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 Q06: Acoustics: General |
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Chair: Likun Zhang, University of Mississippi Room: 205 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q06.00001: Acoustic radiation force and scattering series expansions for spheres at low frequencies. Philip L. Marston Even in the case considered here where spheres are taken to be in inviscid fluids and all mechanisms for power absorption are neglected, it can be helpful to consider leading-order corrections to Rayleigh scattering. Using series expansions of scattering partial-wave phase shifts (that depend on material properties) quantities of interest can be expanded in powers of kR where k is the acoustic wave number and R is the sphere radius. This has been accomplished in a unified way for several cases, though it is necessary for kR to be below all resonances in each case. Situations considered include fluid and solid spheres and empty elastic shells in traveling and standing acoustic plane waves [1, 2]. The method is easily generalized to spheres in certain acoustic beams. There has also been renewed interest in low kR expansions of the quadrupole projection of the acoustic radiation stress based on Rayleigh scattering since that is relevant to the equilibrium shape of nearly-spherical acoustically-trapped objects [3]. [1] P. L. Marston, J. Acoust. Soc. Am. 145, EL39--EL44 (2019). [2] P. L. Marston, J. Acoust. Soc. Am. 146 (accepted). [3] P. L. Marston et al., J. Acoust. Soc. Am. 69, 1499-1501 (1981). [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q06.00002: Numerical investigation of heat transfer induced by an oscillatory flow within a thermoacoustic engine core Kazuto Kuzuu, Shinya Hasegawa Quantification of heat transfer phenomena in a thermoacoustic engine is a key to the successful improvements in the engine performance. From this point of view, Piccolo and Pistone (\textit{Int. J. Heat Mass Tran. 49 (2006), pp. 1631-1642}) presented a computational method for heat transfer analysis of the thermoacoustic engine. In their method, a time-averaged energy conservation equation is solved for the acoustic field fixed at thermoacoustic engine core, and for simplification, the acoustic field was assumed to be a standing wave. However, the actual thermoacoustic device is often designed so that a travelling wave could be achieved at the engine core part, and furthermore, there are interactions between gas motion and temperature field. The present study proposes a computational model which can consider such interactions and travelling wave. In this model, both the Rott's acoustic approximation equation (\textit{Thermoacoustics, ASA Press, pp.102}) and the time-averaged energy conservation law are simultaneously solved in two-dimensional space. From the results, heat transfer within a thermoacoustic engine core is discussed. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q06.00003: Linear and nonlinear propagation of single-frequency dissipative waves in ducts with slowly-varying cross section Pablo L. Rendon, Nigel Peake Finite-amplitude sound waves propagating in ducts are subjected to a variety of competing effects. In the context of the acoustics of wind instruments, nonlinear steepening is known to occur in trombones and trumpets when these instruments are played loudly, and in aircraft engine intakes buzz-saw noise is also associated with nonlinear propagation. The main dissipative mechanisms in these ducts tend to be losses at the duct walls rather than viscothermal attenuation. These losses are due to a Stokes boundary layer which oscillates as the waves pass. In both these contexts, the duct cross-section varies slowly, and this typically means that the wavenumber will change along the length of the duct. We use a WKBJ method to obtain equations which describe acoustic wave propagation subject to combinations of these effects, and for ducts of different geometries. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q06.00004: 3D Thermoacoustics in a Microwave Plasma Seth Pree, John P Koulakis, Seth Putterman Pulsed microwaves directed at an acoustic cavity filled with partially ionized gas may generate intense sound fields. These fields have been shown to center and confine the hottest portions of the gas to the center of a spherical cavity via a generalization of acoustic radiation pressure we have called the pycnoclinic acoustic force. Because partially ionized gas is luminous, the sound field is rendered visible by luminosity and temperature oscillations caused by the acoustic field's periodic adiabatic compression. This observation indicates that the microwave absorptivity of the gas may also fluctuate as sound passes through it. If microwave absorption increases in phase with the acoustic compression, the conditions for acoustic amplification may be met. This would allow the generation of high amplitude sound and possibly confinement with a continuous wave microwave source. We will describe the apparatus, present evidence of acoustic plasma confinement, and outline the theoretical conditions for microwave plasma thermoacoustics. We will also discuss how sustaining such a high amplitude sound field and the pycnoclinic acoustic force with this 3D thermoacoustic effect may enable a new laboratory model of convection in a central force. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q06.00005: Experimental study of one-dimensional acoustic metamaterials of sound-soft inclusions. Camila Horvath, Mar\'ia Luisa Cordero, Agn\`es Maurel We are studying the properties of a one-dimensional acoustic metamaterial. The metamaterial consists in a periodical array of rectangular cross section pillars, made of a sound penetrable material (air) built-in a medium with higher acoustic impedance (Polydimethylsiloxane). We are focused on the acoustic response, when the periodicity of the array is several orders of magnitude lower than the wavelength of the incident wave. The problem is being studied experimentally and theoretically, using homogenization methods (Marigo, 2018, RSPA; Maurel, 2014, JASA) to simulate acoustic response of the array and comparing these results with the experimental measurements. Using lithography and soft lithography techniques, we fabricate the metamaterial, shaping a microscale periodical structure of rectangular cross section air bubbles within a Polydimethylsiloxane media. In order to reproduce a one-dimensional array, the air bubbles are pillars with a length, normal to its cross section, four orders of magnitude larger than the periodicity of the structure. We study experimentally the acoustic wave transmitted and reflected by the metamaterial. We find that it behaves as a perfect reflector for a wide range of frequencies. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q06.00006: Analysing the structure of the acoustic analogy based-high frequency Green’s function in non-axi-symmetric sheared flows via a Ray tracing solver Sarah Stirrat, Mohammed Afsar, Adrian Sescu The chevron nozzle remains a popular approach aimed at reducing jet noise. It works by breaking up large turbulence structures and by increasing mixing, but it also effects propagation of sound. In this study, we investigate the effect of chevron-type mean flow in an acoustic analogy model where the wave propagation reduces to the solution of the Rayleigh equation and is calculated using a ray theory model for a jet represented by a transversely sheared mean flow. Since the generalised acoustic analogy (GAA) shows that the acoustic pressure is given by the convolution product of a rank-2 tensor propagator and the fluctuating Reynolds Stress, we determine the propagator (that is related to the vector adjoint Green's function of the linearised Euler operator) using the high frequency Ray theory developed by Goldstein (J. S. V., Vol. 80, p. 499, 1982) under an isotropic model of the fluctuating Reynolds stress. We calculate the Rayleigh equation Green’s function at high frequencies for a series of chevron mean flow patterns with multiple lobes. Our results reveal that the chevron jet introduces much more non-periodic spatial modulation of the Green’s function with a local minima in amplitude within the jet. We conclude by discussing how this explains the observed reduction in sound. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q06.00007: Outer streaming within a two-dimensional channel Kyle Pietrzyk, Ilenia Battiato Acoustic streaming is the net time-averaged flow that results from nonlinearities in an oscillating flow. Extensive research has sought to identify different physical mechanisms and regimes of acoustic streaming in systems of various geometries. While streaming in a channel maintains a simple geometry, it requires an appropriate set of scales to capture the multiple regimes of streaming that can occur. This study aims to define a set of scales and dimensionless numbers for general outer acoustic streaming within a channel. The chosen scales are validated through the recovery of slow streaming equations describing Rayleigh streaming in a channel and Eckart streaming for the case of infinitely far away channel walls. Using the scales and the time-averaged momentum equations, fast streaming is then analyzed and nonlinear Reynolds numbers, which indicate whether the streaming is nonlinear or linear, are found. With this analysis, a brief procedure for identifying regimes of acoustic streaming within a channel is provided for future analyses involving streaming in complex multi-scale systems. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q06.00008: Acoustic propagation in random media using polynomial chaos expansions Alexandre GOUPY, Christophe MILLET, Didier LUCOR Sound propagation in the atmosphere is highly dependent on the information to specify the waveguide parameters. For real-world applications, there is considerable uncertainty regarding this information, and it is more realistic to consider the wind and temperature profiles as random functions, with associated probability distribution functions. Even though the numerical methods currently-in-use allow accurate results for a given atmosphere, high dimensionality of the random functions severely limits the ability to compute the random process representing the acoustic field, and some form of sampling reduction is necessary. In this work we use polynomial chaos (gPC)-based metamodels to represent the effect of large-scale features onto the acoustic normal modes. The impact of small-scale atmospheric structures is modelled using a perturbative approach of the coupling matrix. This~two-level approach allows to estimate the statistical influence of each mode as the frequency varies. An excellent agreement is obtained with the gPC-based propagation model, with a few realizations of the random process, when compared with the Monte Carlo approach, with its thousands of realizations. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q06.00009: Reduced Order Modeling of Spray Flame Response to Harmonic Velocity Fluctuations Vishal Acharya Modern combustion systems are all susceptible to thermoacoustic combustion instabilities. To understand these instabilities, reduced order models are developed for the dynamics of the flame when subjected to various source fluctuations, such as due to velocity, mixture ratio, pressure etc. Prior research has significantly focused on modeling for gaseous premixed flame dynamics with recent research also increasing focus on gaseous non-premixed diffusion flames. However, reduced order modeling for spray flames has received no attention and thus this work presents a modeling framework for the dynamics of spray flames with a focus on the velocity coupled response. The response is characterized using the Flame Transfer Function (FTF). The paper uses the classical Burke-Schumann diffusion flame configuration as a basis with the fuel introduced in the form of a spray of liquid droplets. The space-time dynamics in the model uses the fast-chemistry limit applied to the mixture fraction equation for both the gaseous and liquid phases. These equations are coupled through evaporation of the liquid droplets and results in new control parameters such as a vaporization Damkohler number in addition to parameters pertaining to the spray and droplets themselves. Collectively, the effects of these new parameters on spray flame dynamics can be understood. [Preview Abstract] |
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