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A metamorphosis of three-dimensional wave structure in transitional and turbulent boundary layers

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Laminar-turbulent transition in boundary layers is characterized by the generation and metamorphosis of flow structures. The early transition is usually associated with a process of the evolution from a three-dimensional (3-D) wave to a Λ-vortex. To develop a deeper understanding of the spatiotemporal wave-warping process and its roles in precipitating the development of other structures (e.g. hairpin-like structure and turbulent spot), we present numerical studies of both K-regime transition and bypass transition. In this talk, I will first illustrate a qualitative comparison of flow visualizations between a K-regime zero pressure gradient case and an adverse pressure gradient case, based on the method of Lagrangian tracking of marked particles. The underlying vortex dynamics will be presented using a proposed method of Lagrangian-averaged enstrophy. Next, I will draw attention to the 3-D wave structures in bypass transition and early turbulent boundary layer and will describe similar flow behaviours between transitional and turbulent boundary layers. Finally, I will discuss a path to transition, which hypothesizes that the amplification of a 3-D wave precipitates low-speed streaks and rotational structures in wall-bounded flows.

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