Statistics and dynamics of the boundary layer reattachments during the drag crisis transitions of a circular cylinder

Time series of pressure measured on the periphery of the central section of a circular cylinder of aspect ratio 22 are used to investigate the dynamics of the local reattachments during the drag crisis transitions. A succession of multi-stable dynamics are identified and characterized through conditional statistical analysis as the Reynolds number is increased. The first transition marking an abrupt weakening of the periodic pressure fluctuations associated to the global shedding dynamics is accompanied by the appearance of symmetric and bistable perturbations. Afterwards, two scenarii of asymmetric and symmetric boundary layer reattachments are found. The asymmetric scenario leads to two transitions. During the first transition, the flow explores randomly three stable states to eventually stabilize on the state corresponding to the permanent reattachment on one side of the cylinder. The second transition is bistable and leads to the permanent reattachment on both sides. For the second symmetric scenario, the boundary layer reattachments occur simultaneously on both side of the cylinder. In that case, the flow explores randomly two stable states to eventually stabilize on the state of full reattachment. Pressure distributions of all of these states are characterized as well as their corresponding probabilities during the drag crisis transitions of the critical regime. (C) 2015 AIP Publishing LLC.