MANIPULATION OF FREE SHEAR FLOWS USING PIEZOELECTRIC ACTUATORS

An air jet emanating from a square conduit having an equivalent diamet er of 4.34 cm and a centreline velocity of 4 m/s is forced using four resonantly driven piezoelectric actuators placed along the sides of th e square exit. Excitation is effected via amplitude modulation of the resonant carrier waveform. The flow is normally receptive to time-harm onic excitation at the modulating frequency but not at the resonant fr equency of the actuators. When the excitation amplitude is high enough , the excitation waveform is demodulated by a nonlinear process that i s connected with the formation and coalescence of nominally spanwise v ortices in the forced segments of the jet shear layer. As a result, th e modulating and carrier wave trains undergo spatial amplification and attenuation, respectively, downstream of the exit plane. Strong insta bilities of the jet column are excited when the jet is forced at phase relationships between actuators that correspond (to lowest order) to the azimuthal modes m = 0, +/- 1, +/- 2, and - 1 of an axisymmetric fl ow. The streamwise velocity component is measured phase locked to the modulating signal in planes normal to the mean flow. Resonantly drivin g the actuators with different carrier amplitudes results in a distort ed mean flow having a featureless spectrum that can be tailored to pro vide favourable conditions for the introduction and propagation of des irable low-frequency disturbances.