Dynamics of tip vortex flow over three dimensional hydrofoils by LDV measurements

Abstract

Tip vortices are observed downstream of the flow past the three-dimensional hydrofoils used in propeller, wind and tidal turbines and they largely determine the forces exerted on such structures. In this study, we present the Laser Doppler Velocimetry (LDV) measurements in the flow past three-dimensional hydrofoils namely NACA 4412, NACA 16020, NACA 66(2)415 inclined at an angle of 10° with respect to the free-stream velocity, U∞. The three-dimensional hydrofoils have a semi-span, s = 60 mm, chord lengths, c = 48 mm, 60 mm & 80 mm which correspond to three aspect ratios, ∧ = 2.5, 2.0 & 1.5, respectively, at the chord length based Reynolds number, Re ranging from Re = 1.92 × 105 ？ 6.39 × 105. LDV measurements were performed at several coordinate points in zy？plane perpendicular to the stream-wise, x direction, measuring the mean stream-wise, u(z, y) and mean velocity component along the z-axis, w(z, y) together with the root-mean square of their fluctuations in the plane located at a stream-wise distance, x/c = 1.0. The swirling strength of the tip vortices are qualitatively evident from the measurements of z？ velocity component field, w(z, y) for all the hydrofoils, at various aspect ratios and Reynolds numbers. It is observed that the magnitude of normalized axial velocity deficit, (u(z, y) ？ U∞)/U∞ and normalized turbulent kinetic energy, TKE(z, y) are dependent on the type of hydrofoil, their aspect ratio, ∧ and the Reynolds number, Re. TKE within the region of the wake and vortex contain about 99% of the peak magnitude of TKE observed in the x/c = 1.0 plane. The vortex center, vortex position and vortex radius are shown to be dependant on the hydrofoil shape, aspect ratio and Reynolds number. ？ 2022 Elsevier Ltd