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Fluidic oscillators show promise for use in aerodynamic flow control applications, with many studies reporting oscillation frequencies in the 1–10,kHz range. Spyropoulos[1] introduced a 'sonic' oscillator whose oscillation frequency depends on the inlet flow rate. The purpose of this paper is to demonstrate the existence of a second mode of operation (Mode II) for such an oscillator, with a separate physical mechanism to the traditional, flow rate-dependent mode (Mode I). Mode II is shown to be a back-pressure driven oscillation that is largely independent of flow rate once instigated. This is explained by a stationary wave formed along the outlet paths, and occurs when conditions on the degree of back pressure and the weakening of the Coanduă attachment strength are met. For a fixed device geometry, the conditions reduce to a minimum flow rate threshold, so that the combination of high flow rate and constant oscillation frequency could make Mode II an attractive flow control solution in an industrial context where minimising device size is often critical.

The goal of this paper is to present the behaviour of a jet shear layer in response to acoustic excitation from a signal processing perspective. The main idea is that the vortices that roll-up in the jet shear layer are similar to the discrete samples of a digital control system, and hence that the Nyquist-Shannon sampling theorem should apply. We further hypothesize that the strength of a vortex is determined by the mean amplitude of the excitation waveform during its creation. We also argue that, at least in some cases, demodulation occurs as a result of the vorticity signal generated by the convection of discrete vortices past a point in the shear layer. This vorticity signal is related to the amplitude modulation (AM) excitation waveform by a half-wave rectification operation, a common implementation of an AM demodulator. To investigate these ideas, a free, round jet that is excited upstream of the nozzle is studied using particle image velocimetry (PIV). Experiments are conducted that confirm that the sampling theorem applies and an aliased response is observed when the Nyquist limit is exceeded. Previous authors have attributed demodulation to a vortex merging mechanism, but we demonstrate that merging is not always required for demodulation, and suggest that it is one of two mechanisms at play.

Shear layers act as demodulators when subjected to amplitude modulated, acoustic perturbations. Recent work explained that the demodulation is a result of the hypothesized relationship between the excitation waveform and the vorticity signal of the large scale structures, which was represented by a half-wave rectification model. In this paper, this model is explored by overmodulating the excitation signal amplitude. The rectifier model predicts several effects of overmodulation on the flow response, including a doubling of the demodulated response frequency. To validate these predictions, a free, round jet that is excited acoustically upstream of the nozzle is studied using particle image velocimetry (PIV). Analytical results from the model are confirmed using a second experimental setup where a jet that emerges from a nozzle and attaches to an adjacent, inclined wall is excited acoustically. Beyond the insight it provides into shear layer vortex dynamics, overmodulation serves as a useful excitation technique for practical applications, where the shear layer response frequency can be increased at the expense of the response amplitude.

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