DIRECT NUMERICAL-SIMULATION OF PARTICLE DEPOSITION ONTO A FREE-SLIP AND NO-SLIP SURFACE

We consider here the direct numerical simulation (DNS) of channel flow with two different surfaces: a no-slip, fixed wall and on the opposit e side a free-slip, free surface. The simulated velocity field agrees well with the experimental data for a free-surface flow obtained by Ko mori er al. [Int. J. Heat Mass Transf. 25, 513 (1982)]. The DNS is use d to simulate particle trajectories, which are computed with a dynamic particle equation in which only the drag force given by the Stokes la w is taken into account. For the particle time scale, nondimensionaliz ed in terms of the fixed-wall friction velocity and the kinematic visc osity, we use the values tau(+) = 5 and tau(+) = 15. A statistically s tationary condition is studied that is obtained by the introduction of a uniform distribution of particles at the beginning of the channel a nd by continuous removal through deposition at the two walls. The stea dy-state concentration distribution is nonuniform across the channel w idth, primarily due to the process whereby particles are trapped close to the surface. Moreover, we find that the wall-normal concentration profiles are self-similar. The deposition on both the no-slip and the free-slip wall can be described by a constant deposition coefficient, with for. tau(+) = 5 the larger value on the free-slip wall and for ta u(+) = 15 the opposite, i.e., the larger value over the no-slip wall. To study the deposition process in more detail we consider the cross c hannel particle fluxes and velocity statistics that are conditioned on deposition events. By means of instantaneous near-wall particle distr ibutions we also consider the patterns of particles and their accumula tion in certain areas of the flow. For a no-slip surface the well-know n result that particles tend to collect in the low-speed streaks is co nfirmed. The patterns of particles near the free-slip surface are comp letely different, which can be explained in terms of the different typ es of coherent structures that are present near this surface. (C) 1998 American Institute of Physics.