Correlations between the instantaneous velocity gradient and the evolution of scale-to-scale fluxes in...

Using high-resolution particle tracking velocimetry and filter-space techniques, we study the links between the scale-to-scale transfer of energy and enstrophy and instantaneously rotational and straining regions as determined by the classic Okubo-Weiss parameter in a quasi-two-dimensional laboratory flow. Although the Okubo-Weiss parameter has shortcomings, we find that, when suitably conditioned, it is surprisingly a good predictor for the future evolution of the spectral fluxes. By studying Lagrangian correlation functions, we explain our findings by showing that both the spectral fluxes and the Okubo-Weiss parameter are independently correlated for long times along fluid-element trajectories and thus that any predictive capacity of the Okubo-Weiss parameter arises because it is coupled to fluid advection. Our results suggest potential strategies for forecasting in complex flows by looking for quantities with long Lagrangian correlation times.