TRANSITION IN WALL-BOUNDED FLOWS

In this paper, wepresent direct comparisons of experimental results on transitionin wall-bounded flows obtained by flow visualizations, hot-filmmeasurement, and particle image velocimetry, along with a briefmention of relevant theoretical progresses, based on a criticalreview of about 120 selected publications. Despite somewhatdifferent initial disturbance conditions used in experiments, theflow structures were found to be practi- cally the same. Thefollowing observed flow structures are considered to be offundamental importance in understanding transitional wall-boundedflows: the three-dimensional nonlinear wave packets calledsolitonlike coherent structures (SCSs) in boundary layer and pipeflows, the - vortex, the secondary vortex loops, and the chain ofring vortices. The dynamic processes of the formation of thesestructures and transition as newly discovered by recentexperiments include the following: (1) The sequential interactionprocesses between the-vortex and the secondary vortex loops, whichcontrol the manner by which the chain of ring vortices isperiodically introduced from the wall region into the outer regionof the boundary layer. (2) The generation of high-frequencyvortices, which is one of the key issues for understanding bothtransitional and developed turbulent boundary layers (as well asother flows), of which several explanations have been proposed buta particularly clear interpretation can be provided by theexperimental discovery of secondary vortex loops. The ignorance ofsecondary vortex loops would make the dynamic processes and flowstructures in a transitional boundary layer inconsistent withprevious discoveries. (3) The dominant role of SCSs in allturbulent bursting, which is considered as the key mechanism ofturbulent production in a low Reynoldsnumber turbulent boundarylayer. Of direct relevance to bursting is the low-speed streaks,whose formation mechanism and link to the flow structures inwall-bounded flows can be answered more clearly than before interms of the SCS dynamics. The observed SCSs and secondary vortexloops not only enable revisiting the classic story of wall-boundedflow transition, but also open a new avenue to reconstruct thepossible universal scenario for wall-bounded flow transition.