ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

1pPA4. Effects of atmospheric stratification on sonic-boom propagation.

Robin O. Clevel

Appl. Res. Labs., P.O. Box 8029, Austin, TX 78713-8029

Sonic-boom propagation is affected by stratification, geometrical spreading, nonlinear distortion, absorption and dispersion, and turbulence. Stressed in this paper is stratification, in particular its indirect effect on distortion and absorption. The stratification of the density and sound speed leads to refraction and impedance variation, which play a major role in determining the amplitude of the waveform on the ground. Stratification, and associated spreading, can also control the amount of nonlinear distortion a sonic boom suffers. In extreme cases the amount of nonlinear distortion is finite---a phenomenon known as waveform freezing. Analysis shows that for aircraft in the lower 20 km of the atmosphere waveform freezing does not occur. Through their dependence on temperature, pressure, and humidity, absorption and dispersion are also stratified. A new time domain algorithm, based on a Burgers-type equation, was developed to analyze the effect of the stratification of absorption [ 3275 (A) (1994)]. The code is used to predict sonic-boom waveforms on the ground. It is demonstrated that sonic-boom shocks are affected not just by local absorption but also by absorption along path for the past 5 km or so. [Work supported by NASA.]