Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FH: Acoustics I |
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Chair: Peter Oshkai, University of Victoria Room: Salt Palace Convention Center 250 B |
Monday, November 19, 2007 8:00AM - 8:13AM |
FH.00001: Experimental Study of Acoustically-Coupled Cavity Flows: Effect of Resonator Geometry on the Acoustic Source Structure Alexey Velikorodny, Peter Oshkai Digital particle image velocimetry in conjunction with unsteady pressure measurements is employed to investigate flow-acoustic coupling due to turbulent flow over coaxial deep cavities (side branches) mounted in a duct. Global, quantitative instantaneous and time-averaged flow patterns provide insight into the underlying physics. In addition, structure of the acoustic noise source is characterized in terms of patterns of generated acoustic power. A semi-empirical approach that involves numerical calculation of the acoustic (irrotational) velocity and experimental measurements of total velocity is employed for acoustic power calculation. The present study focuses on the effects of the resonator geometry on the associated flow patterns. Streamlined or bluff bodies placed in the vicinity of the side branch openings have significant influence on the degree of separated shear layer interaction. Moreover, Strouhal mode of the shear layer oscillations also has a significant effect on spatial structure and strength of the acoustic source. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FH.00002: An Experimental Study of Oscillating and Pulsating Flow in a 2-D Diffuser using TRPIV Cameron King, Barton Smith Separating oscillating and pulsating flow in 2-D diffusers of various angles is studied experimentally. Time-Resolved PIV measurements and simultaneous pressure measurements reveal that during the accelerating portion of the cycle, the flow may remain attached in spite of a very large adverse pressure gradient. Separation is observed to begin high in the diffuser and propagate downward. ~Separation is found to occur earlier in the cycle with increasing displacement amplitude. The time-varying pressure measurements are used to determine the resultant minor losses for the flow in each direction. These measurements, together with the velocity measurements, are used to calculate the acoustic power dissipation and acoustic impedance. The minor losses and acoustic power dissipation are found to be a decreasing function of the Reynolds number and an increasing function of displacement amplitude. The impact of the addition of a steady-flow component in the direction of increasing flow area is assessed, as well as the effect of diffuser angle. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FH.00003: Flow Excited Helmholtz Resonator - Theory and Experiments Scott Morris, Paul Slaboch, Ruolong Ma Flow over the orifice of a Helmoltz resonator can result in a self excited resonance. The present research has focused on understanding this vortical-acoustic coupling by considering a simple control volume momentum analysis. A forcing term can then be identified, and considered in terms of a combined hydrodynamic-acoustic scaling. Direct measurements of the forcing were obtained using PIV for a range of speeds and orifice geometries. These measurements have motivated a simplified model for the forcing which allows accurate predictions of both the frequency and amplitude of the cavity pressure fluctuations. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FH.00004: ABSTRACT WITHDRAWN |
Monday, November 19, 2007 8:52AM - 9:05AM |
FH.00005: Modeling the acoustic excitation of a resonator Shreyas Mandre, Lakshminarayanan Mahadevan The sounding of a beverage bottle when blown on is a familiar but very little understood phenomenon. A very similar mechanism is used by musical wind instruments, like organ pipes and flutes, for sound production. This phenomenon falls under the general umbrella of flow induced oscillations and is representative of a more generic mechanism. The modeling of this phenomenon essentially involves two components. The first is the resonator, which bears the oscillations and this component is very well understood. The resonator, however, needs an external energy input to sustain the oscillations, which is provided by the jet of air blown. The dynamics of the jet and its interaction with the resonator is the primary focus of this talk. In particular, we provide a linearized model based on first principles to explain the feedback of energy from the jet to the resonator and compare the predictions with experimental results. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FH.00006: Numerical Simulations on Self-sustained Oscillations of Flows past Cavities Yasushi Watanabe Sound is generated by the interaction of a vortex with the corner of a cavity-like configuration during take-off and landing of an aircraft. This vortex formation may be coupled with a resonant acoustic mode of the cavity. Numerical simulations have been performed by solving compressible Navier-Stokes equations. These simulations were performed over a wide range of Reynolds number and thickness of the inflow boundary layer in the low subsonic regime. For the case of a relatively thin boundary layer at high Reynolds number, strong pressure oscillation is accompanied by a standing wave in the cavity. As the Reynolds number becomes lower, or the thickness of the boundary layer becomes sufficiently large, the standing wave inside cavity disappeared. The effect of this type of vortex-surface interaction on the sound radiation will be addressed. Moreover, the consequence of damping on the pressure oscillation will be described in terms of Reynolds number. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FH.00007: On Centrifugal Instabilities and Wake Mode in the Flow over an Open Cavity Guillaume Bres, Tim Colonius Three-dimensional Direct Numerical Simulations of the full compressible Navier-Stokes equations are performed for open cavities that are homogeneous in the spanwise direction. The formation of oscillating spanwise structures is observed inside the cavity. This 3D instability arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. In general, the three-dimensional mode has a spanwise wavelength of approximately the cavity depth and oscillates with a frequency an order-of-magnitude lower than 2D Rossiter (flow/acoustics) instabilities. The 3D mode properties are in excellent agreement with predictions from our previous linear stability analysis. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. We connect these results with the observation of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows. Preliminary results on the connections between the 3D centrifugal instabilities and the presence/suppression of the wake mode are also presented. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FH.00008: Prediction of sound from human vocal folds Daniel Bodony, Haoxiang Luo, Rajat Mittal The creation of voiced sounds in humans depends on the flow-induced vibration of the vocal folds within the larynx. The vocal folds, which are a complex structural system of cartilage and tissue, create an oscillatory ``glottal jet'' whose harmonic content partially determines the tone of the speech. In this work we will discuss the process of sound generation in the laynx by combining a fully coupled two-dimensional fluid-structure simulation of the incompressible flow field in the vicinity of the vocal folds to an acoustic analogy description of the sound field. The structural dynamics of the vocal folds are based on physically realistic properties and are coupled to the motion of the fluid via an immersed boundary method. Relationships between the sound produced and the vocal fold dynamics will be discussed. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FH.00009: Development of a new \textsc{ABS Acoustic Bubble Spectrometer}$^{\mbox{{\textregistered}}\copyright }$ system Xiongjun Wu, Chao-Tsung Hsiao, Georges Chahine Dynaflow has developed an acoustic based device, the ABS Acoustic Bubble Spectrometer{\textregistered}{\copyright}, that measures bubble size distributions and void fractions in liquids based on the measurement of sound propagation through the liquid. In the original system, a pair of hydrophones is used to transmit and receive short monochromatic bursts of sound at different frequencies through the liquid. These signals are processed and analyzed to obtain a frequency dependent attenuation and phase velocities of the acoustic waves. Subsequently, the bubble size distribution is obtained following solution of an inverse problem. In the new system, we have utilized multiple hydrophone pairs that have different frequency response ranges to cover a wider range of bubble size measurement. A transmission signal amplifier is integrated into the system to improve the signal noise ratio. We have also implemented an adaptive control scheme that automatically adjusts the transmitting signal strength and acquisition resolution to optimize the measurement process and used a rectangular and a sine acoustic wave pattern to improve accuracy of signal analysis.~ These changes result in improved bubble size detection and higher void fraction limit for detection. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FH.00010: Optimized translation of microbubbles driven by acoustic fields. Jean Toilliez, Andrew Szeri A single acoustically driven bubble translating unsteadily in a fluid is considered. The inverse Reynolds number is identified as small perturbation parameter in the translation equation. A closed-form, leading order solution for the bubble translation is obtained, assuming nonlinear radial oscillations and a pressure field as the forcing term. The result is the ability to predict and understand the rapid and slow transients of bubble displacement, which is proportional to the average acoustic radiation force. The periodic attractor of the Raleigh-Plesset equation serves as basis for an optimal acoustic forcing designed to achieve maximized bubble translation over one dimensionless period. At moderate acoustic intensity, a maximized radial variance leads to displacement many times larger than the case of purely sinusoidal forcing. Shape stability issues are considered. Together, these results suggest new ways to predict some of the direct and indirect effects of the acoustic radiation force in biomedical applications: e.g., targeted drug delivery and bubble accumulation. [Preview Abstract] |
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