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In order to provide fundamental data on the acoustic energy released by combustion processes, measurements were made of the noise arising from open turbulent flames. These studies of the acoustic radiation from premixed open flames, which relate the characteristics of the noise to the various geometric and fluid-flow parameters defining the flames, are described. Examination of the combustion-noise spectra showed that their peaks could be expressed by a constant value of the Strouhal number (fmax⋅D/Ub), where D is the burner-port diameter and Ub the normal burning velocity. The acoustic power (P) generated by a turbulent flame was found to be related to the fluid-flow parameters by the expression P∝q2⋅Ufn(Ub), where q is the mixture flow rate and U the flow velocity. In this expression, the exponent of the mixture velocity varies in such a way that the rate of increase in acoustic-power output with mixture velocity is greatest for low values of the combustion velocity, and is reduced as the combustion velocity increases. [This work was supported by the Joint Research Committee of the Gas Council and by The University of Leeds.]

In order to provide fundamental data on the acoustic energy released by combustion processes, measurements were made of the aerodynamic sound generated by premixed open turbulent flames. Studies are described that relate the characteristics of the noise to the various geometric and flow parameters defining the flame, and it is shown that the sound radiated may be considered to arise from a statistical distribution of monopole sources throughout the zone of combustion. Examination of the combustion-noise spectra indicated that their peak frequencies may be expressed by a constant value of a Strouhal number, in terms of the burner-port diameter, the mixture flow velocity, and the flame speed. The acoustic power radiated by a turbulent flame was found to be related to the flow parameters by the expression per P ∝ (ρU4D2/c)(Ub/U)2. This relationship is shown to be similar to the basic equation for the acoustic-power output of a monopole source.

The Hartmann whistle, in its most basic configuration, consists of a flat-bottomed, cylindrical cavity which is axially aligned with a supersonic air jet of the same diameter. Discrete-frequency oscillations of the enclosed air column are driven at large amplitudes when the cavity is located within certain regions of the cellular structure of the jet. An optical and acoustical study of the phenomenon is described, together with that of the Hartmann 'pulsator'. In the latter form the whistle has the small cavity replaced by a large Helmholtz-type resonator with the same orifice diameter, resulting in a large-amplitude aeroacoustic oscillator with a periodic time of several orders of magnitude greater than for the regular whistle. The underlying cause of the newly discovered bistable condition of the normal 'shock-disc' located in the alrstream between the nozzle and the cavity orifice is an important aspect which makes possible a (presently qualitative) theory of operation which accounts for the principal features of the Hartmann whistle and its direct derivatives. Some other aspects still requiring further elucidation and which are the subject of continuing effort are mentioned. The Report includes a brief review of the currently available literature pertaining to the phenomenon.

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