Vortex shedding from blunt and oblique trailing edge hydrofoils
The phenomenon of vortex shedding behind a hydrofoil is an important issue from both scientific and technical point of view. The resulting fluctuating forces may lead to excessive vibrations and premature cracks in the hydraulic machines. According to previous studies, it is well known that an oblique trailing edge, also called "Donaldson cut", reduces the vibration in comparison with a blunt trailing edge; however the physics of the problem is still poorly understood. The purpose of the present work is the experimental investigation of vortex shedding dynamics in the wake of oblique and blunt trailing edge NACA0009 hydrofoils. Experiments are performed at zero incidence angle and high Reynolds numbers, ReL =5 10 5 - 2.9 10 6. The wake velocity profile is measured by two-component Laser Doppler Velocimetry. Cavitation occurrence in the core of the vortices is used as a mean of wake visualization with the help of a high speed camera. We have found that vortex induced vibration is significantly reduced for oblique trailing edge hydrofoil in comparison with the truncated one, which is in agreement with former reports. A disorganization of the Karman vortex street in the near wake is believed to be the reason of this vibration reduction. The high speed movies clearly show that the alternate shedding of the vortices turns into almost simultaneous shedding at the hydrofoil trailing edge. As a result, the upper and lower vortices pair with a significant thickening of the lower vortex core and a reduction of its strength. Consequently, the fluctuating lift, which is the cause of the structural vibration, is also reduced by the oblique truncation. We believe that this result stands for a basis to better optimize the trailing edge of turbine blades in order to further decrease the flow induced vibration.