Experimental characterization of drag-reducing riblets in High-Reynolds number turbulent pipe flow

Nowadays, the need for energy-efficient transportation is more pressing than ever, and the study of aerodynamics focused on drag reduction has become a critical research area. As a matter of fact, finding effective strategies to reduce drag can lead to significant environmental and economic benefits. Extensively investigated over recent decades, riblets represent a proven passive drag-reduction technology with significant potential for enhancing aerodynamic efficiency. However, a substantial gap persists between laboratory-scale Reynolds numbers and those encountered in practical engineering applications. This thesis aims at bridging this gap by investigating riblet physics at high-Reynolds numbers in the Long-Pipe wind-tunnel in the CICLoPE laboratory. Experiments were conducted employing Hot-Wire Anemometry (HWA) to characterize the drag-reduction capabilities of adhesive triangular riblets with a tip distance of 166 μm and a tip height of 82 μm spanning a broad range of high-Reynolds numbers, from about 10000 up to 30000. Mean-velocity profiles and turbulent statistics have been exploited to estimate the riblets’ drag-reducing performances. Although several anomalies were observed, mainly related to uncertainties in the estimation of the friction velocity and structural setup issues, the qualitative results of the study are encouraging and seem to validate the effectiveness of riblets at high-Reynolds numbers. Addressing these specific experimental challenges will be fundamental to transition from qualitative observations to a precise quantification of riblets’ drag-reduction capabilities in high-Reynolds number regimes.