Jet switching phenomenon for a periodically plunging airfoil

An experimental investigation has been carried out on rigid and flexible airfoils oscillating in still fluid. It was found that the vortex pairs generated by the oscillating airfoil move at an 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 increasing heave frequency and increasing amplitude. Over the range of frequency, amplitude, and stiffness 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 with the particle image velocimetry measurements. The deflection of the jet, and thus the jet switching effect, was found to diminish with increasing free stream velocity (decreasing Strouhal number).