The paper describes experiments on a sinusoidally pitching NACA 0012 hydrofoil with reduced frequency $0.16\le k\le 2.51$ and angle of attack amplitude $6^{\circ }\le {\alpha }_m\le {34}^{\circ }$. It extends the chord Reynolds number range with respect to earlier studies to $\text{Re}=5.9\times {10}^4,1.2\times {10}^5,1.9\times {10}^5,\text{and}2.4\times {10}^5$. Lift, drag, and moment are directly measured and compared with predictions from linear inviscid theory, while wake flowfields are visualized by phase-locked particle image velocimetry. Three vortex shedding regimes, i.e., the undulating wake, the reverse von Kármán vortex street, and the leading-edge vortex, are identified from the phase-averaged vorticity fields at different $k$ and ${\alpha }_m$. The nonlinear effect on statistical features of the force and moment responses gradually increases with ${\alpha }_m$. The viscous effect leads to relatively large deviations of the measured responses from the linear inviscid models at low $k$. With increasing Re the measured values gradually approach those predicted by the models. Moreover, linear inviscid theory cannot capture the ${\alpha }_m$-dependent behavior of the drag and moment phase lags ${\psi }_d$ and ${\psi }_m$, respectively, resulting in a larger deviation from the measured values. A linear variation of ${\psi }_m$ with ${\alpha }_m$ is found at $k$ and Re for which the reverse von Kármán vortex street dominates. Within the trailing-edge shedding regimes of the undulating wake and the reverse von Kármán vortex street, the force and moment responses are largely insensitive to the wake pattern at varied ${\alpha }_m$. However, in the leading-edge vortex regime, at low $k$ and large ${\alpha }_m$, the flow remains no longer attached and the time-dependent pitching moment is found to be strongly affected by the leading-edge vortex over the wing.

Close

PrevNext

View figure in article

• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 

• Aerodynamics
• Flow-structure interactions
• Vortex flows