CONDITIONAL FLUID-PARTICLE ACCELERATIONS IN TURBULENCE

Simple closures for average fluid-particle accelerations, conditional on fixed local fluid velocity, ate considered in isotropic, homogeneou s and stationary turbulence using exact probability density transport equations and are compared with direct numerical simulations (DNS). Su ch accelerations are common ingredients in Lagrangian stochastic model s for fluid-particle trajectories in turbulence. One-particle accelera tions are essentially trivial, so the focus is on two-particle relativ e accelerations, which are important in the relative dispersion proces s. The closure is simply a quadratic form in the velocity variable and this special form also defines the Eulerian velocity probability dens ity function (pdf), and comparisons with DNS (for grids up to 512(3)) of both the acceleration closure and velocity pdf's are encouraging.