The Opposing Planar Jet Oscillator

Abstract

The fundamental nature of the flow oscillations which are generated by two opposing planar jets is investigated. Particular attention is given to the underlying mechanism which sustains the oscillations over a wide range of flow parameters. The jet columns are observed to undergo large lateral deflection oscillations once in each direction per cycle, in an asymmetric manner. Extensive characterization of the jet oscillations over a wide range of flow parameters is established, including both the aeroacoustic response, as well as the unique flow features which are synchronized with the oscillations. The impingement region and circulation regions in each quadrant of the flow field are shown to play essential roles in sustaining the oscillations, as the pressurization of the impingement region causes the jets to initially deflect away from the centerline, while the low-pressure regions which form in the circulation zones drive the jet columns back towards, and ultimately across, the centerline. A number of interesting observations are made regarding the oscillation characteristics, including a dependence of the oscillation frequency on the jet aspect ratio, which helps explain much of the discrepancy in the Strouhal numbers reported in the literature to date. Furthermore, the nature of the sound-source field is investigated including the directionality of the various frequency components which are radiated.

Unique mitigation strategies of the opposing planar jet oscillations are also explored by attempting to disrupt the circulation regions through the use of splitter plates. The oscillations are weakened considerably as the development and convection of the circulation zones is impeded. Preventing the circulation flow from interacting with the jet exit region drastically increases the effectiveness of the splitter plates, as even short splitter plates are shown to completely eliminate the oscillations. This demonstrates a very effective mitigation strategy of the opposing planar jet oscillator which is ideal for a variety of practical applications.

One of the main challenges of the current investigation into the opposing planar jet oscillator is the extent to which the detailed time-varying pressure field can be resolved. Since it is not possible to experimentally detail the time-varying pressure field of the opposing planar jets, a novel PIV-based pressure field mapping technique is developed and benchmarked. A separate apparatus consisting of a planar jet impinging on a v-shaped plate is utilized to benchmark the proposed technique. This technique effectively resolves the features of the time-varying pressure field which are synchronized with the flow oscillations and helps circumvent many of the challenges which existing PIV pressure field mapping techniques face. It also provides a valuable tool for researchers to simultaneously determine the kinematic and dynamic aspects of various flow phenomena in a variety of fields, especially those in the area of aeroacoustics and fluid-structure-interaction.