Experimental Investigation into the Drag Performance of Chevron-Shaped Protrusions in Wall-Bounded Turbulence

Chevron-shaped protrusions have been proposed in the literature for turbulent skin friction reduction. However, there is no consensus on the performance of this passive flow control technique; both an increase and a decrease in drag have been observed in previous studies. There is also no experimental evidence to support the working mechanism behind the drag reduction effect that has been postulated in the literature. In this study, direct force measurements were used to replicate experiments from the literature and, in addition, were used to test new array configurations to characterise the effect of individual design parameters on drag performance. A total of 23 different protrusion configurations were investigated in a turbulent boundary layer flow. In addition to the integral force measurements, particle image velocimetry was used to measure wall-parallel velocity fields in order to extract the statistical sizing and energy of the near-wall cycle turbulence. All configurations increased the drag between 2% and 10% for a friction Reynolds number of 1700. The drag reduction reported in the literature could not be replicated; however, these findings agreed with an experimental and numerical study that reported drag increase. The trend observed in the low-speed streak spacing from the PIV experiments was consistent with that observed in the balance data. Nevertheless, no evidence was found to support the working mechanism proposed in the literature. These results cast doubt on the proposed drag reduction potential of chevron-shaped protrusions. In the authors' view, the results of this study strengthen previous conclusions regarding their minor increase in drag. Future studies to further approach a consensus are proposed.