Flow over airfoil model covered by bio-inspired herringbone riblets

Abstract

Flight feathers of birds are featured by the typical herringbone pattern, which is consisted of a central shaft and divergent barbs on both sides. In this work, bio-inspired herringbone riblets are embedded into the suction side of a NACA0012 airfoil model, with an attempt to explore their roles on the flow and fluid force. Experiments are conducted in a water tunnel at a Reynolds number of Re = 2 × 10⁵, based on incoming freestream velocity and airfoil cord length c. While the lift and drag forces of the airfoil model are measured by a load cell, flow fields over the suction side of the airfoil model are captured by the particle image velocimetry (PIV) technique. The herringbone-ribbed suction side of the airfoil model is defined by the divergent angle β (= 60°) of the riblets, the spanwise wavelength λ (= 0.2c and 0.4c) of the repeating herringbone pattern, as well as the riblet height h (= 0.6 %c and 1.2 %c). Results from the force measurements reveal that the airfoil models with the herringbone-ribbed suction side outperform their smooth counterparts and the baseline NACA0012 model, with the stall being significantly postponed from 10° to over 16° while the maximum time-mean lift coefficient being remained nearly unaffected. This is attributed to the transition from laminar to turbulent boundary layers, thus associated with substantially suppressed flow separation, over the airfoil models with the bio-inspired riblets being covered on the suction side. On the other hand, it is observed that the time-mean lift coefficient is considerably reduced whilst the drag coefficient is marginally increased at the angle of attack α < 12° for the airfoil models with the bio-inspired riblets being covered on the suction side, compared with those of their smooth counterparts and the baseline NACA0012 model.