Transition in Wall-Bounded Flows

Abstract

In this paper, we present direct comparisons of experimental results on transition in wall-bounded flows obtained by flow visualizations, hot-film measurement, and particle-image velocimetry, 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 solitonlike coherent structures (SCSs) in boundary layer and pipe flows, the Λ-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 the following: (1) The sequential interaction processes between the Λ-vortex and the secondary vortex loops, which control the manner by which the chain of ring vortices is periodically introduced from the wall region into the outer region of the boundary layer. (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. (3) The dominant role of SCSs 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. The observed SCSs and secondary vortex loops not only enable revisiting the classic story of wall-bounded flow transition, but also open a new avenue to reconstruct the possible universal scenario for wall-bounded flow transition.

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“…"Vortex Breakdown" has been widely spread in many research papers and textbooks [5,[9][10][11]. However, our analysis found that "Vortex Breakdown" is theoretically impossible.…”

“…In practice, "Vortex Breakdown" can never happen. It is found that "Vortex Breakdown" described by exiting publications [5,9,10] is either misinterpreted by 2D visualization or by using the pressure center as the vortex center.…”

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“…However, it was recent studies, such as [68][69][70][71][72][73][74][75], that have revealed much of the detail of such streak formation, in particular their vortical structures. Among them [75] proposed a well-explained streak formation, revealing detailed vortical dynamics as shown in figure 24. The transitional flow structures consist of the T-S wave and Lambda vortex, the soliton-like coherent structures (SCSs), the secondary closed vortex, the chain of ring vortices and the resultant stream-wise vortices.…”

“…Streak structure and breakdown: (a -c) typical hydrogen bubbles in laminar streak formation illuminated by a horizontal laser sheet at different heights [75]; (d) three-dimensional structure reconstructed from these detailed photos [75]. rsfs.royalsocietypublishing.org Interface Focus 5: 20150020 zone as indicated in figure 27; then this breakdown triggers immediately neighbouring streaks, starting breakdowns with tiny time delays, forming the tip of the wedge shape.…”