Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BK: Acoustics II |
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Chair: Scott Morris, University of Notre Dame Room: Hilton Chicago Joliet |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BK.00001: Relation between instability waves and coherent sound in a subsonic jet investigated with phased microphone arrays Takao Suzuki, Tim Colonius To investigate the role of instability waves for noise generation in a subsonic round jet, we measure instability waves with a conical phased-microphone array in the near field and compare them with coherent sound in the far-field. By introducing a beam-forming algorithm with eigenfunctions obtained from linear stability analysis, we determine amplitudes of the axi-symmetric and first two azimuthal modes. In addition, data acquired in the near field provide streamwise evolution as well as local wavenumbers (i.e. phase velocity) of instability waves. We compare these results with coherent sound signals obtained from a far-field microphone array at NASA Glenn Research Center. Cross spectral matrices obtained from the far-field array provide sound radiation patterns of POD modes. We focus on the azimuthal mode balance at frequencies near the peak radiation and investigate the effects of chevron nozzles in this presentation. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BK.00002: Characterization Of Side Branch Resonators Using Particle Image Velocimetry Peter Oshkai, Ting Yan Acoustically-coupled flow past a coaxial deep cavity (side branch) resonator mounted in a duct is investigated using digital particle image velocimetry. Various resonator geometries are characterized in terms of spatial structure and strength of the corresponding acoustic power source. Emphasis is on the effect of the separation between the coaxial side branches on the interaction between separated shear layers that form across the side branch openings. Phase-averaged images of the flow in conjunction with unsteady pressure measurements are evaluated in order to provide insight into the mechanisms of acoustic power generation. When the coaxial side branches are located relatively far away from each other, each of them forms an independent acoustic source. As the distance between the side branches decreases, interaction between the associated oscillating shear layers results in formation of a single acoustic source of complex spatial structure. In addition to the effects of the main duct width, the structure and strength of the acoustic source also depend on the stage of transverse oscillations of the separated shear layers. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BK.00003: Noise Sources in Baseline and Controlled High Reynolds Number and High Speed Jets J. Kastner, J.-H. Kim, M. Samimy The importance of dynamic processes involving large coherent structures in entrainment, mixing, and noise generation in free jets and shear layers is well established. Active control of dynamics of these structures is an effective and desirable way of manipulating such processes in these flows. Localized Arc Filament Plasma Actuators (LAFPA) developed at the Gas Dynamics and Turbulence Laboratory (GDTL) have high bandwidth and amplitude and are uniquely suited for such a task. Eight actuators were distributed around the exit of a 2.54 cm axisymmetric nozzle and operated over a wide range of frequencies to cover the jet column and the initial shear layer instabilities. Two jets were examined, a Mach 0.9 jet (Re {\#} of 7.6 x 10$^{5})$ and a Mach 1.3 ideally expanded jet (Re {\#} of 1.1 x 10$^{6})$. The jets were forced with azimuthal modes 0, 1, $\pm $1 {\&} $\pm $2. A three-dimensional microphone array with 8 microphones located in the jet's far field was used to investigate noise sources. The array could locate the source of high amplitude noise events in space and time and also provide metrics including SPL and correlations. Detailed results of this work will be presented and discussed. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BK.00004: Ducted Rotor Acoustics David Stephens, Scott Morris The effect of blade loading on the far field acoustics of a subsonic ducted rotor was studied using an experimental model. Ducted rotors create dipole sound through many mechanisms that lead to a complex acoustic signature with broadband and tonal features. These sound producing mechanisms include unsteady blade loading due to ingested turbulence, tip flows, and rotor blade boundary layer effects. Modeling these sources requires an understanding of the interaction between turbulent flows and the rotating blades. The scale and structure of the rotor blade boundary layers are dependent on the blade loading, and turbulence in the boundary layer scatters off the trailing edge to produce noise. In the present work hot wire anemometry was used to document relevant length scales and turbulence levels in the rotor wake in order to better understand these sources of sound. Far field microphone measurements were acquired in an anechoic chamber to correlate blade loading effects with acoustic signatures. Measured acoustic spectra are compared with dipole source models from the literature to resolve variations in sources due to the effects of blade loading. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BK.00005: Wall-Pressure Correlations to Velocity Field Events for Subsonic Cavity Flows Nathan Murray, Lawrence Ukeiley Subsonic flow over an open cavity with a length to depth ratio of 6 is studied experimentally using a syncronous pressure-PIV setup that creates a database of PIV velocity snapshots and their corresponding surface pressure fluctuations. The two measurements are syncronized by recording the image acquisition trigger signal (TTL pulse at approximately 10Hz rate) simultaneously with the wall-pressure signals at a 90kHz sample rate. Then, the pressure values corresponding to a given PIV snapshot are extracted from the data by referencing the image acquisition trigger during post-processing. Data were collected for five free stream Mach numbers: 0.2, 0.3, 0.4, 0.6 and 0.75. Using the resulting database, the effect on the surface pressure from various acoustic analogy-type source terms is evaluated. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BK.00006: Sound and vibration produced by an airfoil tip in a turbulent boundary layer flow with an elastic end wall. Paul Slaboch, Scott Morris Predicting the far field acoustics of an airfoil tip immersed in a turbulent boundary layer over an elastic end plate is a complicated problem. The unsteady lift of the airfoil and the local end wall vibration pattern form a coupled system of hydrodynamic and acoustic pressures. An experiment has been performed, incorporating as much of the flow physics as possible while maintaining the simplest possible boundary conditions. Specifically, a stationary airfoil tip in a turbulent boundary layer over an elastic end wall was used in an anechoic wind tunnel to relate the unsteady lift of the airfoil to the vibration patterns of the end wall. The vibration patterns were acquired using a scanning laser Doppler vibrometer. This talk will present results of experiments completed with both a rigid and elastic end wall. Preliminary data suggest that the unsteady lift of the airfoil is dependent upon the vibrational characteristics of the end wall. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BK.00007: An Experimental Investigation of Trailing Edge Acoustics Daniel W. Shannon, Scott C. Morris Measurements of the convected vorticity field in the near wake of a blunt asymmetric trailing edge has lead to the hypothesis that large scale turbulence related to a vortex shedding modulates the broadband sound produced by smaller scale turbulent motions. This paper will focus on efforts to support this hypothesis through the simultaneous measurement of the unsteady pressure on the model surface and the far field acoustic pressure. The acoustic data were acquired in an anechoic wind tunnel utilizing a pair of phased microphone arrays containing 40 condenser microphones each. Correlations between the surface pressure and the acoustic pressure suggest that the tonal noise is more closely related to the unsteady surface pressure on the attached pressure side of the model and that the broadband noise is correlated with the surface pressures over the separated suction side of the trailing edge. An analysis of the broadband noise as a function of the phase of the vortex shedding process suggests that the both the surface pressure and the acoustic pressure are modulated by the vortex shedding motions. [Preview Abstract] |
Sunday, November 20, 2005 12:27PM - 12:40PM |
BK.00008: Flows around Oscillating Bodies at Low Reynolds Numbers Charlotte W. Kotas, Peter H. Rogers, Minami Yoda Understanding the acoustically induced fluid flows inside the fish ear, specifically near the otolith, may give insights on how fish localize underwater sound sources and lead to compact, directionally sensitive underwater acoustic sensors. To explore such acoustically induced flow fields, spheres, as well as prolate and oblate spheroids of various aspect ratios, were oscillated at Reynolds numbers based upon the oscillation frequency and body half-dimension ranging from 6 to 120 and normalized oscillation amplitudes of 0.05--0.2. Spheroids oscillated with different orientations (measured with respect to their principal axes) were also investigated to determine how these flow patterns are affected by the direction of the incident sound. Both oscillatory flow within a single period and steady streaming flow were studied using flow visualization and particle-image velocimetry (PIV). The effect of various parameters on the inner flow region (of primary interest in sensing applications) is explored at these intermediate Reynolds numbers. The experimental results are compared with asymptotic perturbation analyses and numerical results. [Preview Abstract] |
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