How oblique trailing edge of a hydrofoil reduces the vortex-induced vibration

Abstract

The effect of hydrofoil trailing edge shape on the wake dynamic and flow induced vibration is investigated at high Reynolds number, Re=0.5 × 106–2.9 x 106. Two NACA 0009 hydrofoils with blunt and oblique trailing edges are tested. The velocity field is surveyed with the help of Laser Doppler Velocimetry (LDV), and Particle-Image-Velocimetry, (PIV). Proper-Orthogonal-Decomposition (POD) is used to extract coherent structures from PIV data. Besides, flow induced vibration measurements and high-speed visualization are also performed. A significant reduction of vortex induced vibration is obtained with the oblique trailing edge, in accordance with former reports. High speed videos clearly demonstrate that for both tested hydrofoils, the alternate vortices detach from upper and lower corners of the trailing edge. Due to the oblique truncation, the lower detachment location is shifted upstream with respect to the upper one. Therefore, as the upper vortex rolls up, it coincides with the passage of the lower vortex, leading to their collision. This strong interaction leads to a redistribution of the vorticity, which no more concentrates within the core of Karman vortices. The analysis of the phase locked average of velocity profiles reveals that the oblique truncation leads to a thickening of the core of upper and lower vortices as well as a disorganization of the alternate shedding in the near wake, recovers downstream. We strongly believe that the collision between upper and lower vortices and the resulting vorticity redistribution is the main reason of the vibration reduction obtained with oblique trailing edge. This result paves the way for further optimization of the trailing edge shape.