SELF-SIMILARITY OF BALLISTICALLY SEPARATING MOTIONS IN TURBULENT RELATIVE DISPERSION

摘要

Dynamical properties of passive particle pairs are investigated in two-dimensional free convection turbulence by direct numerical simulation. In terms of the exit-time .statistics, it is confirmed that the growth of relative separations r(t) is consistent with the prediction of the Bolgiano-Obukhov scaling in the inertial range, (r2(t.)) ex. V'. Furthermore, by looking into the probability density function (PDF) of exit-time, motions of relative separations of particle pairs are classified into two types: ballistic and diffusive motions, both of which satisfy the Bolgiano-Obukhov scaling. The probability density function of exit-time of diffusively separating motions is described by Richardson's diffusion equation, and that of ballistically separating motions corresponds to the PDF of the Lagrangian velocity increment. Our results also indicate that the PDF of the Lagrangian velocity increment relates to that of the stretching rate of a relative separation in the dissipation range.