Simulation of N-wave and Shaped Supersonic Signature Turbulent Variations PDF Download

Author: Trevor Stout
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Languages : en
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Book Description
One of the design paradigms for the next generation of civil supersonic aircraft targets a reduction in sonic boom amplitude and impulsivity, creating a shaped signature at the ground which is likely more acceptable to the public. The unshaped, conventional N-wave sonic boom is well known to distort due to atmospheric turbulence or wind and temperature fluctuations, randomly increasing or decreasing the loudness perceived by listeners. However, the effect of turbulence on the shaped signature is not well understood. No acoustic databases from powered, fully-shaped aircraft yet exist because these prototypes are currently in development. Thus, the present work uses a numerical model based on an augmented nonlinear KZK propagation equation and simulated shaped signatures to analyze the turbulence effects. The KZK equation accounts for absorption, nonlinearity, advection due to turbulence and resultant diffraction, as well as other propagation effects. A novel implementation of a fully time-domain solution is put forth. As is typically done in the literature, the solution to the KZK equation in two dimensions is predominantly used in the present work, though a computationally-intensive 3D algorithm is applied to a limited case. As part of the recently-completed Sonic Booms in Atmospheric Turbulence (SonicBAT) project, supersonic flyover measurement campaigns were conducted to produce the first database of its kind with acoustic data of N-waves from powered, manned aircraft and concurrent atmospheric turbulence and weather data. One objective was to provide suitable inputs for the present numerical model so that the simulated output statistics could be compared with the measured. This comparison shows reasonable agreement between the model and the measurement for most cases across a wide range of atmospheric and turbulence parameters, suggesting that the KZK model is suitable for predicting full-scale sonic boom loudness variations. Using the validated algorithm, turbulence effects on simulated shaped signatures are analyzed in many hypothetical atmospheres and compared to the effects on the N-wave, revealing that the boom shaping tends to reduce loudness variations. Finally, the prevailing use of 2D simulations in the literature and the present work is qualified by comparison to comparable 3D simulations, finding that inclusion of the third dimension somewhat augments the turbulence effects and increases the predicted probability of high-amplitude N-wave booms.