Jet Switching Phenomenon for a Plunging Airfoil

An experimental investigation has been carried out on rigid and flexible airfoils oscillating in still fluid. Laser Doppler Velocimetry and Particle Image Velocimetry measurements were taken over a range of airfoil flexibilities, plunge frequencies and plunge amplitudes. It was found that the vortex pairs generated by the oscillating airfoil move at angle to the chordwise direction. The deflection angle of the induced jet was observed to change periodically in time. The switching period was found to increase with increasing airfoil stiffness and to decrease with plunge frequency and amplitude. Over the range of frequencies, amplitudes and stiffnesses tested the switching period was found to be two orders of magnitude greater than the heave period. The development of the vorticity field for upward and downward deflected jets, as well as the transition between the two modes, was captured. Jet switching was still observed in experiments when the side walls were removed, indicating that wall effects are not responsible for the switching phenomenon.

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