The existence of very-large scale motions or `superstructures' in wall-bounded turbulent flows
Seminar Room 1, Newton Institute
Recent experiments in the logarithmic and wake regions of wall-bounded turbulent flows have revealed the existence of very large-scale motions (with instantaneous length-scales up to 20 boundary layer thicknesses). This regime of very long meandering positive and negative streamwise velocity fluctuations (and associated roll-modes) appear to be universal features of all wall-bounded turbulent flows, having been recently documented in turbulent boundary layers, pipes, channels and atmospheric surface layers. Importantly, these structures appear to maintain a presence or footprint in the near-wall region, seeming to modulate or influence the near-wall cycle. As Reynolds number increases, not only is there an increase in scale-separation between these large-scale motions and the near-wall cycle, but also an increase in the overall energy of these long meandering features as compared to the viscous scales. This would seem to imply that these features will play an increasingly dominant role in high Reynolds number wall-bounded turbulent flows.