William C. Ward

Two-sensor power measurements in lossy ducts

Theory and measurements of the use of two adjacent pressure sensors to measure acoustic power flow of a simple harmonic sound wave in a duct are presented. This theory differs from the usual intensity-times-area formulation of this problem by including the phase shift between pressure gradient and velocity, which is caused by viscous drag on the gas at the duct wall. For high standing-wave ratios, the power obtained by this method differs significantly from the product of mid-duct intensity and duct area. These measurements confirm the method to an accuracy of 5%, even at high amplitudes where the acoustic flow is turbulent and the theory might not necessarily be valid.

Design environment for low‐amplitude thermoacoustic energy conversion (DeltaEC)

The Los Alamos thermoacoustics code, available at www.lanl.gov/thermoacoustics/, has undergone extensive revision this year. New calculation features have been added to the original Fortran computational core, and a Python‐based graphical user interface wrapped around that core provides improved usability. A plotter routinely displays thermoacoustic wave properties as a function of x or tracks results when a user‐specified input variable, such as frequency or amplitude, is varied. The Windows‐like user interface provides mouse‐based control, scrolling, and simultaneous displays of plots and of several categories of numerical values, in which color indicates important features. Thermoacoustic phenomena can be calculated with superimposed steady flow, and time‐averaged pressure gradients are calculated. In thermoacoustic systems with toroidal topology, this allows modeling of steady flow caused by gas diodes (with or without time‐averaged heat transfer) and Gedeon streaming. Thermoacoustic mixture separatio...

Simple harmonic analysis of regenerators

A simple harmonic analysis of the performance of regenerators is described. Computer calculations based on the simple harmonic analysis run orders of magnitude faster than time-integration methods, with acceptably small reductions in accuracy. This simple harmonic analysis method is based on previous one-dimensional differential equations of heat and mass flow, with the additional assumption of steadystate operation and with the key approximations that only the magnitudes and phases of the fundamental components of time-dependent variables are of interest and that the instantaneous friction-factor and heat transfer coefficients are determined from the instantaneous velocity via steady-flow correlations. This permits a rapid computation scheme in which complex amplitudes of oscillatory variables are used, without numerical time integrations. The method is presented here using the Kays and London correlations for stacked-screen regenerators, but it can be used for any regenerator matrix for which steady friction-factor and heat transfer correlations exist.

Visualizing industrial CT volume data for nondestructive testing applications

This paper describes a set of techniques developed for the visualization of high-resolution volume data generated from industrial computed tomography for nondestructive testing (NDT) applications. Because the data are typically noisy and contain fine features, direct volume rendering methods do not always give us satisfactory results. We have coupled region growing techniques and a 2D histogram interface to facilitate volumetric feature extraction. The new interface allows the user to conveniently identify, separate or composite, and compare features in the data. To lower the cost of segmentation, we show how partial region growing results can suggest a reasonably good classification function for the rendering of the whole volume. The NDT applications that we work on demand visualization tasks including not only feature extraction and visual inspection, but also modeling and measurement of concealed structures in volumetric objects. An efficient filtering and modeling process for generating surface representation of extracted features is also introduced. Four CT data sets for preliminary NDT are used to demonstrate the effectiveness of the new visualization strategy that we have developed.

Cost‐effective electrodynamic drivers with improved efficiency for thermoacoustic refrigerators

Commercial loudspeakers were made practical for laboratory thermoacoustics by placing one in the cold end of the resonator [S. R. Murrell and G. Mozurkewich, J. Acoust. Soc. Am. 94, 1772(A) (1993)], but the efficiency of such devices suffers from the opposed directions of traveling wave/standing wave heat pumping effects and from additional thermal loads at the cold heat exchanger. Efficient designs using drivers at the higher temperature end typically require special materials and fabrication. In the proposed compliant configuration, a driver with mechanical resonance below the operating frequency (which is typical of available loudspeakers) is placed close to the ambient heat exchanger and the volume backing the driver is tuned to achieve resonance and a high electrical efficiency. Numerical results show that the current ×BL product can be as little as 25% of the required driver effort. This bypasses requirement for the expensive, high flux‐density magnets needed in successful efficient designs with hig...

DP: a Fast Median Filter Approximation

We present a new non-discrete algorithm that quickly approximates a median filter. This new algorithm proves to be faster than our implementations of many other fast median filter algorithms.

Initial tests in AT&T Bell Labs’ Varechoic Chamber

Results are available for the first measurements in the recently completed Varechoic chamber, a digitally controllable variable acoustics facility at ATT this talk will present full 1/3‐oct decay rates of the room. Some interesting impulse responses, and comparisons with image model calculations, will also be given.

Thermoacoustic power conversion for space power applications

Thermoacoustic engines are a recent class of devices that convert between heat and sound energy without moving parts. When coupled to a suitable transducer, thermoacoustic prime movers can produce electric power with high reliability and efficiency in lightweight packages that feature low vibration levels. This paper begins with an introduction of thermoacoustics and an overview of design and optimization and then considers analytical results for two transducers that also have no moving parts: piezoelectric transduction for a helium‐based engine, and magnetohydrodynamic (MHD) transduction in an engine with liquid sodium working fluid.

Test results and design analysis for a thermoacoustic underwater projector

An experimental thermoacoustic projector (a heat‐driven sound source without moving parts) produced source levels near 190 dB at 120 Hz during recent tests at the Navy’s Seneca Lake facility. These data were taken near 60 m depth; in thermoacoustic projectors, the source level increases linearly with depth. The device is composed of two coupled vertical tubes. The upper driver tube is filled with helium and contains a thermoacoustic stack with hot and cold heat exchangers. The lower tube is an impedance matching device filled with water up to a variable level. The tube opening is necessarily small compared to wavelength. For a radiation impedance with such a small resistive component, the test device demonstrated a resonant mode ambiguity before reaching the optimum tuning point. This effect reduced the maximum source level by 3–5 dB below the design level, and had a similar effect on the overall efficiency. This presentation will give an overview of the experimental results and introduce design modificat...

Materials for thermoacoustic generation of sound

Thermoacoustic generation of sound underwater represents a technology distinctly different from conventional underwater transduction. By proper design, there is potential for high‐power, low‐frequency generation of continuous‐wave, pulsed, or modulated output. From a materials point of view, there are three major components: the stack, which maintains the temperature gradient responsible for pumping the acoustic oscillations; the working medium, which supports the internal acoustic oscillations; and the heat exchangers, which supply energy from an external source to the stack. Each of these subsystems has critical thermal and acoustical requirements that constrain the selection of materials and the overall source configuration. Thermal conductivity, viscosity, and specific heat replace piezoelectric constants and stress‐strain relationships as fundamental properties. In addition, the acoustic performance is modeled more effectively as a waveguide than as a lumped‐parameter system. Many of the advantages a...