No abstract

No abstract

A rod in crossflow is a technique known for its simple and effective means of suppressing cavity tones. Although several hypotheses have been put forward regarding its working principle, no validated explanation exists. In the present study we investigate whether the cylinder, through its wake, changes the stability characteristics of the shear layer that develops over the cavity. The present study pertains to a cavity of length to depth ratio L/D=2, for subsonic Mach numbers ranging from 0.5 to 0.8. The upstream boundary layer was found to be turbulent for all cases considered. We use linear stability theory in the spatial, compressible and inviscid formulation for our study. We construct artificial velocity profiles that are prototypical of the experimentally measured velocity profiles, to investigate how the wake of the cylinder influences the stability of the shear layer. Parametric analysis of these profiles revealed several interesting stability characteristics, viz., different stability modes, the dominant feature of the velocity profile that influences the stability characteristics of the shear layer. We also calculate integrated growth rates, based on a series of profiles measured along the length of the cavity, for a variety of baseline and control configurations. Comparison of these integrated growth rates with the acoustic suppression data showed that the link between the two is weak. Thus the ability of the rod to suppress the cavity resonance is not directly explained by linear stability analysis of the modified shear layer, for the configurations considered.

This work is an experimental study of high subsonic jet impingement tone suppression. We begin by documenting the characteristics of the impingement tone for various Mach numbers and standoff (nozzle exit to ground plate) distances. The results revealed frequency staging and the presence of two types of impingement tones. A novel feature of our work is the use of four miniature high-frequency actuators known as powered resonance tubes that were located circumferentially around the main jet nozzle. The powered resonance tubes were capable of producing high amplitude acoustic excitation over a range of frequencies, up to 17.5 kHz. Our target excitation frequency range was about 3-5 times that of the natural flow instability. Using high-frequency excitation, tonal suppression levels as high as 20 dB and broadband suppression levels as high as 5-10 dB were obtained. The mass addition rate from the powered resonance tubes was of the order of 2% of the mass flow rate from the main jet. Mass flow reductions could be obtained under conditions when the powered resonance tube resonated strongly. Our results suggest that appropriately designed miniature powered resonance tube actuators have potential for use in flow control applications. Nomenclature a 1 = speed of sound corresponding to the freestream conditions C 1 , C 2 = correction factors for the main jet velocity and the acoustic velocity, respectively D = main nozzle exit diameter f = frequency of the acoustic wave h = standoff distance between the nozzle exit and ground plate M = Mach number n = number of the impingement stage p = phase shift associated with acoustic wave reflection u = flow velocity = acoustic wavelength Subscripts j = main jet properties PRT = powered resonance tube

No abstract