Index of content:
Volume 133, Issue 4, April 2013
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
133(2013); http://dx.doi.org/10.1121/1.4792140View Description Hide Description
The influence of size polydispersity on the resonant acoustic properties of dilute emulsions, made of fluorinated-oil droplets, is quantitatively investigated. Ultrasound attenuation and dispersion measurements on various samples with controlled size polydispersities, ranging from 1% to 13%, are found to be in excellent agreement with predictions based on the independent scattering approximation. By relating the particle-size distribution of the synthesized emulsions to the quality factor of the predicted multipolar resonances, the number of observable acoustic resonances is shown to be imposed by the sample polydispersity. These results are briefly discussed into the context of metamaterials for which scattering resonances are central to their effective properties.
Nusselt numbers of laminar, oscillating flows in stacks and regenerators with pores of arbitrary cross-sectional geometry133(2013); http://dx.doi.org/10.1121/1.4792138View Description Hide Description
Though widely used in steady-flow heat transfer applications, the Nusselt number—a dimensionless heat transfer coefficient—has not been studied as thoroughly in oscillating flows and is therefore not generally used in thermoacoustic applications. This paper presents expressions for the Nusselt numbers of laminar oscillating flows within the pores of stacks and regenerators, derived from thermoacoustic theory developed by Rott and Swift. These expressions are based on bulk (velocity-weighted, cross-sectionally averaged) temperature, rather than the cross-sectionally averaged temperature. Results are shown for parallel plates, circular pores, rectangular pores, and within the boundary layer limit. It is shown that bulk temperature does not become infinite during an acoustic cycle and that the Nusselt number is a complex constant at all times. In addition, steady-flow Nusselt numbers are recovered when velocity and temperature profiles are like those in steady flows.
133(2013); http://dx.doi.org/10.1121/1.4794396View Description Hide Description
A method for synthetic aperture flow imaging using dual stage beamforming has been developed. The main motivation is to increase the frame rate and still maintain a beamforming quality sufficient for flow estimation that is possible to implement in a commercial scanner. This method can generate continuous high frame rate flow images with lower calculation demands than the full synthetic aperture flow imaging. The performance of the approach was investigated using Field II simulations and measurements with the experimental scanner SARUS. A laminar flow with a parabolic profile was generated by a flow rig system. The flow data were acquired by a commercial MHz linear array transducer. Four emissions were transmitted sequentially and repeated times corresponding to emissions. Flow with a peak velocity of m/s was measured, the relative standard deviation was , and the bias was ( and for the simulations). A parameter study revealed that emission spacing, number of cross-correlation functions used for averaging, and the length of the velocity searching range influence the performance. Compared to the full synthetic aperture flow imaging the total number of beamformed samples are reduced by a factor of times, and the frame rate is much higher than the conventional method for the same velocity estimation accuracy.
133(2013); http://dx.doi.org/10.1121/1.4794367View Description Hide Description
A time domain analytical solution is presented to calculate the pressure response along the axis of a paraboloidal reflector for a normally incident plane wave. This work is inspired by Hamilton's axial solution for an ellipsoidal mirror and the same methodology is employed in this paper. Behavior of the reflected waves along reflector axis is studied, and special interest is placed on focusing gain obtained at the focal point. This analytical solution indicates that the focusing gain is affected by reflector geometry and the time derivative of the input signal. In addition, focused pressure response in the focal zone given by various reflector geometries and input frequencies are also investigated. This information is useful for selecting appropriate reflector geometry in a specific working environment to achieve the best signal enhancement. Numerical simulation employing the finite element method is used to validate the analytical solution, and visualize the wave field to provide a better understanding of the propagation of reflected waves. This analytical solution can be modified to apply to non-planar incident waves with axisymmetric wavefront and non-uniform pressure distribution. An example of incident waves with conical-shaped wavefront is presented.