EFFECT OF TURBULENCE ON THE PARTICLE SETTLING VELOCITY IN THE NONLINEAR DRAG RANGE

Particle settling velocity in the nonlinear drag range is investigated using a Monte Carlo simulation for particles in a low Reynolds number , isotropic, Gaussian, pseudo turbulence. The settling velocity is aff ected by both the trajectory bias, which enhances the settling velocit y, and the nonlinearity of the drag associated with the turbulence, wh ich reduces the settling velocity. The effect of the trajectory bias i s important in an almost frozen turbulence when the settling velocity is comparable to the turbulence and particle motion in the creeping-fl ow regime. For a nonfrozen turbulence, the effect of the trajectory bi as on the settling velocity may be overwhelmed by the effect of the no nlinear drag associated with the turbulence. The ensemble average of t he second invariant of the turbulence deformation tensor, [II(d)], alo ng the particle trajectories is obtained to characterize statistically the trajectory bias and the correlation between the particle concentr ation and turbulence structure. [II(d)] attains its maximum value at z ero settling for a given particle inertia. The effect of increasing th e settling velocity leads to an exponentially decreasing [II(d)] for a large settling rate, and hence a significant reduction in the traject ory bias and the concentration-structure correlation. For very small o r large particle inertia, [II(d)] vanishes.