Sonic boom postprocessing functions to simulate atmospheric turbulence effects.

by Lance L. Locey

Two methods of representing the effects of atmospheric turbulence on sonic boom propagation are developed. The first method involves estimating a finite impulse response (FIR) filter which converts a sonic boom measurement made at altitude into an estimate of the same sonic boom when measured on the ground, after it has propagated through a portion of the atmosphere. The filter functions model the linear propagation between the measurement points in space and time. The filters can be applied as post processing functions to sonic boom waveforms based on computational fluid dynamics (CFD) predictions to simulate realistic turbulent propagation. Three experimental data sets are used to estimate turbulent filter functions.;The second method for developing filter functions employs a numerical algorithm to propagate an N-wave. The propagation algorithm used is the nonlinear progressive wave equation, or NPE. The NPE was modified to include a statistical representation of turbulence based on the method of Fourier mode summation, using a modified von Karman spectrum. The results from the numeric propagation can be used to estimate filter functions in the same way that the experimental data sets were used.;The filters are then applied to various shaped sonic boom waveforms. The shaped sonic booms are the result of CFD calculations for aircraft specifically designed to produce low level sonic booms, or low-booms. When the various filters are applied to the low-booms results are presented in terms of Steven's Mk VII Perceived Level (PL) of noise, which is a metric which describes the perceived loudness of a sonic boom. It is shown that the PL values are usually decreased by turbulence, but the PL values sometimes increase.

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