Aerodynamic sources of acoustic resonance in a duct with baffles

Journal of Fluids and Structures

Cody

et al. 2009

Shear flow past a deep cavity can generate self-sustained oscillations, including locked-on flow tones due to coupling between the inherent instability of the separated shear layer and an acoustic mode of the cavity resonator. This investigation focuses on the pressure amplitude response within a deep cavity, as a function of the streamwise length of the cavity opening; for each length, the pressure amplitude response is characterized over a wide range of inflow velocity.Criteria for locked-on flow tones are assessed; they include: a measure of the strength of lock-on, SoL; and the quality factor Q. All self-excited oscillations are assessed using both of these criteria, in order to interpret dimensionless forms of the fluctuation pressure amplitude.The pressure amplitude response of the cavity involves several successive regimes, due to variations of streamwise length L of the cavity opening. These regimes are defined in relation to L/θ, where θ is the momentum thickness of the inflow boundary layer. Below a minimum value of L/θ, flow tones cannot be generated. Furthermore, these regimes are defined in terms of the possible hydrodynamic modes (stages) of the unsteady shear layer and the acoustic modes of the deep cavity.

Section: Flow-acoustic Coupling In Engineering Configurationsmentioning

confidence: 99%

Generation of tones due to flow past a deep cavity: Effect of streamwise length

Yang

Journal of Fluids and Structures

Cody

et al. 2009

“…They include a jet through a system of orifice plates (Hourigan et al, 1990;Stoubos et al 1999), a separated shear layer past a cavity resonator (DeMetz & Farabee, 1977;Elder, 1978;Elder, Farabee & DeMetz, 1982;andNelson et al 1981, 1983), and a separated layer past a resonant side branch in the form of a duct or pipe (Pollack, 1980;Bruggeman et al 1989Bruggeman et al , 1991Kreisels ef al. 1995;Ziada & Biihlmann, 1992;and Ziada & Shine, 1999).…”

Section: Overview Of Flow Tonesmentioning

confidence: 99%

Imaging of the Self-Excited Oscillation of Flow Past a Cavity During Generation of a Flow Tone

Geveci

Oshkai

et al. 2002

Flow through a pipeline-cavity system can give rise to pronounced ff ow tones, even when the inflow boundary layer is fully turbulent. Such tones arise from the coupling between the inherent instability of the shear flow past the cavity and a resonant acoustic mode of the system. A technique of high-image-density particle image velocimetry is employed in conjunction with a special test section, which allows effective laser illumination and digital acquisition of patterns of particle images. This approach leads to patterns of velocity, vorticity, streamline topology and hydrodynamic contributions to the acoustic power integral. Comparison of global, instantaneous images with time-and phaseaveraged representations provides insight into the small-scale and large-scale concentrations of vorticity, and their consequences on the topological features of Streamline patterns, as well as the streamwise and transverse projections of the hydrodynamic contribution to the acoustic power integral. Furthermore, these global approaches allow the definition of effective wavelengths and phase speeds of the vortical structures, which can lead to guidance for physical models of the dimensionless frequency of oscillation.

“…The critical features of power generation according to this relationship are described by a variety of discrete models of the vortices in the shear layer (Howe, 1975;Nelson et al, 1983;Bruggeman, 1989) as well as distributed representations of vorticity (Hourigan et al, 1990;Kriesels et al, 1995;Hohans, 1998). Radavich et al (1999) numericalIy solved the compressible Navier-Stokes equations, thereby accounting far the interaction between the mean flow and the acoustic field.…”

Section: Overview Of Flow Tonesmentioning

confidence: 99%

“…Previous studies have provided considerable insight into the generation of flow tones. The variety of configurations addressed in these studies include a jet through a system of orifice plates (Hourigan et al 1990;Stoubos et al 1999), a separated shear layer past a cavity resonator (DeMetz & Farabee, 1977;Elder, 1978;Elder et al, 1982;and Nelson et al 198 1, 1983), and a separated layer past a resonant side branch in the form of a duct or pipe (Pollack, 1980;Bruggeman et al 1989Bruggeman et al , 1991 Kreisels et al 1995 ;Ziada & Buhlmann, 1992;and Ziada & Shine, 1999). Rockwell (1983) and Blake (1986) describe the common elements associated with purely hydrodynamic oscillations.…”

Section: Overview Of Flow Tonesmentioning

confidence: 99%

See 1 more Smart Citation

Imaging of acoustically coupled oscillations due to flow past a shallow cavity: effect of cavity length scale

Oshkai

Geveci

Journal of Fluids and Structures

et al. 2005

Flow-acoustic interactions due to fully turbulent inflow past a shallow axisymmetric cavity mounted in a pipe are investigated using a technique of high-imagedensity particle image velocimetry in conjunction with unsteady pressure measurements. This imaging leads to patterns of velocity, vorticity, streamline topology, and hydrodynamic contributions to the acoustic power integral. GlobaI instantaneous images, as well as time-averaged images, are evaluated to provide insight into the flow physics during tone generation. Emphasis is on the manner in which the streamwise length scale of the cavity alters the major features of the flow structure.These image-based approaches allow identification of regions of the unsteady shear layer that contribute to the instantaneous hydrodynamic component of the acoustic power, which is necessary to maintain a flow tone. In addition, combined image analysis and pressure measurements allow categorization of the instantaneous flow patterns that are associated with types of time traces and spectra of the fluctuating pressure. In contrast to consideration based soIeiy on pressure spectra, it is demonstrated that lockedon tones may actually exhibit intermittent, non-phase-locked images, apparently due to low damping of the acoustic resonator. Locked-on flow tones (without modulation or intermittency), locked-on flow tones with modulation, and non-locked-on oscillations with short-term, highly coherent fluctuations are defined and represented by selected cases. Depending on which of,these regimes occur, the time-averaged Q (quality) -factor and the dimensionless peak pressure are substantially altered.