Flat plate boundary layer transition induced by a controlled near-wall circular cylinder wake

The flat plate boundary layer transition induced by the wake of a circular cylinder close to the wall is experimentally investigated using particle image velocimetry (PIV) and hydrogen bubble visualization techniques. The wake of the circular cylinder is controlled by a slot synthetic jet at the rear stagnation point of the circular cylinder. It is found that when the synthetic jet is actuated, the wake can be greatly modified. When the excitation frequency of the synthetic jet is set at the natural shedding frequency of the cylinder wake, the symmetrical shedding pattern can be observed. While the excitation frequency increases to be twice of the natural shedding frequency, the wake appears to be antisymmetrical again, but with the shedding frequency locked onto half of the excitation frequency. Flow visualizations show that spanwise secondary vortices can be induced in the near wall region by these large scale vortices in the wake. It is found that the secondary vortices destabilize into streamwise stretched. vortices as they convect downstream. After the introduction of the synthetic jet, the destabilization process is promoted. By investigating the disturbance growth inside the boundary layer, it reveals that the synthetic jet can cause earlier initialization of the disturbance growth, thus promoting the transition process of the boundary layer. An explanation is provided that the low frequency components of the wake disturbances, which interact with the boundary layer, are enhanced by the introduction of the synthetic jet. Therefore, the destabilization of the secondary vortices is promoted, and disturbance growth in the boundary layer initiates earlier. (C) 2015 AIP Publishing LLC.