报告题目： The shortest route from freestream noise to bypass transition
报告人: Dr. Xuerui Mao
Associate Professor, University of Nottingham,UK
Bypass transition is well observed and the understanding of this scenario is critical to improve the performance of turbomachinery, aeroplanes and high-speed trains. Bypass transition roughly consists of three stages: 1) the generation of velocity streaks induced by low-frequency free-stream noise, 2) the secondary instability of streaks activated by high-frequency noise, and 3) further nonlinear development to turbulence. These three stages were separately studied in the past, but are integrated in the present work. The nonlinear optimal (most energetic) variation of the inflow noise with respect to time and space is computed in order to reveal the shortest route from free-stream perturbation to bypass transition. In particular, a nonlinear lift-up mechanism, which occurs after the well documented linear lift-up stage, is observed to be essential in the breadown of streaks. The computation involves the integration of an adjoint equation, which requires the full trajectory of the velocity field.