Single-particle dispersion in stably stratified turbulence

NICOLAS SUJOVOLSKY; MARK RAST; PABLO MININNI

Tireste

Congreso; Turbulence, mixing and beyond 2017; 2017

ICTP

One of the main properties of turbulent flows is the efficiency to transport and mix quantities advected by the fluid. In particular, in stratified turbulence, a proper understanding of transport processes is relevant, e.g., to correctly model the diffusion of pollutants in the atmosphere, the suspension of cloud droplets, and hail formation. In these flows the presence of restitutive forces associated with the stratification allow excitation of internal gravity waves, which interact between themselves and with eddies, modifying the dynamics and, in particular, the transport of particles. We performed simulations of stably stratified turbulence with Lagrangian particles, varying the Brunt-Väisäla frequency (corresponding to the frequency of internal gravity waves, and associated with the degree of stratification of the fluid), and the external mechanical forcing. We show that the Lagrangian vertical velocity spectrum is compatible with a Garrett-Munk spectrum, as observed in the ocean, and with a maximum in the frequency of internal gravity waves. This spectrum can be explained from the statistics of waiting times considering that vertical motions are dominated by internal gravity waves. We also derive a simple model for the evolution of the probability density function of particle displacements in the horizontal direction, using a continuous random walk model that also takes into account the presence of horizontal winds in the flow.