Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session S06: Acoustics: Aeroacoustics II |
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Chair: Aaron Towne, University of Michigan Room: 205 |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S06.00001: Aeroacoustic source localization and source level estimation during wind tunnel testing of a flat plate with and without a gap Alexander Douglass, Natasha Chang, David Dowling Acoustic measurements in wind tunnels are notoriously difficult because of machinery noise from the tunnel, and aeroacoustic noise generated within the tunnel and by the test model; and because the potentially-weak signals of interest may share a common frequency range with these noise sources. Thus, when model changes are made, localizing any new aeroacoustic source(s) and determining their level(s) are challenging tasks. This presentation provides such experimental results for the aeroacoustic source associated with addition of a 6 mm gap in a 0.5-m-by-1.0-m flat plate aligned with the flow direction. When present, the gap was located 0.40-m from the plate's leading edge. The measurements were collected in the Anechoic Flow Facility of the Naval Surface Warfare Center -- Carderock Division at nominal air speeds of 19 and 30 m/s using a \textasciitilde 0.6-m-diameter spiral microphone array with 24 elements placed 1.24 m from the plate and separated from the air flow by a thin barrier. Measurements in the 4-to-12 kHz frequency range were processed using conventional and high-resolution beamforming methods with and without noise-reference subtraction. Source location and level estimation for the gap-induced aeroacoustic sound source was successful at nominal signal-to-noise ratios of --20 dB. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S06.00002: Mixed-porosity airfoil acoustics Aren Hellum Porous materials have been shown to reduce the noise produced by lifting surfaces. Acoustic and wake velocity measurements have been made on several arrangements of poroelastic material including full-chord impermeability, full-chord porosity, and multiple chordwise variations of “mixed” porosity. The "mixed" porosity arrangements are produced by changing the area fraction constructed of porous material. Two different porous materials have been employed. The results are made nondimensional and compared to published data. These comparisons indicate that the noise reduction associated with poroelastic foils is associated with the porosity rather than the elasticity of the material. A percolation-based physical model to explain elevated noise production at high frequencies is also proposed. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S06.00003: Approximate Solutions for Nonlinear Acoustic Pulses Propagating Upstream in a Subsonic Flow Fatemeh Bahmani, Mark Cramer, Tomas Gronstedt We have studied acoustic pulses traveling upstream in one dimensional subsonic flow. A sinusoidal pressure pulse is imposed at the right boundary and the transient wave propagation is studied. The undisturbed flow is assumed to be inviscid with uniform density and entropy. The flow velocity is taken to be uniform. N-waves are observed to form after the shock formation time. Due to interactions of the shocks with the waves in front and behind it, the wave amplitude decreases and the wavelength increases as the pulse wave propagates upstream. The variation of pressure coefficient with time and the strength of the pressure disturbances are presented. This solution provides a rough estimation of upstream traveling pulses and can be used to guide and check computations in many practical fluid mechanics applications. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S06.00004: Reduced-Order Investigation of Volumetric PIV for Noise Source Characterization Adam Nickels, Jeff Harris, Alexander Mychkovsky, James Wiswall, Kristin Cody, Ted Bagwell Flow induced noise sources are often highly three-dimensional, turbulent phenomenon that require knowledge of the three-dimensional velocity-gradient tensor over significant spatial and temporal domains to fully characterize. To address these needs, volumetric-PIV is used to measure the cross-section of a turbulent jet, providing direct measurements of the time-dependent velocity-gradient tensor. Synchronously obtained acoustic pressure measurements allow for correlation of near field quantities with acoustic features of the flow. To better elucidate the spatially and temporally coherent flow features related to the acoustic source, spectral-POD is applied to the velocity field and correlated with the acoustic pressure. [Preview Abstract] |
(Author Not Attending)
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S06.00005: Flow-based decomposition of turbulent-jet noise Marco Raiola, Daniele Ragni Turbulent jet noise is a complex phenomenon characterized by the interaction of modal and non-modal features in the flow field, the former usually defined as wave-packets in literature. In this work the velocity field of a turbulent round jet at Re$=$30000 is measured using low-repetition rate tomographic Particle Image Velocimetry. Features in the flow field are extracted on a statistical basis using Proper Orthogonal Decomposition, revealing both non-modal features and features with a clear modal behavior in the streamwise direction which are compatible with the wave-packets reported in literature. The temporal evolution of both velocity and pressure of these features, not readily available from the measurements, is estimated through the Galerkin projection of advection equation and of the Euler pressure equation. This approach is advantageous since it also provides estimates of the interaction between several features/modes. Finally, a similar Galerkin projection is attempted on the Lighthill's equation in order to provide a flow-based decomposition of the far-field noise produced by the jet and to estimate the noise emission due to the cross-interaction of the flow-field modes. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S06.00006: Lagrangian Analysis of Intermittent Sound Sources in a Flow-through Square Duct Containing Two Circular Orifice Plates Vineeth Nair, C.P. Premchand, K.V. Reeja, P.R. Midhun, Manikandan Raghunathan, Raman I. Sujith Tonal sound production in pipe flow through orifices is a manifestation of self-sustained pressure oscillations that occur when positive feedback is established between an acoustic mode of the pipe and vortex shedding. These oscillations, which can be established in segmented solid rocket motors, gas-transport pipelines, and automobile exhaust systems are undesirable as they result in structural damage due to fatigue failure. The onset of such oscillations (instability) in turbulent flows for changes in operating conditions from stable operation happens via an intermittent regime comprising bursts of pressure oscillations that are governed by slow (hydrodynamic) and fast (acoustic) time scales. In this study, we focus on the establishment of self-sustained oscillations in the flow through a square pipe containing two circular orifice plates by varying the mass flow rate through the pipe. We extract the coherent structures responsible for sound production at the acoustic time scale from measured time-resolved velocity fields using dynamic mode decomposition (DMD) and the framework of Lagrangian Coherent Structures (LCS). We find that there are noticeable similarities in the sound-producing coherent structures obtained during the bursting stage of intermittency and instability. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S06.00007: Trailing edge noise modification due to edge porosity Yoas Zachary, Paul Trzcinski, Michael Krane It is believed that trailing edge porosity characteristics of large owl wings is the cause of their observed quiet flight. This hypothesis was tested in the ARL Penn State anechoic chamber by measuring sound radiated by the interaction of a vortex ring with a single fixed edge. Both porous and rigid trailing edges were tested. Vortex ring motion was characterized by Schleiren. Typical rings measured 9 mm in diameter, and convected at speeds ranging from 30 m/s to 90 m/s. Rigid and porous edge radiated noise was then compared across this range of vortex ring speeds to show degree of attenuation due to edge porosity. [Preview Abstract] |
Tuesday, November 26, 2019 12:02PM - 12:15PM |
S06.00008: The Ballad of Water Entry Rafsan Rabbi, John Allen, Jesse Belden, Tadd Truscott Not all water splashes are similar, nor do they sound the same. From the pitter-pattering sound of raindrops on lakes and pools to the thunderous rumble of falls plunging into the rivers flowing below, these sounds can elucidate the mechanisms and evolution behind their origins. Particularly, solid objects can create different sounds when they impact the water surface water depending on their size, shape, speed, etc. Herein, we study these sounds by dropping hydrophobic spheres into a quiescent water-filled tank and capture the accompanying sounds with in-air microphones and sub-surface hydrophones. Spheres (diameter: 10 mm-24 mm) impacting water at low to high velocities (1- 6 m/s) were tested, revealing that the specific sealing phenomena of the cavities have distinct acoustic signatures. Synchronizing high-speed images of these impacts with audio signals captured by acoustic sensors reveal interesting and unique distinct sounds created by the unique cavity sizes, bubble shedding patterns and in some cases the rebounding Worthington jets. These observations help tie the water entry hydrodynamics and acoustics together in a definitive way. [Preview Abstract] |
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