Waves and the role of helicity in rotating and stratified turbulence

P. D. MININNI

Cancún

Congreso; AGU Meeting of the Americas; 2013

AGU

Invariants of the equations of motion play an essential role in the behavior of turbulent flows. The cascade of energy to the small scales in three dimensional hydrodynamic turbulence, associated with the conservation of energy in the ideal case, is a well-known example. Less understood is the role played by the helicity, which embodies the global correlations between the velocity field and the vorticity. In ideal rotating flows, helicity is conserved, and waves in rotating and stratified flows are associated with helical flows, to the point that helical-wave decompositions are often used to study their dynamics. Helicity is also believed to be important in certain atmospheric processes, such as rotating convective (supercell) thunderstorms, and in conducting flows for the generation of large-scale magnetic fields. We present results from large direct numerical simulations of rotating and/or stratified turbulent flows. In the case or rotating flows, helicity in the flow affects scaling laws and the mixing of scalar quantities. At sufficiently small scales, the time scale of the eddies becomes of the order of the wave period, and isotropy is recovered with the eddies dominating the dynamics and the flow displaying Kolmogorov spectrum. In stratified and rotating flows, helicity emerges spontaneously from the joint action of eddies and of inertia-gravity waves, and it occurs when the waves are sufficiently strong. For Burgers number smaller than 3, the amount of helicity produced is correctly predicted by a quasi-linear balance equation. Extensions of these results to other systems, and possible experimental measurements will be briefly discussed.