The settling velocity of heavy particles in an aqueous near-isotropic turbulence

The ensemble-average settling velocity, V-s, of heavy tungsten and glass particles with different mean diameters in an aqueous near-isotropic turbulence that was generated by a pair of vertically oscillated grids in a water tank was measured using both particle tracking and particle image velocimetries. Emphasis is placed on the effect of the Stokes number, St, a time ratio of particle response to the Kolmogorov scale of turbulence, to the particle settling rate defined as (V-s-V-t)/V-t where V-t is the particle terminal velocity in still fluid. It is found that even when the particle Reynolds number Re-p is as large as 25 at which V-t/v(k)approximate to10 where v(k) is the Kolmogorov velocity scale of turbulence, the mean settling rate is positive and reaches its maximum of about 7% when St is approaching to unity, indicating a good trend of DNS results by Wang and Maxey (1993) and Yang and Lei (1998). This phenomenon becomes more and more pronounced as values of V-t/v(k) decrease, for which DNS results reveal that the settling rate at V-t/v(k)=1 and Re-p<1 can be as large as 50% when Stapproximate to1. However, the present result differs drastically with Monte Carlo simulations for heavy particles subjected to nonlinear drag (Re-p>1) in turbulence in which the settling rate was negative and decreases with increasing St. Using the wavelet analysis, the fluid integral time (tau(I)), the Taylor microscale (tau(lambda)), and two heavy particles' characteristic times (tau(c1),tau(c2)) are identified for the first time. For St<1, tau(c1)