A nonlinear room mode determines the operating conditions of a large-cavity synthetic jet actuator at low frequencies

TL;DR

This paper identifies a third low-frequency resonant mode in large-cavity synthetic jet actuators, showing its impact on performance and how cavity modifications can control it, extending the design considerations for these devices.

Contribution

It reveals a previously unreported low-frequency resonant mode in large-cavity SJ actuators and demonstrates how cavity modifications influence this mode and overall performance.

Findings

A third low-frequency resonant mode exists in large-cavity SJ actuators.

Cavity obstacles can dim and focus this resonant mode.

The mode dominates at higher sound intensities, affecting performance.

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 performance of an SJ actuator results 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 third resonant mode that develops at very low frequencies due to a cavity much larger than the volume displaced by the actuator. We present evidence that the large-cavity dynamics determine the SJ performance in combination with the well-described formation criteria. We compare the intensity of this resonant mode with the first room modes using standard frequency analysis. Unlike typical room modes, the distribution of this resonant mode is very biased to lower frequencies. 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. This mode overcomes the Helmholtz resonance, dominating the dynamics for higher sound intensities. 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 smaller stroke lengths, characterized as a fixed relation between the Reynolds and the Stokes numbers. We consider that the large-cavity dynamics is an additional element that, if integrated as design criteria, could extend the optimum frequency response of SJs.