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IF statement in Bondary Condition
Posted Jul 5, 2012, 12:31 p.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.3 1 Reply
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Hi friends,
Is it possible to use the conditional statement in a Boundary condition ( Like the wetted wall) ?
I want to define to different contact angles based on the velocity.
1) if (v>=0, Pi/3, Pi/4)
2) Pi/3*(v>=0)+Pi/4*(v<0)
which one is correct ?
If I generally use the velocity (v) in my model, Does the COMSOL automatically used the velocity magnitude related to my boundary condition (on the wall) OR should I first define the velocity on the wall as a separate variable ?
BEST REGARDS,
Is it possible to use the conditional statement in a Boundary condition ( Like the wetted wall) ?
I want to define to different contact angles based on the velocity.
1) if (v>=0, Pi/3, Pi/4)
2) Pi/3*(v>=0)+Pi/4*(v<0)
which one is correct ?
If I generally use the velocity (v) in my model, Does the COMSOL automatically used the velocity magnitude related to my boundary condition (on the wall) OR should I first define the velocity on the wall as a separate variable ?
BEST REGARDS,
1 Reply Last Post Jul 6, 2012, 2:47 a.m. EDT
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Posted:
1 decade ago
Hi
normally both are equivalent, but you must notice that you have a step function there, hence at the switching time/position/value you have a non defined derivative, which will give you some issues or solver ringing when you pass the transition in time, or value in sweep mode. Therefore it's better to use the step(), or ramp() or other built in functions that have also optional smooth transitions that will give a define derivative all along the scan process.
Variables are confusing in COMSOL (I'm from the "old" shool one should distinguish all items clearly;) therefor in COMSOL a variable "v" can be a global scalar variable defined everywhere, a local field variabe v(x,y,z,t...) (and applied to the local dx*dy*dz element of the selected Entities) or a variable that maps DIFFERENT fields on Respective Different ENTITIES.
It's as the confusion often appearing with the notation of derivatives and vector compoennts, Ex is the "x" component of the electric field vector, while Vx is the x derivative of the Voltage scalar, so you the user, must know which applies. Basically it means you nee to know your physics, but some better convention for differenting these could be usefull, nobody is perfect, and anyhow you get used to it
--
Good luck
Ivar
normally both are equivalent, but you must notice that you have a step function there, hence at the switching time/position/value you have a non defined derivative, which will give you some issues or solver ringing when you pass the transition in time, or value in sweep mode. Therefore it's better to use the step(), or ramp() or other built in functions that have also optional smooth transitions that will give a define derivative all along the scan process.
Variables are confusing in COMSOL (I'm from the "old" shool one should distinguish all items clearly;) therefor in COMSOL a variable "v" can be a global scalar variable defined everywhere, a local field variabe v(x,y,z,t...) (and applied to the local dx*dy*dz element of the selected Entities) or a variable that maps DIFFERENT fields on Respective Different ENTITIES.
It's as the confusion often appearing with the notation of derivatives and vector compoennts, Ex is the "x" component of the electric field vector, while Vx is the x derivative of the Voltage scalar, so you the user, must know which applies. Basically it means you nee to know your physics, but some better convention for differenting these could be usefull, nobody is perfect, and anyhow you get used to it
--
Good luck
Ivar