Wednesday, 5 June 2002: 9:45 AM

Enhancement of inertial particle clumping by velocity-gradient intermittency

Although it has been recognized for some time that the impact of
velocity-field intermittency on droplet collision in
turbulent clouds is potentially significant (Tennekes and Woods,
QJRMS, 1973), a corresponding impact of intermittency on droplet
clumping (preferential concentration) has only recently been
hypothesized. In particular, Shaw et al (JAS, 1998) argue that
intermittency increases droplet clumping via the complex geometrical
interaction of vortex tubes and the
inertia-induced slip of particle motion. I present a quantitative
analysis of the effect of
non-Gaussian velocity-gradient statistics on inertial particle
clumping at high Reynolds number (Re). I demonstrate that the
Re-dependence of particle clumping is a function of three 4th-order
velocity-gradient scalar invariants: mean-square dissipation,
mean-square enstrophy and the dissipation-enstrophy covariance. An
Re-dependent effective Stokes number (St

_{eff}) is derived that is proportional to the square-root of the flatness factor of the longitudinal velocity derivative. In the atmospheric boundary-layer, St_{eff}is approximately 2.7 times larger than the usual Stokes number. These results support Shaw et al's hypothesis that velocity field intermittency tends to increase preferential concentration. However, in contrast with Shaw et al, I demonstrate that, in real turbulence, vortex tubes do not statistically affect St_{eff}and, hence, preferential concentration.Supplementary URL: