A resonant interaction between a large aspect ratio rectangular jet and a flat-plate is addressed in this experimental study. The plate is placed parallel to but away from the direct path of the jet. At high subsonic conditions and for certain relative locations of the plate, the resonance accompanied by an audible tone is encountered. The trends of the tone frequency variation exhibit some similarities to, but also marked differences from, corresponding trends of the well-known edgetone phenomenon. Under the resonant condition flow visualization indicates a periodic flapping motion of the jet column. Phase-averaged Mach number data obtained near the plate's trailing edge illustrate that the jet cross-section goes through large contortions within the period of the tone. Farther downstream a clear 'axis switching' takes place. These results suggest that the assumption of two-dimensionality should be viewed with caution in any analysis of the flow.

This study addresses unwanted high intensity noise sometimes encountered in engine test facilities. Model-scale experiments are conducted for a round jet discharged into a cylindrical duct. In most cases, the unwanted noise is found to be due to longitudinal resonance modes of the duct excited by the random turbulence of the jet. When the 'preferred mode' frequency of the jet matches a duct resonant frequency there can be a locked-in 'super-resonance' accompanied by a high intensity tone or 'howl'. Various techniques are explored for suppression of the unwanted noise. Tabs placed on the ends of the duct are found ineffective; so are longitudinal fins placed inside the duct. A rod inserted perpendicular to the flow ('howl stick') is also found generally ineffective; however, it is effective when there is a super-resonance. By far the most effective suppression is achieved by a wire-mesh screen placed at the end of the duct. The screen not only eliminates the superresonance but also the duct mode spectral peaks. Apparently the screen works by dampening the velocity fluctuations at the pressure node and thereby weakening the resonant condition.

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[Abstract] A multiple-point Rayleigh scattering diagnostic is being developed to provide mass flux measurements in gas flows. Spectroscopic Rayleigh scattering is an established flow diagnostic that has the ability to provide simultaneous density, temperature, and velocity measurements. Rayleigh scattered light from a focused 18 Watt continuous-wave laser beam is directly imaged through a solid Fabry-Perot etalon onto a CCD detector which permits spectral analysis of the light. The spatial resolution of the measurements is governed by the locations of interference fringes, which can be changed by altering the etalon characteristics. A prototype system has been used to acquire data in a Mach 0.56 flow to demonstrate feasibility of using this system to provide mass flux measurements. Estimates of measurement uncertainty and recommendations for system improvements are presented.

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