Streamwise evolution of coherent structures in wall-bounded turbulence under the influence of synthetic jets

To reveal the regulatory mechanisms of synthetic jets on coherent structures in a flat-plate turbulent boundary layer at a low friction Reynolds number of 489, single-slot and double-slot synthetic jet configurations were designed. Particle image velocimetry measurements were coupled with three analytical methods, including two-point correlation, space-time correlation, and linear stochastic estimation, to systematically investigate the streamwise evolution of coherent structures and associated drag reduction influences. Experimental results indicate that the double-slot jets achieve an average drag reduction rate of 21.76%, obviously outperforming the single-slot jets, which attain 14.45%. The peak drag reduction rate reaches 25.60% for the double-slot configuration, compared to 20.18% for the single-slot counterpart, while showing weaker streamwise attenuation in control efficacy. Synthetic jets suppress turbulent bursting and interlayer momentum exchange by lifting near-wall low-momentum fluids, compressing the spatiotemporal scales of coherent structures, reducing their migration velocity, and augmenting sweep velocity. The secondary disturbance of the double-slot jets extends the streamwise control range and enhances the disruption of large-scale hairpin vortex packets. This study offers robust experimental and theoretical support for the development of low-power-consumption drag reduction technologies for underwater vehicles.