Effects of submerged synthetic jet on the coherent structures in turbulent boundary layer

The research focuses on the active control scheme of a fully-developed turbulent boundary layer (TBL) forced by the submerged synthetic jet, and the particle image velocimetry (PIV) system is used to capture the two components velocity signals in the flow field. The mean velocity profiles near the wall obtained by the single-pixel ensemble correlation (SPEC) algorithm evidence the decline of streamwise velocity gradient, indicating the reduction of skin-friction drag. The deficit of streamwise velocity as well as turbulence attenuation in the cross flow attests that the perturbation effects are persistent in a certain range downstream of the synthetic jet injection section. The spatial topologies of the coherent structures detected by the improved quadrant splitting method (IQSM) reveal that the sweep events, responsible for the significant wall friction, are repressed under the action of synthetic jet. By employing the method of two-point correlation, destruction of the near-wall streaks is observed, and the control mechanism acts in a way to suppress the extension along the streamwise direction of the correlation coefficient. Based on the linear stochastic estimation, it is inferred that the influence of force is such the near-wall hairpin vortex detaches from the wall, and its ability to induce environmental fluid is vastly weakened.