Laminar separation delay by optimal streaks

Boundary layers are prone to separation when subjected to adverse pressure gradients. Flow separation often reduces the performance of aerodynamic and hydrodynamic vehicles. Vortex generators have been utilized successfully as passive flow control devices; however they often trigger turbulence, which increases drag and heat transfer. Moreover, finding the optimal parameters for their design is oftentimes based on trial and error. In this study we seek by means of nonlinear optimization the velocity disturbance at a given upstream position, which maximizes the downstream location of the separation point. Our approach leverages insights gained from the non-modal growth of disturbances in boundary layers and their role in generating a mean-flow distortion which augments the shear at the wall. Optimal laminar separation delay is achieved for a disturbance described by counter-rotating vortices, which generate streamwise streaks downstream. The optimal spanwise wavelength of the vortices is found and its connection to the mean-flow distortion is discussed.