Richard Raspet

General formulation of thermoacoustics for stacks having arbitrarily shaped pore cross sections

Theoretical treatments of thermoacoustics have been reported for stacks with circular pore and parallel plate geometries. A general linear formulation is developed for gas‐filled thermoacoustic elements such as heat exchangers, stacks, and tubes having pores of arbitrary cross‐sectional geometry. For compactness in the following, F represents the functional form of the transverse variation of the longitudinal particle velocity. Generally, F is a function of frequency, pore geometry, the response functions and transport coefficients of the gas used, and the ambient value of the gas density. Expressions are developed for the acoustic temperature, density, particle velocity, pressure, heat flow, and work flow from knowledge of F. Heat and work flows are compared in the short stack approximation for stacks consisting of parallel plates, circular, square, and equilateral triangular pores. In this approximation, heat and work flows are found to be greatest for the parallel plate stack geometry. Pressure and spe...

Acoustic streaming in closed thermoacoustic devices

A derivation of acoustic streaming in a steady-state thermoacoustic device is presented in the case of zero second-order time-averaged mass flux across the resonator section (nonlooped device). This yields analytical expressions for the time-independent second-order velocity, pressure gradient, and time-averaged mass flux in a fluid supporting a temperature gradient and confined between widely to closely separated solid boundaries, both in the parallel plate and in the cylindrical tube geometries (two-dimensional problem). From this, streaming can be evaluated in a thermoacoustic stack, regenerator, pulse tube, main resonator of a thermoacoustic device, or in any closed tube that supports a mean temperature gradient, providing only that the acoustic pressure, the longitudinal derivative of the pressure, and the mean temperature variation are known.

Calculation of turbulence effects in an upward refracting atmosphere

In an upward refracting atmosphere, measured values of excess attenuation (50–500 Hz) seldom exceed 20 to 30 dB at a range of 1 km. Calculations of excess attenuation for a steady (nonturbulent) atmosphere predict a deep shadow zone with much higher excess attenuation. This paper investigates the contribution of atmospheric turbulence to decreasing the predicted excess attenuation. Since no convenient analytical method presently exists which can simultaneously account for turbulence, upward refraction, and a finite‐impedance ground surface, a parabolic equation method is used to numerically simulate sound propagation. As an initial test, the calculation is compared to Daigles theoretical and experimental results for a homogeneous atmosphere [J. Acoust. Soc. Am. 65, 45–49 (1979)]. For a test with upward refraction, the calculation is compared to the experimental results of Wiener and Keast [J. Acoust. Soc. Am. 31, 724–733 (1959)].

Investigation of the mechanisms of low‐frequency wind noise generation outdoors

Simultaneous measurement of wind noise and the instantaneous wind speed were performed for bare and screened microphones outdoors. Analysis of these measurements demonstrates that the dominant source of pressure fluctuations at the microphone outdoors is the intrinsic turbulence in the flow. This is in contrast to the results of measurements performed in low‐turbulence environments by Hosier and Donavan [Natl. Bur. Stand. Rpt. NBSIR79‐1599 (Jan. 1979)] and by Strasberg [J. Acoust. Soc. Am. 83, 544–548 (1988)]. For low‐turbulence conditions the fluctuating wake of the screen is the dominant noise source. This finding has important implications for windscreen design for outdoor measurements since the principles described by Hosier and Donavan apply only to low‐turbulence conditions.

A numerical method for general finite amplitude wave propagation in two dimensions and its application to spark pulses

The general equations of finite amplitude acoustics, including classical absorption effects and second‐order nonlinear effects, are written in a form suitable for two‐dimensional numerical solution. A finite difference scheme then is applied to numerically solve the equations. To demonstrate the method, examples are given of spherical free‐field propagation, normal plane reflection from a hard surface, and oblique spherical reflection from a hard surface for spark pulses. This method has an advantage over Burgers’ equation methods, one‐way wave equation methods, and Pestorius type algorithms in that it can predict the interaction of multiple finite amplitude acoustic waves at arbitrary propagation angles.

COMPARISON OF COMPUTER CODES FOR THE PROPAGATION OF SONIC BOOM WAVEFORMS THROUGH ISOTHERMAL ATMOSPHERES

A numerical exercise to compare computer codes for the propagation of sonic booms through idealized atmospheres is reported. Ground waveforms are calculated using four different codes or algorithms: (1) weak shock theory, an analytical prediction; (2) SHOCKN, a mixed time and frequency domain code developed at the University of Mississippi; (3) ZEPHYRUS, another mixed time and frequency code developed at the University of Texas; and (4) THOR, a pure time domain code recently developed at the University of Texas. The codes are described and their differences noted. They are then used to calculate propagation for two different source waveforms, for both a uniform and isothermal (varying density) atmosphere, with and without the presence of molecular relaxation. In all cases the results of THOR, SHOCKN, and ZEPHYRUS are in excellent agreement. Because the weak shock theory algorithm does not include the effect of ordinary absorption, it does not predict the shock structure provided by the other codes. This l...

A fast‐field program for sound propagation in a layered atmosphere above an impedance ground

This paper studies sound propagation in a layered atmosphere bounded by a ground, whose impedance is described by the Delany–Bazley–Chessell’s empirical model. The problem is formulated in terms of a Green’s function integral in the spectral domain, and is numerically evaluated by a Fast Field Program (FFP). Numerical results are included to show that (i) in simple test cases, the FFP solution is in excellent agreement with existing asymptotic solutions; (ii) numerical overflow arises when the number of layers is large and/or the frequency is high, and a method to circumvent this difficulty is described; and (iii) the FFP is a most powerful tool in solving propagation problems in layered media bounded by complex impedances.

Evaporation–Condensation Effects on Resonant Photoacoustics of Volatile Aerosols

In determining the optical properties of the atmosphere, the measurement of light absorption by aerosols is particularly challenging, and yet it is important because of the influence of strongly absorbing black carbon on climate and atmospheric visibility. The photoacoustic method obtains aerosol light absorption in situ, without use of filters, by acoustic measurement of the heat generated from aerosol light absorption, and its transfer to the surrounding air. However, in the general case, volatile aerosols heated by light absorption may also cool by evaporation (mass transfer). In this paper, the limiting case of the photoacoustic response of a volatile aerosol is compared with that of a dry aerosol to further the understanding of the data obtained with photoacoustic instruments. While the theory of photoacoustics of volatile aerosols for low-frequency, nonresonant cells has already been developed, current methods employ high-frequency, acoustically resonant photoacoustic instruments for quantifying atmospheric aerosol light absorption and vehicle exhaust mass concentration associated with black carbon. In this paper, a complete theory of photoacoustics for volatile aerosols is developed that includes additional terms to allow for higher-frequency devices, large particles, and high particle densities. Numerical calculations are used to determine the limits of various approximations.

Working gases in thermoacoustic engines

The best working gases for thermoacoustic refrigeration have high ratios of specific heats and low Prandtl numbers. These properties can be optimized by the use of a mixture of light and heavy noble gases. In this paper it is shown that light noble gas-heavy polyatomic gas mixtures can result in useful working gases. In addition, it is demonstrated that the onset temperature of a heat driven prime mover can be minimized with a gas with large Prandtl number and small ratio of specific heats. The gas properties must be optimized for the particular application of thermoacoustics; it cannot be assumed that high specific heat ratio and low Prandtl number are always desirable.

Impedance formulation of the fast field program for acoustic wave propagation in the atmosphere

In an earlier paper, the present authors’ work in adapting the fast field program (FFP) formulation to atmospheric propagation above a complex impedance boundary was described. It was found that numerical overflow problems for high frequencies and multiple layers limited the utility of the FFP in solving atmospheric problems. In this paper is a description of a new formulation which eliminates the overflow problems inherent in the earlier formulation. The results of these two formulations are compared for a test case and the superiority of the new formulation is demonstrated. Results of the FFP2 for a simple atmospheric profile are compared with field measurements and the applicability discussed.