Ok. I _think_ this has all the elements I’m looking for, just have to break it down. “Patterns and flow in frictional fluid dynamics” came up when I searched for “structure emerges from flow” and well, these are the kinds of elements I’d hoped to find. Now to see what my model is missing and/or has extra that can fold in.
“(a) Air is slowly injected into a linear Hele-Shaw cell loaded with polydisperse glass beads (∼100 μm diameter) submersed in a water/glycerol solution. The gap is Δz=0.5 mm and the cell forms a channel 20 cm wide and 30 cm long. The granular material settles after loading. (b) The invading air/fluid interface accumulates a front of close-packed grains in the gap between the plates.”