Uncovering causality in isotropic turbulence by massive machine manipulation

Flow structures important for the dynamics of isotropic turbulence are identified by running an unprecedentedly large number of direct numerical simulations (DNS) seeded with localized perturbations. The perturbations zero either the local velocity fluctuations or the local vorticity within a region of a given size $L$. Some perturbations grow up to $10^3$ times more than others after one turnover, in terms of the global $L_2$-norm of the perturbation field. Because perturbations are local in space, the procedure classifies regions of the flow according to how much they grow. The most `reactive' regions are studied, showing that they tend to contain strong events, either strong vortices or strong velocity perturbations. Large perturbations $(L \gtrsim 60\eta)$ grow or not based on the contribution of the region to the turbulent kinetic energy, whereas smaller perturbations $(L \lessim 60 \eta)$ do so based on the vorticity the local flow induces in the rest of the flow field. Differences found between the most and the least growing perturbations hint that while both of them affect the small scales of the flow, only the former contaminate the integral scales, resulting in a much stronger effect on the overall flow.