An experimental study regarding the effect of streamwise vorticity on trailing edge vortex induced vibrations of a hydrofoil

Abstract

A set of experiments testing the effect on vortex induced vibrations from three different trailing edge designs on a generic hydrofoil has been carried out at the Waterpower Laboratory of NTNU. One of the designs, a Donaldson type semi-blunt trailing edge not a-typical for use in hydraulic machinery serves as a reference. The other two, a trailing edge with boundary layer vortex generators and a trailing edge with rounded serrations cut into the blade, both introduce streamwise vorticity in the wake of the hydrofoil, as shown by particle image velocimetry measurements. The trailing edge of the hydrofoil is made exchangeable, so that a direct comparison of the strain intensity levels measured with a structurally embedded strain gauge bridge located close to the trailing edge can be made. The results indicate that the vortex generators are effective at mitigating the lock-in state of resonance and significantly reduces the maximum strain intensity measured, compared to the more conventional reference design. Velocity measurements both at lock-off conditions as well as close to resonance reveal a de-correlation of the streamwise velocity fluctuations in the wake for this design, suggesting that the von Kármán type vortex shedding is weakened even at resonance conditions. A de-correlation of the streamwise velocity fluctuations in the wake of the serrated trailing edge is also observed at lock-off conditions, but the vortex shedding appear to become gradually more coherent, as well as strengthened, for this design as the foil vibrations increase closer to resonance.