A rotating drum with a few hundred balls in it. There is one knob that matters, the spin, and one number that decides everything: √(g/R). For a half-metre drum that is 4.43 rad/s. Below it gravity wins and the bed falls back. Above it the wall holds a grain even at the top of the circle. Four videos, one per regime, in order of speed.
Watch on YouTube ↗The bed is dragged up a grain at a time until the whole face lets go at once. Not a flow: a series of separate collapses. 8 % of the grains get above the axis.
Watch on YouTube ↗The collapses merge. The lower half travels with the wall as a solid, a thin layer runs back down its face, and all the mixing happens in that layer. 18 %.
Watch on YouTube ↗Grains stop rolling back and start falling across the cavity onto the far side of the bed. 31 %, fastest grain 4.17 m/s.
Watch on YouTube ↗The bed sticks to the wall as a ring and mixing stops. 59 %.
Nothing here is scripted. Every grain is a body, contacts are a spring and a dashpot with Coulomb-capped friction, and the wall, the end caps and the lifter plates are a moving boundary carrying ω×r. The regimes are not modes you switch between - they are what falls out of one speed knob.
One thing the videos would hide if I let them: the fourth regime needs the lifters. Set them to zero and at the same 9 rad/s a smooth wall slips under the bed, which stays at the bottom - 3 % above the axis instead of 59 %. √(g/R) says whether a grain stays up there. It says nothing about getting it up there.
Granular tumbler is one of 132 modules in Makarov Physics Suite.
Posted by the game’s developer or publisher to its Steam community hub and published by Valve through Steam’s public news API. Sifter reproduces it unedited and writes none of it.