The near-wake structure resulting from the interaction between the water and the cylinder is researched. The wake characteristics for the smooth cylinder and the cylinder with different numbers of grooves are measured by high-speed particle image velocimetry (PIV) at a Reynolds number of 7400. From the measured data, it is observed that the v-groove increases the Reynolds shear stress and the turbulent kinetic energy; thus, the formation of the Karman-like vortices is prevented and the recirculation region is reduced. The number of grooves has a limited effect on the wake characteristics. In addition, a one-dimensional orthogonal wavelet decomposes the fluctuation velocities in the cylindrical wake region to study the multi-scale wake characteristics behind the v-groove cylinders. The large-scale structures show that the Karman-like vortices of the grooved cylinder are closer to the trailing edge of the cylinder. The intermediate-scale structures show more vortices behind the 32-groove cylinder. The small-scale structures show that the phase difference between the upper and lower oscillation positions has become smaller. Meanwhile, the large-, intermediate- and small-scale Reynolds shear stress distributions suggest that the shear layer instability in the near-wake region creates vortices, which increase the turbulent kinetic energy and narrow the wake region.

This study develops a novel tree bark-inspired secondary groove cylindrical structure for wake flow regulation and drag reduction of cylindrical bluff bodies. Combined particle image velocimetry and large eddy simulation were adopted to investigate the wake characteristics and drag reduction performance at a subcritical Reynolds number of Re = 7400. Key experimental results show that, compared with a smooth cylinder, the double-secondary-groove cylinder achieves the optimal performance, reducing its recirculation length by ∼18.75% and the mean drag coefficient by 10.3%. Subsequent flow field analysis revealed that groove crests disrupt the regular periodic shedding of large-scale coherent vortex structures by exciting small-scale three-dimensional turbulent structures, whereas groove valleys exhibit a more significant suppression effect on turbulent kinetic energy in the near-wake region. To elucidate the multi-scale flow control mechanism of the secondary-groove structure, fast Fourier transform and multi-resolution dynamic mode decomposition were applied to perform quantitative frequency analysis and multi-scale modal decomposition of the wake flow structures, which reduced the dominant wake vortex shedding frequency and weakened the periodicity of large-scale vortex shedding. Notably, the results indicated that groove valleys primarily suppress large-scale low-frequency coherent structures in the wake, while groove crests excite high-frequency small-scale turbulent pulsations, thus driving efficient energy cascade in the wake flow field. This study provides new insights into passive flow control strategies based on multi-scale bio-inspired surfaces.

This study investigates the drag reduction mechanism of a submersible equipped with hair-strand appendages. After confirming Reynolds number independence, hair-strand appendages with varying length ratios (Lh/Li = 0.5, 1, 1.5, 2, and 2.5) and number ratios (Nh/N0 = 1/11, 3/11, 7/11, 1, and 14/11) were applied to the submersible at a Reynolds number of 19 572 for passive flow control, and drag force was measured using torque sensors. Experimental results demonstrate that hair-strand appendages achieve a maximum drag reduction of 4.7% at Lh/Li = 1 and Nh/N0 = 1; moreover, the drag coefficient is more sensitive to changes in the length ratio of hair-strand appendages, and the number ratio governs the overall rigidity of the hair-strand appendage array. Particle image velocimetry experiments reveal that as the number of hair-strand appendages increases, the recirculation zone, turbulent kinetic energy, and Reynolds shear stress all decrease. The oscillation of hair-strand appendages dissipates turbulent kinetic energy while reducing Reynolds shear stress. Spectral analyses, including continuous wavelet transform, discrete wavelet transform, and multi-resolution dynamic mode decomposition (mrDMD), reveal that the hair-strand appendages enhance momentum exchange. To further resolve the multi-scale vortical behavior in the wake, Liutex-based vorticity fields were analyzed using mrDMD. These results confirm that hair-strand appendages accelerate turbulent energy transfer by converting large-scale coherent kinetic energy into high-frequency, microscale turbulence.