## Startup

Open up **COMSOL Multiphysics 4.0a (Classkit License) **and we will have a screen like this:

## Model Selection

If you need a general procedure for how to open models in COMSOL head over to the previous post . For this particular problem let’s use the *Chemical Species Transport* and the sub-model *Transport in Porous Media (chpm)*. Select *Steady State* and finish your model by clicking on the flag. You could do multiple physics to solve this problem… and in fact Matthew Bible and Isaac Keebler have a very nice solution using the *Porous Media *and the *Dilute Species Transport.* Their solution is cool but I’m going to keep it simple.

## Building the Model

Refer to the previous post and see how to generate geometries… here we will be making three rectangular shapes to represent our problem. With three rectangles that are 0.2 m wide and 0.8 m tall. Although maybe not required I would union the geometries. Union by selecting *Geometry* in the *Model Builder* and selecting all the geometries under your *Geometry *heading and then pressing the *Form Union* item on the list. Follow your nose to get the union. Here’s the geometry:

## Boundary Conditions

Basically you want two concentration conditions on the vertical boundaries at x=0 and x=0.6, open boundaries at the top and bottom of your geometry in you second rectangle (that’s the lines and where , a high diffusivity in the middle of your geometry (~1), a large porosity in your middle geometry (~1), and lower diffusivity and porosity on either side of your middle region (I used the defaults).

That’s it, solve your model. Solution screen shot, maybe soon.

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