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 (Steff) is derived that is proportional to the square-root of the flatness factor of the longitudinal velocity derivative. In the atmospheric boundary-layer, Steff 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 Steff and, hence, preferential concentration.