Influence of amplitude and frequency modulation on flow created by a synthetic jet actuator

Synthetic jet actuator is a compact device that has applications in drag reduction, heat transfer enhancement and controlled mixing. An isolated synthetic jet is produced by the interaction of a train of vortices that are formed by alternate ejection and suction of fluid across an orifice such that the time-averaged mass flux is zero. A unique feature of a synthetic jet is that it is formed entirely from the working fluid of the flow system in which it is utilized. The temporal and spatial characteristics of the synthetic jet are dependent on the actuator operating conditions. Amplitude and frequency modulation of the actuator offer possibility of better flow control. No systematic study on the effect of amplitude modulation on synthetic jet characteristics is available in the literature. The present study reports the properties of the synthetic jet as a function of both excitation voltage and modulation frequency. Three modulating frequencies namely, 10 Hz, 25 Hz, and 50 Hz and excitation voltages in the range 5–50 V has been considered. Schlieren visualization has been used to characterize the jet spread while hotwire anemometry provides detailed time-averaged and fluctuating velocity fields. The present study shows a significant impact of amplitude modulation on the spatial and temporal evolution of synthetic jets. The rms velocity fluctuations increase due to amplitude modulation and the effect is more pronounced at low modulating frequencies. The behavior of a synthetic jet in the presence of a neighboring synthetic jet, i.e. the behavior of a dual synthetic jet configuration has also been investigated. The spreading of the jet is seen to increase considerably in the presence of a second neighboring synthetic jet.