Discussion Forum

There should be no problem in modeling what you describe. For instance, see these models :
- "shell diffusion" model explains what's behind the scene with tnagential derivatives.
- "transport and adsorption" mix a 2D and a 1D approach. This 1D approach in a 2D geometry is not restricted to straight lines, you can solve it on a curved shape. Idem in higher dimensions.
- for fully integrated solutions, the "heating circuit" model integrates shells in a 3D geometry where electric potential, heat transfer and structural mechanics are solved in the shell and fully coupled to the volume.

Note that for a number of physics (including heat transfer) this feature is directly integrated to the boundary conditions, you don't necessary need to add another equation to make the coupling yourself. Some modules might be required depending on the physics you want to solve.

These mixing of dimensions are directly available for heat transfer, structural mechanics, ACDC (potential or electromagnetics), reaction-diffusion, CFD, electrochemistry, modal analysis on boundary in RF or acoustics, surface reactions... I forget some for sure.

The pipe flow module gives extra capabilities 3D/1D or 3D/2D/1D coupling for mixing pipes with volumic flow (and diffusion, heat transfer or acoustics on top of fluid flow).

In the context of COMSOL, this kind of feature is really important as you see this is shared by multiple applications. With the possibility to define "non local" couplings between nD and (n+1)D makes the technique useful for more applications than those listed by COMSOL.

It relates sometimes to a "simple" relation between the unknown or its derivatives in up and down side of the boundary. For instance, a contact resistance in heat transfer problem makes the unknown discontinuous through the boundary, or the new slightly more complicated "Screen" internal boundary that is used for wire gauzes or perforated plates. By "simple", I mean there is actually no extra equation involved, this is an algebraic relation between the unknown or its derivatives in up and down side of the boundary. This class of problem is found directly in boundary condition for each physics. See as well in the doc the up() and down() operators.

In other circumstances, it might need to solve a new PDE in the boundary by its own : you generally have access to this equation in the list of physics (thin film flow, electric current in shells, heat transfer in thin shells, etc...).

These features typically require access to a specific module, unless you want/can model it with PDE, lower dimensions equation available with COMSOL Multiphysics.

I hope this helps,
Eric

Hello Adam, in addition to Eric's answer about predefined couplings,
you also have the possibility to freely couple existing physics of different dimensions, by using coupling operators. This is very powerful since it gives you complete freedom to tailormake your coupling, using off-the-shelf physics interfaces already available.

First add the two models you want to set up, in the same mph model file. If you want to combine a 2D model with a 3D: start with creating 3D model. Then, right-click the top (root) node in the model builder tree, and select Add model. Select 2D in the Wizard and finish your 2D physics selection.

Then, define couplings between the two.
Go to the model from which you want to read the information (say a potential or electric field), right-click Definitions>Model Couplings and add General Extrusion (or the simplified Linear extrusion, depending on you model topology). In the Coupling settings you can define which variables should be coupled and between which geometries (models).

Depending on your modeling setup, you may need to add several couplings, to from each of the geometries.

regards Niklas

Hi,

This is very useful information. However, there is something I do not exactly understand (it might be because I am a beginner).

I have carefully read the example from the Chemical Reaction Engineering Module (Packed bed reactor) as it is very similar to the problem I want to solve.

- How do you define the 'Destination map' and 'Source' in the options of the general extrusion?

- Is it necessary that the two geometries you want to couple have the same length units (e.g: m) so that you have then to re-scale one of your equations?

Thanks in advance

Borja