For this enhancement process to occur despite the pressure force that tends to “smear” these maxima out, it is necessary that there be a background solid-gas drift, and also Coriolis forces that help establish a geostrophic balance with the pressure force. The role of Coriolis forces would appear to indicate that one-dimensional toy models do not adequately capture the instability.

Larger amplitude vertical flows will (when outside of the linear regime) transport density away from maxima and hence limit the growth of the instability. and would compete with other nonlinear effects like particle clumping (’traffic jams’). Incidentally, another process in the nonlinear saturation that will work against particle clumping is particle pressure, not usually modelled. The large local densities of nonlinear clumps will lead to enhanced collision frequencies and consequently to a particle pressure which will resist further concentration. Attempts to establish the conditions for self-gravitational collapse of such clumps should in principle include this effect.

Approximating dust as a pressureless fluid coupled to the gas
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It is of interest to investigate in some generality how the description of dust as a pressureless fluid, used in many simulations, arises. As CY10 point out, the usual criteria for gas molecules 