Thanks for this, Prasanth. I've actually been going through the Eulerian Multiphase Granular tutorial (not sure where it is online to link, sorry), which is very similar to my project, except that it has an axis spinning instead of a stirrer bar.

I'm having a problem with my mixing in my project, in that my Fluent won't mix my hydrogel and calcium when I run the model; it keeps them in their separate zones. I can set up three different surfaces in DesignModeler by drawing three different sketches and generating the surfaces, but they won't mix in Fluent as they are separated by walls instead of interfaces. When I ty to make all of my walls interfaces, I get an error in Ansys (presumably because I don't have any walls to hold the fluid in, as in I should only have the interfaces where the fluids connect). When I try to draw the walls that I need to have interfaces as separate sketches and try to make a surface out of those sketches in DesignModeler (Calcium is rectangle on top, hydrogel is the large section):
 it won't allow me to create the surfaces (error "!"):
(I initially had the sketches connected to the origin by two 5 mm dark blue lines (and had the rest of my sketches dark blue too), but I deleted them in the hope that that might help. Doing that changed all of my sketches to pale green. Unfortunately, I still had the same issue.)
When I do the drawings in Inventor and create a STP file, Fluent won't pick up on the different drawings, only the three surfaces (patches) from Inventor, which gives the same issue for me, i.e. calcium/hydrogel not intermixing.
How do I recify this? I imagine it's something to do with how the separate sketches connect?