Experimental study on the modified coherent structures in the turbulent boundary layer forced by a synthetic jet actuator

An experimental investigation is carried out to characterize the modification of the coherent structures in the turbulent boundary layer controlled by a synthetic jet actuator (SJA) with a spanwise slit exit at the friction Reynolds number of 479. Planar particle image velocity is applied to capture the instantaneous velocity field in the streamwise-wall-normal plane. The time-averaged velocity field shows that the SJA introduces an apparent upward motion downstream of the actuator, accompanied by a reduced streamwise velocity. The viscous shear stress is largely suppressed by the SJA in the near-wall region, due to the weakened streamwise velocity gradient. On the other hand, the Reynolds shear stress is significantly enhanced by the SJA-induced spanwise vortices centered at approximately 0.15 boundary layer thickness away from the wall, where the swirling strength is apparently increased. Phase averaging is conducted based on the wall-normal velocity above the SJA, giving insight into the spatial-temporal development of the SJA-induced spanwise vortices. The variable-interval space-average (VISA) analysis shows that the bursting frequency is dramatically increased downstream of the actuator, while it drops gradually with further increase in the streamwise location. In addition, the conditional averaged velocity field based on the VISA detection indicates that the strength of the bursting events is enhanced under control, contributing to the intensified turbulent activities.