Volume 104, Issue 4, October 1998
Index of content:
- AEROACOUSTICS, ATMOSPHERIC SOUND 
104(1998); http://dx.doi.org/10.1121/1.423721View Description Hide Description
A new ray-tracing method for predicting the sound field near a flat impedance ground in a refracting atmosphere is developed that includes the effect of vector wind and turbulence explicitly. Improvements on previous ray-tracing schemes include the use of a generalized Snell’s law, an integration by means of a Gaussian quadrature, and a bracketing method for finding the ray paths. For the turbulence calculations, the interray coherence is determined from turning point heights rather than from integrals along the ray paths. A more efficient algorithm is developed for computing the sound field above an impedance plane in a realistic atmosphere. Despite the inherent limitations of the ray-trace approach, particularly in respect of logarithmic wind profiles, the algorithm is valid over a wide range of practical situations.
104(1998); http://dx.doi.org/10.1121/1.423722View Description Hide Description
The feedback mechanism of low-speed edgetones is analyzed by using the jet–edge interaction model in which reaction of the edge is modeled by an array of dipoles. From the jet–edge interaction model the surface pressure of the edge and the upstream wave are estimated by assuming the downstream disturbance as a sinuously oscillating flow with a constant convection speed. The surface pressure distribution on the edge is found to increase from zero at the edge tip to a peak value around a quarter wavelength downstream, which may be regarded as the effective source point of the upstream-propagating sound wave. From the condition that the two wave trains should be phase-locked at the nozzle lip, is obtained for low-speed edgetones in the phase criterion of the form, where h is the stand-off distance, Λ and λ are the wavelengths of downstream and upstream, respectively, and n is the stage number. Based on the phase criterion, the ratio of the convection speed, to the jet speed, is estimated from the experimental data for low-speed edgetones and found to be about and to be almost independent of frequency. Finally, an approximate model for the frequency characteristics has been obtained in the form, where d is the width of the two-dimensional nozzle, the Mach number of the jet velocity, St is the Strouhal number, and f is the frequency. The present model is confirmed substantially in comparison with available experimental data.