Laminar Streaks in Oscillating Boundary Layers

The growth of laminar streaks in a flat-plate boundary layer is investigated in the presence of both spanwise wall oscillations and steady streamwise-dependent spanwise wall forcing. The laminar streaks are forced by free-stream vortical disturbances, which interact with the boundary layer to produce disturbances, which first grow and then decay. The oscillating plate produces a base flow that matches Blasius boundary layer in the streamwise and wall-normal directions, but produces a generalized Stokes layer (GSL) in the spanwise direction. This depends on the Blasius flow and tends to the classical Stokes layer (CSL) in the asymptotic limit of high oscillation frequency or large downstream distance. Spanwise oscillations of the plate and spanwise wall forcing can both reduce the total energy and maximum amplitude of the disturbances. Their relative effects are discussed.