INVESTIGATION OF LAGRANGIAN COHERENT STRUCTURES IN A WAKE-INDUCED BOUNDARY LAYER TRANSITION etc15 Tracking Number 105Presentation: Session: Lagrangian aspects of turbulence 3 Room: Room F Session start: 15:00 Thu 27 Aug 2015 Guosheng He hgs_05@sina.com Affifliation: Fluid Mechanics Key Laboratory of Ministry of Education, Beijing University of Aeronautics and Astronautics, Beijing, China Chong Pan panchong@buaa.edu.cn Affifliation: Fluid Mechanics Key Laboratory of Ministry of Education, Beijing University of Aeronautics and Astronautics, Beijing, China Qi Gao qigao@buaa.edu.cn Affifliation: Fluid Mechanics Key Laboratory of Ministry of Education, Beijing University of Aeronautics and Astronautics, Beijing, China Lihao Feng lhfeng@buaa.edu.cn Affifliation: Fluid Mechanics Key Laboratory of Ministry of Education, Beijing University of Aeronautics and Astronautics, Beijing, China Jinjun Wang jjwang@buaa.edu.cn Affifliation: Fluid Mechanics Key Laboratory of Ministry of Education, Beijing University of Aeronautics and Astronautics, Beijing, China Topics: - Vortex dynamics and structure formation, - Wall bounded flows, - Lagrangian aspects of turbulence Abstract: The evolution of coherent structures in a flat plate boundary layer transition induced by the cylinder wake is investigated using the particle image velocimetry (PIV) technique. The finite-time Lyapunov exponent (FTLE), which characterizes the amount of stretching about the flow trajectory, is used to extract the Lagrangian coherent structures. It is revealed that secondary vortex is induced by the cylinder wake vortices in the near wall region,which would evolve into hairpin vortex as it convects downstream. The subsequent evolvement of the hairpin vortex, characterized by the regeneration of offspring hairpin vortex upstream of it, leads to the appearance of the hairpin packet and the boundary layer finally reaches a turbulent state. |