TPIV study for near-field tip vortex from an elliptical hydrofoil

Tip vortex cavitation （TVC） is a common type of cavitation in hydraulic machinery and marine propulsion. Since TVC inception is highly relevant to the vortical flow around the blade tip of turbines and propellers, it is essential to give more insights into the flow field of the tip vortex to reveal the inherent relationship between flow properties and TVC. Measurement for the tip vortex from an elliptical hydrofoil has been conducted in a high-speed cavitation tunnel utilizing tomographic particle image velocimetry （TPIV） with high time-resolution. The results show that the wandering motion of the tip vortex is noticeable in the near field. The time-averaging process without taking into account wandering motion can bring extra errors into the time-averaged flow field. Therefore, it is necessary to filter out the wandering motion for the quantitative analysis on vortex characteristics. The tip vortex is under roll-up process and can be greatly affected by the shear layer from the hydrofoil, which contributes to the asymmetric circumferential velocity distribution and a high-axial-velocity area between the shear layer sheet and the vortex core. The tip vortex contains the most of the turbulence energy within its core and the turbulence energy is dominated by the vertical and spanwise velocity fluctuations which are considered as the main source of the fluctuating pressure in the core center combining with previous researches.