Evidence of rare backflow and skin-friction critical points in near-wall turbulence using micropillar imaging

The recent discovery of rare backflow events in turbulent boundary layer flows based on the analysis of simulation data has again raised the need of experimental visualizations of wall-shear stress fields in unsteady flows. The localization of critical points, which are thought to strongly correlate with large-scale events in the log-layer, is of importance. Up to now, there is no experimental proof of these rare events and their topological patterns. Their existence in a turbulent boundary-layer flow along a flat plate is shown herein by means of imaging with 2D arrays of flexible micropillars attached at the wall.