Direct numerical simulation (DNS) can be used to extract information implicit in the equations of fluid-particle motion. We have investigated the lift-off of a single particle and many particles in pressure driven flows by 2D DNS (N.A. Patankar, Huang, Ko & Joseph 2001a, Ko, N.A. Patankar & Joseph 2001, Joseph, & Ocando 2001, Choi & Joseph 2001, N.A. Patankar, Ko, Choi & Joseph 2001b). We show ...