Impact of spanwise extent of transverse grooves on drag reduction in boat-tailed bluff bodies: an experimental study

The paper describes the first experimental study on the application of small contoured grooves in boat-tailed bodies characterized by vortex shedding. In particular, we experimentally investigate the flow-separation delay and drag-reducing performance of spanwise-extruded and spanwise-discontinuous grooves. For this purpose, we consider groove geometries similar to those proposed and numerically investigated by Mariotti et al. (Eur J Mech B/Fluids 74:351–362, 2019) and Pasqualetto et al. (Fluids 7:121, 2022a). The Reynolds number, based on the freestream velocity and the model crossflow dimension, is \(\mathrm Re=9.6\cdot 10^4\). In addition to serving as an experimental confirmation of previous numerical studies, an important difference is that the present experiments were conducted with a freestream turbulence intensity of 0.9%, whereas the simulations were carried out with a freestream without turbulence. This extends the applicability of this flow control device to a situation closer to real-world or industrial applications. In the experiments, we measure pressure-drag variations for different configurations and flow correlations in the spanwise direction through pressure and hot-wire measurements. The results confirm the good performance of the grooves as passive flow-control devices and the capability of grooves to delay flow separation even in a turbulent freestream. The experiments elucidate the physical mechanism leading to the enhanced performance, specifically the reduction of friction losses due to the local recirculation embedded in the groove region. However, the experiments reveal a different behavior in terms of vortex shedding correlation in the spanwise direction with the introduction of grooves of different spanwise extents. Interestingly, the spanwise-extruded grooves exhibit a weaker increase in spanwise correlation of vortex shedding in experiments compared to simulations. This difference is likely due to the presence of freestream turbulence in the wind tunnel, which is absent in simulations. As expected, the introduction of the spanwise-discontinuous groove reduces vortex shedding correlation. Consequently, in experiments the adoption of spanwise-discontinuous grooves yields fewer benefits than those previously found numerically.