Multi-scale wake characteristics around a cylinder with the secondary groove

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.