint force expression from test(V) to test(V)*1.00000001 (of V=0 pointwise boundary) - setting Null-space function(nullfun) to (Explicit-)Sparse - Using Constant constraint Jacobian *for getting V=0 pointwise boundary work again Nice Greetings

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