The Effect of Nonlinear Drag on the Motion and Settling Velocity of Heavy Particles

Abstract

The effects of nonlinear drag on the motion and settling velocity of heavy particles in a turbulent atmosphere are investigated. The authors approach the problem rather systematically by first considering the response of particles to much simpler fluid motions that are subprocesses of the more complex turbulent field. The authors first consider the motion and time response of particles falling under gravity in still fluid. Then the effects of a sudden gust or step change in relative velocity between a falling particle and its surrounding fluid are investigated. The authors demonstrate that horizontal relative motion produced by a sudden gust tends to reduce the settling velocity of a panicle. In simple oscillating fluids it is shown that the reduction of settling velocity increases with increasing amplitude of fluid oscillation. The authors also explore the effects of oscillation frequency on the settling velocity and show that if the period of fluid oscillation is less than the particle response time, then the settling velocity reduction becomes independent of oscillation frequency. Finally, the authors explore the motion of heavy particles within simulated isotropic turbulence and show that the effect of nonlinear drag is to produce a slowing of particle settling velocity.