Single-particle, particle-pair, and multiparticle dispersion of fluid particles in forced stably stratified turbulence

Abstract

The dispersion of fluid particles in statistically stationary stably stratified turbulence is studied by means of direct numerical simulations. Due to anisotropy of the flow, horizontal and vertical dispersion show different behavior. Single-particle dispersion in horizontal direction is similar to that in isotropic turbulence for short times, but shows a long-time growth rate proportional to t2.1±0.1, larger than the classical linear diffusion limit. In vertical direction, three successive regimes can be identified: a classical t2-regime, a plateau that scales as N−2, and a diffusion limit where dispersion is proportional to t. By forcing the flow and performing long-time simulations, we are able to observe this last regime, which was predicted but not observed before in stratified turbulence. This diffusive regime is caused by molecular diffusion of the active scalar (density). The mean squared separation of particle pairs (relative dispersion) in vertical direction shows two plateaus that are not present in isotropic turbulence. They can be associated with the characteristic layered structure of the flow. In the long-time limit again a linear regime is found as for single-particle dispersion. Pair dispersion in horizontal direction behaves similar to that in isotropic turbulence except for long times. Finally, the study of multiparticle statistics in stably stratified turbulent flows is reported. The evolution of tetrads gives an impression of the shape of particle clouds. It is found that with increasing stratification, the volume of the tetrads decreases, and they become flatter and more elongated.

Vertical dispersion of light inertial particles in stably stratified turbulence: The influence of the Basset force

“…This diffusive regime is caused by molecular diffusion of the active scalar ͑density͒ and it is only observed for statistically stationary stably stratified turbulence. 4,10 Recently, Lindborg and Brethouwer 11 reported a theoretical derivation of the scaling behavior in the intermediate N −2 -regime and of the irreversible mixing in the diffusive regime. Also in the horizontal direction the dispersion of fluid particles in stably stratified turbulence is altered.…”

Preferential Concentration of Heavy Particles in Stably Stratified Turbulence

“…Some properties of the flow are given in Table I; an elaborate description of the resulting flow plus a justification of the forcing method can be found in Ref. [14]. …”

“…The nonlinear vortex mode is responsible for this anisotropy. The linear wave mode acts in the vertical and causes suppressed vertical particle dispersion, fluid particles remain within a layer around their equilibrium height [12,14]. This study is performed by means of direct numerical simulations.…”

“…This study is performed by means of direct numerical simulations. A pseudospectral code is used that solves the full Navier-Stokes equations with Boussinesq approximation on a triple-periodic domain [14,15]. In a precomputation a divergence-free homogeneous isotropic turbulent initial velocity field is created.…”

Particle transport in incompressible MHD Kolmogorov flow

“…Superdiffusive transport for VFL is explained by these two maxima -particles may be caught in one vertical stream or the other, and they can experience long flights. Superdiffusive transport was observed for stratified turbulence in [12,13], but it is not obvious if the generating mechanism is similar (for instance the forcing is different here). Because these particles have a fixed velocity amplitude, all three PDFs have finite support.…”