Miniature vortex generator control of turbulent boundary layers

Miniature vortex generators (MVGs) are a promising passive flow control technique for viscous drag reduction by producing large-scale vortical motions that manipulate turbulence structures in turbulent boundary layers (TBLs) without significant device drag. This study conducts hot-wire anemometry experiments to investigate the influence of the Reynolds number and the ratio of MVG height $h$ to TBL thickness $\delta _0$ (MVG height ratio $h/\delta _0$ ) on turbulence structures. Experiments encompass two MVG height ratios, $h/\delta _0=0.09,\;0.18$ , friction Reynolds numbers ranging from ${\textit{Re}}_\tau =400$ to 2000 and measure the velocity information at various downstream stations. Spectral analysis confirms the MVG-induced vortices amplify large-scale structures in the outer region, sustaining up to 100 times the MVG height downstream. The MVGs are also found to attenuate turbulence energy across a wide range of turbulence structures below the amplification location in the logarithmic region, connected with the MVG-induced spanwise motion. Increasing the friction Reynolds number from ${\textit{Re}}_\tau =400$ to $900$ or doubling the MVG height ratio causes the amplified structures to develop into longer motions and move away from the wall, while increasing the turbulence energy attenuation proportion to the log region. Moreover, the energy attenuation amplitude of large-scale structures in the near-wall region increases with a larger MVG height ratio but decreases with increasing Reynolds numbers. The findings indicate that, at friction Reynolds numbers ${\textit{Re}}_\tau \geqslant 900$ , MVGs induce spanwise motions that attenuate near-wall structures and modulate large-scale outer motions. The present configuration does not yield a global viscous drag reduction, but the turbulence modulation trends suggest the potential for viscous drag reduction when the MVG configurations are optimised to enhance favourable buffer-layer spanwise motions.