Hidden Order behind Two Dimensional Random Vortices

Abstract

The vortices that appear repeatedly and suggest turbulent dynamics are crucial to the understanding of sheared turbulence. These vortices produce order out of chaos, benefiting the turbulence modelling that focuses only on statistically stable quantities. In three dimensions, the hairpin vortices [J. Zhou, R. Adrian, S. Balachandar, T. Kendall, J. Fluid Mech., 387, 353 (1999); R. Adrian, Phys. Fluids, 19, 041301 (2007)] play such a fundamental role in the transport of momentum and energy for wall bounded sheared turbulence. In two dimensions where the atmospheric flow has a great play, the fundamental vortices have yet to be observed. Here we report the experimental finding of the slanted vortices in the quasi-two-dimensional sheared soap-film turbulence. We also demonstrate how these slanted fundamental vortices, which result from the interaction between the sheared mean flow and the random turbulent vortices, could possibly connect to the turbulent transport. The finding here might introduce another means of understanding the planetary turbulence where the mean flow is commonly existent and nonuniform.