A novel resonant mode drives the dynamics of a large-cavity synthetic jet actuator

Abstract

Synthetic Jet (SJ) actuators are an intrinsically complex combination of electronics, electric and mechanical systems. When studied theoretically, these elements are often simplified to coupled damped harmonic oscillators (DHO) that induce a pressure field within the cavity and drive momentum exchange. Thus, the dynamics of an SJ actuator result from coupling these DHOs, naturally leading to a few resonant modes. There is good evidence in the specialized literature of two resonant modes developing in SJ actuators: the membrane/piezoelectric mode and the Helmholtz resonance. In this work, we report on the effect of a new resonant mode that dominates the two traditional modes when it develops. We present evidence that the resonant mode develops when the cavity is much larger than the volume displaced by the actuator. The new resonant mode is biased to lower frequencies and has a flatter response along the frequency band than other resonant modes. We show that jet and vortex velocities mimic the sound pressure curve for the low-frequency range. Its effect mitigates for the higher range due to a delve through shorter stroke lengths, characterized through the well-documented formation criteria as a fixed relation between the Reynolds and the Stokes numbers. We further characterize the new resonant mode by comparing its intensity with standard room modes. We also show that the resonant mode may be dimmed and focused by adding an obstacle in different cavity positions for the lower sound intensities. We consider that the large-cavity dynamics is an additional element that, if integrated as design criteria, could extend the applicability of SJs and their optimum response.