This is the “acoustic” RDI: dust moving with a constant acceleration through homogenous, neutral gas. Should be simple, but the dust immediately goes unstable and starts to clump up. Movie shows dust (black) and gas (colors show gas density, from low in blue to high in red, an order-of-magnitude stretch). Left is viewed side-on — the dust is moving up under some external acceleration (e.g. gravity or starlight), while right is viewed “down the barrel” (face-on).