Comparative investigation of the effect of tip fins on vortex flow in non-cavitating and cavitating conditions

Abstract

Tip vortex flow is crucial in designing and operating several engineering applications. In this work, the effect of tip fins on vortex flow in non-cavitating and cavitating conditions is investigated experimentally. Rectangular hydrofoils with and without fin tips are considered. The hydrofoil is placed in a water tunnel at an angle of attack of α = 15° and the velocity field is measured using laser Doppler velocimetry (LDV) at several plane sections. A high-speed camera is used to visualize the tip vortex cavitation (TVC), and pressure transducers are employed for acoustic measurements. The operational chord Reynolds number (Re) ranges from 3.3 × 105–6.0 × 105. The non-cavitating flow results reveal that Re has no discernible effect on the tip vortex characteristics, although the axial velocity at the tip vortex core is significantly affected (accelerated to 1.25 times the inlet velocity behind the hydrofoil without a fin tip, decelerated by a factor of 0.75 for the hydrofoil with a fin tip). The fin tip slightly reduces the turbulent kinetic energy (TKE) and transversal velocity at the vortex core. The LDV measurements indicate that the tip fin affects the development of the tip vortex. The fin tip increases the dissipation of TKE and axial velocity in the tip vortex core downstream of the hydrofoil by causing chaotic flow mixing. The cavitating flow results indicate that TVC first occurs at a cavitation number of σ = 3.30 and Re = 3.71 × 105 around the tip of both hydrofoils. Leading-edge TVC appears at the tip of the hydrofoil without the fin and develops as Re increases, before interacting and merging with the trailing-edge TVC when Re > 4.18 × 105. There is a peak frequency range of the sound pressure level in the foil without a fin, but this disappears in the case of the fin tip.