Sedimenting particles with a k_x = π/ 2-mode in the particle density. We show different details (left) from a larger simulated domain (right), at different times (t = 1,6,9,12t_f), based on runKH. Initially, we added noise in the particle velocity to perturb the boundary between particle-rich and particle-poor regions. We believe that we see a “parasitic” Kelvin-Helmholtz instability appear over time.