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The purpose of the current study is to verify the vortex wake structure results of a flapping piezoelectric plate with the heaving airfoils of previous studies and then explore at low Reynolds numbers (<15000, the designated flight domain of Nano Air Vehicles) how the vortex wake structure depends on advance ratio, angle of attack, flapping amplitude, and by span-wise location on the piezoelectric flapping plate. Using Particle Image Velocimetry, a non-intrusive optical measurement technique, the two dimensional velocity, turbulent kinetic energy, and vorticity information was obtained to better understand the flight regime of unsteady flapping flight. As previously shown in works such as Lai and Platzer (1999), unsteady flapping flight conditions, advance ratio below unity, produces vortex wake structures that generate thrust and increasing advance ratio reduces this benefit. Increasing advance ratio obtained by varying incoming flow velocity or the flapping amplitude was compared by vortex structure as well as the thrust generated by flapping, which was found to decrease more rapidly with decreasing the flapping amplitude. The effect of changing the flapping amplitude has an effect on both the wake structure type and size. Increasing angle of attack causes greatly increased drag but did not significantly change the vortex structure type seen for the angles of attack used in this experiment. The effect of span-wise location was found to be independent of the decreased flapping amplitude and understanding the phenomenon demands further study.

An experimental study of the vortex structures in the wake of a piezoelectric flapping plate for Nano Air Vehicle applications