Discussion Forum

Hi

if you dependent variables have different names, then you must set up your own continuity equations and ensure that the sum of the two fluxes adds to "0"

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Good luck
Ivar

Great, thank you. Is there a way to make the dependent variables the same? I basically have a chemical diffusing first through a porous material, then through another material as a diluted species. Ideally I'd like to let the chemical diffuse through the whole thing on its own, but i can't figure out how to get the 2 physics to connect. How would i set up the continuity equations? Would i need to add the ODE physics then too, or is there a way to enter equations without it?

Great, thank you. Is there a way to make the dependent variables the same? I basically have a chemical diffusing first through a porous material, then through another material as a diluted species. Ideally I'd like to let the chemical diffuse through the whole thing on its own, but i can't figure out how to get the 2 physics to connect. How would i set up the continuity equations? Would i need to add the ODE physics then too, or is there a way to enter equations without it?

Hi Peter,

Also struggling with the same problem, please let me know if u crack it.

Cheers

Hi

making the variable names the same is possible if the geoemtries are fully decoupled and you have two independent systems, else you and up with something like c=2=3 in some locations, mathematically difficult to solve, even for COMSOL ;)

Indeed you need to couple your equations, either by external ODEs or via local BC referring to the other variables, and sort out how to segregate the solver variables, which on efirst and in which order.

COMSOL is great to solve your PDE's but you still need to know what physics and how these mix to set up COMSOL models correctly

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Good luck
Ivar

Hi Ivar,

Thanks for the response. When you talk of decoupling the geometry, does it mean that in the first instance of developing your geometry, it might be wise not to form union too quickly if you know or suspect some continuity in physics? In my instance, i am having a 2D geometry for a microchannel with a catalyst layer coated onto the walls. That means i have 3 domains (gas flow channel, catalyst, and solid wall). There will be heat flux continuity say between the gas channel and catalyst. However on right clicking the heat transfer physics node, i could see that the boundary node for continiuty could not activate to select the interface boundary. Could it be that since i formed union earlier in developing the geometry, Comsol could not decouple the geometry so as to allow to specify this condition?

Hope to hear from you soon,

Cheers,

Steven.