Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E24: Acoustics III |
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Chair: Shreyas Mandre, Brown University Room: 30E |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E24.00001: An Experimental Study of a Nonlinear Acoustic Lens Interfaced with Water Carly Donahue, Paul Anzel, Thomas Keller, Chiara Daraio Acoustic waves are routinely used in imaging and excitation applications such as in ultrasonic imaging or hyperthermia surgery. However, current acoustic technology is limited by focal resolution and maximum amplitude. In this work, we have constructed a nonlinear acoustic lens, which is composed of an array of chains of steel spherical particles supported by a matrix. The nonlinearity of the system originates from the contact interaction between the particles, which enables the formation of solitary waves in the chains. The acoustic lens can be designed and interfaced with a target medium such that when the solitary waves exit the chains, the waves coalesce at a focal point. The highly compact acoustic waves at the focus are called ``sound bullets.'' Additionally, since the solitary wave speed increases as the pre-compression between the spheres increases, the focal point can be controlled mechanically. In this work, we use water as our target medium. Measurements are taken using a hydrophone that is scanned over an area to produce a two dimensional pressure map. The chains are separated from the water using cover plates, the choice of which strongly influences the transmission of the solitary wave into the host medium. [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E24.00002: Multiple Scales Analysis of a Thermoacoustic Heat Pump Michael Miller, Shreyas Mandre Thermoacoustics utilizes the temperature and density oscillations inherent to acoustic vibrations coupled with heat conduction near a wall to produce heat transfer from sound (or sound from a heat source). In the heat pump setup, thermal energy is transferred to the wall from an element of gas during compression and taken from the wall during rarefaction. In thermoacoustic phenomena, acoustic oscillations occur on a very short time scale while heat transfer occurs over many acoustic cycles. Therefore, multiple scales analysis is well suited to describe the physics. We present a multiple scales analysis for a narrow two-dimensional channel between two thin, non-stationary plates resulting in an integral equation for the temperature distribution along the channel as a function of the long time scale. We solved this equation numerically to find a steady state solution for a given set of parameters. [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E24.00003: Improved parabolization of the compressible Euler equations Aaron Towne, Tim Colonius The parabolized stability equations (PSE) are a tool for rapid computation of convectively unstable flows. The efficiency of the method is achieved by solving the equations using a spatial marching technique in the downstream direction. Unfortunately, the PSE operator contains upstream propagating acoustic modes that cause instability in this march unless these waves are numerically damped. Existing damping techniques introduce additional error into the solution and in particular contaminate the acoustic mid- and far-field. We have developed a method to explicitly remove the upstream acoustic mode from the linearized Euler equations. The eigenvalue associated with the upstream mode is zeroed in Fourier-Laplace space, resulting in a non-local propagation equation after the transforms are inverted. The non-locality arises from terms involving the square root of the Fourier-Laplace variables. In order to recover local, real-space PDEs from the transformed equations, these terms are approximated using Pade-type rational approximation. The resulting equations are parabolic in the downstream direction and require no damping for a stable march. We will outline this method and present results that demonstrate its stability and accuracy for different approximations of the square root. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E24.00004: Experimental evaluation of sound produced by two cylinders in a cross flow in various configurations Michael Bilka, Peter Kerrian, Scott Morris Cylinders in a cross flow is a canonical test case that is considered representative of industrial fluid flow problems, such as heat exchangers and aircraft landing gear. The general configurations are cylinders in tandem, parallel or staggered, which lead to several interesting flow phenomena such as wake cavity/wake interaction effects (tandem), symmetric/asymmetric wake behavior (parallel) or wake interaction/coanda effects (staggered), depending on relative location of the cylinders. In many cases, it is important to not only understand the flow interaction between the cylinders but also the acoustic consequence of such configurations. However, information on the acoustical behavior based on these configurations is relatively small compared to that of the steady aerodynamic and flow interaction behavior. The present work investigates the acoustic spectral properties of these configurations in order to characterize the sound produced by cylinder proximity and wake effects. The spectra are measured using an acoustic beam-forming technique to identify the sources and remove spurious content from the spectra. [Preview Abstract] |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E24.00005: Sensitivity of meteor infrasound to atmospheric uncertainties Christophe Millet, Christophe Haynes In recent years, numerous bolide sources have been detected by the IMS infrasound arrays. Even though a variety of waveform data may be extracted from recorded signals, only a few parameters are used throughout meteor research, the most common being the arrival time of a signal. Other data forms include the amplitude and duration of the signal. As the shock wave depends on the properties of the medium in which it travels, a full analysis of the atmosphere for any event is required. In the present work, we model the propagation of a shock wave through a randomly layered atmosphere. In a deterministic or random environment, a generated meteoric shock wave propagates from a strong blast region out to the far-field acoustic limit. Inclusion of a random atmosphere will then affect all possible outcomes of the ray path. The resulting amplitude and period of any N-wave signal at ground level are obtained using both a ray tracing method and a theoretical approach based on Whitham's method. This method becomes particularly relevant when applied to the crater-forming meteorite fall near Carancas, Peru (2007); given that the specific trajectory of the meteor was unknown and that the maximum amplitude of the recorded signal were substantially affected by atmospheric conditions. [Preview Abstract] |
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