Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q19: Aeroacoustics IIAcoustics
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Chair: Charles Tinney, The University of Texas at Austin Room: 702 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q19.00001: Computational Study of Circulation Control Aeroacoustics Yue Wu, Kan Wang, Meng Wang Circulation control is a promising method to argument the lift of airfoils and hydrofoils and enhance vehicle maneuverability. However, its acoustic effect is a significant concern and not well understood. In this study large-eddy simulations of a circulation control airfoil in a low-Mach-number flow are performed at a chord Reynolds number of 650,000 and three jet momentum coefficients. The acoustic radiation is calculated by solving the Lighthill equation using a boundary element method. The predicted sound pressure spectra show good agreement with the experimental measurements of Reger et al.\ ({\it J.\ Sound Vib.} Vol.~388, 2017). The Coanda jet is shown to suppress the low-frequency airfoil vortex-shedding noise, but at the same time causes a significant increase in broadband noise in the mid-to-high frequency range. The amount of increase grows with the jet momentum coefficient. High-frequency tonal noise is generated as a result of strong vortex shedding from the slot lip, and its magnitude and frequency also increase with increasing jet momentum coefficient. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q19.00002: Drone noise. Charles Tinney, Jayant Sirohi A basic understanding of the noise produced by single and multirotor drones operating at static thrust conditions is presented. This work acts as an extension to previous efforts conducted at The University of Texas at Austin (Tinney et al. 2017, AHS Forum 73). Propeller diameters ranging from 8inch to 12inch are examined for configurations comprising an isolated rotor, a quadcopter configuration and a hexacopter configuration, and with a constant drone pitch of 2.25. An azimuthal array of half-inch microphones, placed between 2 and 3 hub-center diameters from the drone center, are used to assess the acoustic near-field. Thrust levels, acquired using a six degree-of-freedom load cell, are then used to correlate acoustic noise levels to aerodynamic performance for each drone configuration. The findings reveal a nearly logarithmic increase in noise with increasing thrust. However, for the same thrust condition, considerable noise reduction is achieved by increasing the number of propeller blades thereby reducing the blade passage frequency and both the thickness and loading noise sources that accompany it. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q19.00003: Acoustic Characterization of a Multi-Rotor Unmanned Aircraft Jordan Feight, Richard Gaeta, Jamey Jacob In this study, the noise produced by a small multi-rotor rotary wing aircraft, or drone, is measured and characterized. The aircraft is tested in different configurations and environments to investigate specific parameters and how they affect the acoustic signature of the system. The parameters include rotor RPM, the number of rotors, distance and angle of microphone array from the noise source, and the ambient environment. The testing environments include an anechoic chamber for an idealized setting and both indoor and outdoor settings to represent real world conditions. PIV measurements are conducted to link the downwash and vortical flow structures from the rotors with the noise generation. The significant factors that arise from this study are the operational state of the aircraft and the microphone location (or the directivity of the noise source). The directivity in the rotor plane was shown to be omni-directional, regardless of the varying parameters. The tonal noise dominates the low to mid frequencies while the broadband noise dominates the higher frequencies. The fundamental characteristics of the acoustic signature appear to be invariant to the number of rotors. Flight maneuvers of the aircraft also significantly impact the tonal content in the acoustic signature. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q19.00004: Beer bottle whistling: a stochastic Hopf bifurcation Edouard Boujo, Claire Bourquard, Yuan Xiong, Nicolas Noiray Blowing in a bottle to produce sound is a popular and yet intriguing entertainment. We reproduce experimentally the common observation that the bottle ``whistles'', i.e. produces a distinct tone, for large enough blowing velocity and over a finite interval of blowing angle. For a given set of parameters, the whistling frequency stays constant over time while the acoustic pressure amplitude fluctuates. Transverse oscillations of the shear layer in the bottle's neck are clearly identified with time-resolved particle image velocimetry (PIV) and proper orthogonal decomposition (POD). To account for these observations, we develop an analytical model of linear acoustic oscillator (the air in the bottle) subject to nonlinear stochastic forcing (the turbulent jet impacting the bottle's neck). We derive a stochastic differential equation and, from the associated Fokker-Planck equation and the measured acoustic pressure signals, we identify the model's parameters with an adjoint optimization technique. Results are further validated experimentally, and allow us to explain (i) the occurrence of whistling in terms of linear instability, and (ii) the amplitude of the limit cycle as a competition between linear growth rate, noise intensity, and nonlinear saturation. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q19.00005: Broadband, slow sound on a glide-symmetric metasurface Joseph Beadle, Timothy Starkey, J. Roy Sambles, Alastair Hibbins The patterning of surfaces to control sound attenuation by the manipulation of a trapped acoustic surface wave has received considerable attention. Here the dispersion of acoustic surface modes on a rigid space-coiled metasurface is studied. A single continuous cavity structure consisting of a 1D periodic labyrinth which forms a glide-symmetric surface, with the property of zero bandgap at the first Brillouin-zone boundary, is explored. Acoustic surface waves supported by such surfaces exhibit constant reduced group velocity, to that of air, over a broad frequency bandwidth. Details of the acoustic surface wave dispersion are obtained experimentally by exciting with a near-field point-like pulsed sound pulse and monitoring the resulting near fields by use of a needle-probe microphone. Results obtained compare very well with the predictions of finite element method model. Such broadband, highly-attenuated surface modes have potential applications in reducing noise created by flow over surfaces. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q19.00006: Infrasonic Emissions From A Tornado Christopher Petrin, Brian Elbing Tornadoes cause dozens of deaths and significant damage throughout the United States every year. Tornado-producing storm systems emit infrasound (sound at frequencies below human hearing) up to 2 hours before tornadogenesis. Weak atmospheric attenuation at these frequencies allows them to be detected hundreds of miles away. Hence, passive infrasonic monitoring may be used for long-range study of tornadogenesis. This requires characterization of infrasound during the life of a tornado and from other background sources. This is being accomplished as part of the Collaboration Leading Operational UAS Development for Meteorology and Atmospheric Physics (CLOUD-MAP) project, a multi-university collaboration focused on the development and implementation of unmanned aerial systems (UAS) and their integration with sensors for atmospheric measurement. This presentation will report findings from a fixed infrasonic microphone that has been continuously monitoring the atmosphere since September 2, 2016. Infrasound from a tornado that occurred 19 km from the microphone on May 11, 2017 will be presented as well as an overview of other infrasonic observations. *This work was supported by NSF Grant 1539070 [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q19.00007: Acoustic Wavepacket-Based Analysis of subsonic and supersonic jet noise radiation S Unnikrishnan, Datta Gaitonde Wavepacket models for jet noise are based on spatio-temporally coherent features in jet cores. The irrotational-isentropic (or “acoustic”) component of momentum fluctuation encapsulates several crucial features required for such models. It manifests as a wavepacket containing the signatures of key acoustic phenomena such as intermittency. The acoustic modes of a Mach 0.9 and a 1.3 jet are extracted from Large-Eddy Simulations, and for both, it is shown that the radiative supersonic spectra correctly represent the noise generating content. While the pressure field in the supersonic jet has similar features as the acoustic mode, that in the subsonic jet contains an energetically dominant, but non-radiating subsonic peak, which masks the radiating component. These observations are bridged by examining the solenoidal component of momentum fluctuations of each jet. An examination of energy-based spectral modes of pressure and acoustic fields, reveals that the leading modes of the latter are a better representation of the radiated nearfield, and follow a simple homogeneous wave propagator model even from as close as 1.5 diameters from the jet axes. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q19.00008: Transmission and scattering of acoustic energy in turbulent flows Datta Gaitonde, S Unnikrishnan Sound scattering and transmission in turbulent jets are explored through a control volume analysis of a Large-Eddy Simulation. The fluctuating momentum flux across any control surface is first split into its rotational turbulent ($(\rho \mathbf{u})'_H$) and the irrotational-isentropic acoustic ($(\rho \mathbf{u})' _A$) components using momentum potential theory (MPT). The former has low spatio-temporal coherence, while the latter exhibits a persistent wavepacket form. The energy variable, specifically, total fluctuating enthalpy, is also split into its turbulent and acoustic modes, $H_H'$ and $H_A'$ respectively. Scattering of acoustic energy is then $(\rho \mathbf{u})'_HH_A'$, and transmission is $(\rho \mathbf{u})'_AH_A'$. This facilitates a quantitative comparison of scattering versus transmission in the presence of acoustic energy sources, also obtained from MPT, in any turbulent scenario. The wavepacket converts stochastic sound sources into coherent sound radiation. Turbulent eddies are not only sources of sound, but also play a strong role in scattering, particularly near the lipline. The net acoustic flux from the jet is the transport of $H_A'$ by the wavepacket, whose axisymmetric and higher azimuthal modes contribute to downstream and sideline radiation respectively. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q19.00009: On the existence of an overlap region between the Green’s function for a locally parallel axi-symmetric jet and the leading order non-parallel flow solution Vasilis Sassanis, Mohammed Afsar, Adrian Sescu, Sanjiva Lele We consider determination of the propagator within the generalized acoustic analogy for prediction of supersonic jet noise. The propagator is a tensor functional of the adjoint vector Green’s function that requires solution of the linearized Euler equations for a given mean flow. The exact form of these equations can be obtained for a spreading jet. However since high Reynolds number jets have small spread rates, $\epsilon$ $<$ $<$ $O(1)$, this parameter can be exploited to formulate an asymptotic model that encompasses mean flow spatial evolution at leading order. Such a model was used by Afsar et al. (AIAA-2017-3030 for prediction of supersonic jet noise. We show the existence of an overlap between this solution (valid at low frequencies) and one based on a locally parallel (i.e. non-spreading) mean flow, valid at $O(1)$ frequencies. It is clear that there must exist an overlap between these solutions, since the former non-parallel solution was determined at the distinguished limit where the scaled frequency $\Omega=\omega/\epsilon=O(1)$ was held fixed. Hence the inner equation shows that as $\Omega\rightarrow\infty$, non-parallelism will be confined to a thin streamwise region of size $O(\Omega^{-1})$ and will, therefore, be subdominant at leading order when $\Omega Y=\bar{Y}=O(1)$. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q19.00010: Adjoint-based Sensitivity of Jet Noise to Near-nozzle Forcing Seung Whan Chung, Ramanathan Vishnampet, Daniel Bodony, Jonathan Freund Past efforts have used optimal control theory, based on the numerical solution of the adjoint flow equations, to perturb turbulent jets in order to reduce their radiated sound. These efforts have been successful in that sound is reduced, with concomitant changes to the large-scale turbulence structures in the flow. However, they have also been inconclusive, in that the ultimate level of reduction seemed to depend upon the accuracy of the adjoint-based gradient rather than a physical limitation of the flow. The chaotic dynamics of the turbulence can degrade the smoothness of cost functional in the control-parameter space, which is necessary for gradient-based optimization. We introduce a route to overcoming this challenge, in part by leveraging the regularity and accuracy with a dual-consistent, discrete-exact adjoint formulation. We confirm its properties and use it to study the sensitivity and controllability of the acoustic radiation from a simulation of a $M=1.3$ turbulent jet, whose statistics matches data. The smoothness of the cost functional over time is quantified by a minimum optimization step size beyond which the gradient cannot have a certain degree of accuracy. Based on this, we achieve a moderate level of sound reduction in the first few optimization steps. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q19.00011: Vortex formation at the open end of an acoustic waveguide Leon Martinez del Rio, Pablo L. Rendon, Carlos Malaga, Roberto Zenit For high enough levels of acoustic pressure inside a cylindrical tube, a nonlinear mechanism is responsible for the formation of annular vortices at the open end of the tube, which results in energy loss. Higher sound pressure levels in the tube lead, in turn, to larger values of the acoustic velocity at the exit, and thus to higher Reynolds numbers. It has been observed [Buick et al, 2011] that, provided the magnitude of the acoustic velocity is large enough, two nonlinear regimes are possible: in the first regime, the vorticity appears only in the immediate vicinity of the tube; for higher velocities, vortex rings are formed at the open end of the tube and are advected outwards. We use a Lattice Boltzmann Method (LBM) to simulate the velocity and the pressure fields at the exit of the tube in 3D, with Reynolds numbers based on the acoustic boundary layer thickness $18>R_\delta>1.8 $. We also conduct experiments with phase-locked particle image velocimetry (PL-PIV) 2D within a range of $25.5>R_\delta>10.2 $. Experimental and numerical results are compared for a range of Womersley numbers. The effects of varying both the tube geometry and the end shape are addressed. [Preview Abstract] |
Tuesday, November 21, 2017 3:13PM - 3:26PM |
Q19.00012: ABSTRACT WITHDRAWN |
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