Boundary layer receptivity to periodically passing vortical disturbances*

The boundary layer receptivity to periodically passing vortical disturbances is investigated with rod/airfoil and rod/flat-plate configurations using time-resolved particle image velocimetry and hydrogen bubble visualization techniques. Compared to the canonical free-stream turbulence (FST), the periodically passing vortical disturbances introduced by the rod wakes own larger length scale, higher intensity, stronger inhomogeneity and anisotropy, leading to the boundary layer receptivity different from that in FST-induced bypass transition. It is found that the receptivity is characterized by the formation of spanwise either single-secondary vortex or double-secondary vortices, depending on whether the disturbances related to the high-order harmonics of the periodically passing frequency could penetrate the boundary layer and are significantly amplified. The formation of these secondary vortices possesses good periodicity, which makes them different from the shed vortices of a separated boundary layer. After combining previous investigations with a multi-element airfoil configuration and current investigations, it is confirmed that the formations of single-secondary vortex and double-secondary vortices are typical behaviors of boundary layer receptivity to periodically passing vortical disturbances.