THE EFFECT OF NONLINEAR DRAG ON THE MOTION AND SETTLING VELOCITY OF HEAVY-PARTICLES

The effects of nonlinear drag on the motion and settling velocity of h eavy particles in a turbulent atmosphere are investigated. The authors approach the problem rather systematically by first considering the r esponse of particles to much simpler fluid motions that are subprocess es 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 ve locity between a falling particle and its surrounding fluid are invest igated. The authors demonstrate that horizontal relative motion produc ed by a sudden gust tends to reduce the settling velocity of a particl e. In simple oscillating fluids it is shown that the reduction of sett ling 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 redu ction becomes independent of oscillation frequency. Finally, the autho rs explore the motion of heavy particles within simulated isotropic tu rbulence and show that the effect of nonlinear drag is to produce a sl owing of particle settling velocity.