## Abstract

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_{1})/ 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} ≈ 10 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 St ≈ 1. 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 (T_{1}), the Taylor microscale (T_{λ}), and two heavy particles' characteristic times (T_{c1}, T_{c2}) are identified for the first time. For St<1, T_{c1} < T_{1} and T_{c2}< T_{λ}, whereas T_{c1} ∼ T_{1} and T_{c2}= ≈ T_{λ} for St ≈ 1. This may explain why the settling rate is a maximum near St ≈ 1, because the particle motion is in phase with the fluid turbulent motion only when St ≈ 1 where the relative slip velocities are smallest. These results may be relevant to sediment grains in rivers and aerosol particles in the atmosphere.

Original language | English |
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Pages (from-to) | 868-880 |

Number of pages | 13 |

Journal | Physics of Fluids |

Volume | 15 |

Issue number | 4 |

DOIs | |

State | Published - Apr 2003 |