Machine identification of causally important events in turbulent channel flow

Flow structures that are highly causal to the future state of wall turbulence are studied using a large number of numerical simulations of turbulent channel flow at Reτ = 600. The domain is divided into small cells, and the velocity fluctuations with respect to the local mean within each cell are removed before the flow is allowed to temporally develop. The causal effect of this operation after a given time is measured by the L₂ norm of the difference between the perturbed and unperturbed velocity or vorticity vectors. Cells resulting in large perturbations are considered highly causal. To avoid pre-assumptions as much as possible, the simulations are repeated many times changing the location and size of the cells. The analysis is then conducted on an ensemble of many realizations. The perturbation effect is found to propagate in the wall-normal direction with a celerity of the order of the friction velocity, although somewhat faster than the advection velocity of conventional turbulent sweeps and ejections. Comparison of different wall-normal cell heights shows that the causal effect of the flow structures becomes more intermittent as the perturbation is applied farther away from the wall. The structure of highly causal cells depends on whether the perturbation is characterized by its magnitude or by its amplification rate. Large magnitudes tend to be associated with velocity interfaces, whereas large amplifications are associated with sweeps. Low amplifications are associated with ejections.