Transition in wall-bounded flows
Seminar Room 1, Newton Institute
In this article we present direct comparisons of experimental results on transition in wall-bounded flows obtained by flow visualizations, hot-film measurement, and particle-image velocimetry (PIV), along with a brief mention of relevant theoretical progresses, based on a critical review of about 120 selected publications. Despite somewhat different initial disturbance conditions used in experiments, the flow structures were found to be practically the same. The following observed flow structures are considered to be of fundamental importance in understanding transitional wall-bounded flows: The three-dimensional nonlinear wave packets called solitons-like coherent structures (SCS) in boundary layer and pipe flows; the É©-vortex; the secondary vortex loops; and the chain of ring vortices. The dynamic processes of the formation of these structures and transition as newly discovered by recent experiments include, among others: (1) The sequential interaction processes between the É©-vortex and the secondary vortex loops, which controls the manner by which the chain of ring vortices is periodically introduced from the wall region into the outer region of the boundary layerĀG (2) The generation of high-frequency vortices, which is one of the key issues for understanding both transitional and developed turbulent boundary layers (as well as other flows), of which several explanations have been proposed but a particularly clear interpretation can be provided by the experimental discovery of secondary vortex loops. The ignorance of secondary vortex loops would make the dynamic processes and flow structures in a transitional boundary layer inconsistent with previous discoveries; and (3) The dominant role of SCS in all turbulent bursting, which is considered as the key mechanism of turbulent production in a low Reynolds-number turbulent boundary layer. Of direct relevance to bursting is the low-speed streaks, whose formation mechanism and link to the flow structures in wall-bounded flows can be answered more clearly than before in terms of the SCS dynamics. We combine these newly observed structures and processes to those well-known ones to form a more integrated physical picture of the transitional dynamics. This not only enables revisiting the classic story of wall-bounded flow transition, but also opens a new avenue to reconstruct the possible universal scenario for wall bounded flow transition.