# Tip Vortex Cavitation and Induced Noise Characteristics of Hydrofoils
## 1 Introduction  
Tip vortex cavitation has been documented since the early works of Sir [PERSON] [1] in the year 1895, with photographs in a cavitation tunnel. Wing-tip vortices are characterized by large swirling motion along the axial direction, low-pressure in the core region and large turbulent fluctuations in the vortex center [2, 3]. [PERSON] [2] performed Laser Doppler Velocimetry (LDV) and Stereo Particle Imaging Velocimetry (SPIV) in a flow over an elliptical hydrofoil with National Advisory Committee for Aeronautics (NACA) hydrofoil section, namely NACA 16-020 and observed large magnitudes of vorticity and large gradients of tangential velocity about the vortex core. By numerical computations, [PERSON] [3] showed large vorticity magnitudes and low pressure regions in the core of the vortex behind an elliptic NACA 16-020 elliptic hydrofoil of a finite aspect ratio. [PERSON] [4] explained the effects of wing planform on the vortex features observed in the leeward side of the wing and the cavitation in the low-pressure vortex core in their experiments. The low pressure vortex core in the leeward side of the hydrofoils undergo cavitation once the pressure in the vortex core falls below a critical value called vapor pressure at its corresponding fluid temperature [5]. Finding the conditions when cavitation inception happens has attracted the attention of many researchers [6]. Based on empirical scaling laws, [PERSON] [7] showed the viscosity and the vortex strength to be crucial in determiningthe cavitation inception. The effects of water quality in determining the cavitation inception has been studied experimentally by [PERSON] [8]. [PERSON] [9] showed the effects of dissolved oxygen percentage and measured water quality in determining the cavitation inception and noise characteristics of a cavitating NACA 0020 hydrofoil experimentally. Cavitation inception number for any hydrofoil section has been shown to be related to its angle of the inclination and the Reynolds number of the flow by semi-empirical observations by [PERSON] [10]. [PERSON] [11], [PERSON] [12] and maines [13] have further experimentally verified the validity of [PERSON]'s power-law scaling relation for the cavitation inception number. Velocity and length scale effects on the cavitation inception number has also been shown experimentally by [PERSON] [12]. Experimentally, [PERSON] [14] has shown the velocity, length, viscous and turbulence scaling effects on the cavitation inception number. He also showed the effects of water quality in determining the cavitation inception number.  
A unique, strong frequency associated with the cavitating hydrofoil, called \"singing\" was shown experimentally by [PERSON] [15] using hydrophone measurements in an elliptic planform hydrofoil in a cavitation tunnel. [PERSON] [16] showed that the \"singing\" occurred over a range of cavitation numbers at a given coefficient of lift. [PERSON] [17] showed a correlation between the deformation of bubble nucleus and the noise generated in tip vortex cavitation. [PERSON] [18] and [PERSON] [19] have shown the sharp noise associated with the tip vortex cavitation in their laboratory experiments. In the current study, the cavitation inception and the noise characteristics of NACA 16-020 and NACA 66(2)-415 hydrofoils are studied using high speed imaging and unsteady pressure measurements. Novelty of the current work is in simultaneous comparison of the high-speed images with the unsteady pressure spectrum at each stage of the cavity evolution with the decrease of the cavitation number.