Preliminary Analysis of Nonlinearity in F/A-18E/F Noise Propagation

Abstract

*† ‡ § Analyses of recent F/A-18E/F military power and afterburner measurements suggest that the noise propagation is nonlinear in the far field. Spectral broadening takes place as the radiated noise propagates from 18 to 150 m, the limits of the measurement range. This broadening phenomenon cannot be readily explained in terms of linear propagation effects. Calculation of a nonlinearity indicator derived by Howell and Morfey supports the assertion that nonlinear propagation effects are present. Furthermore, skewness and kurtosis calculations indicate that the noise data distributions are non-Gaussian over the propagation range at these engine settings. Finally, the measured spectra have been compared against predictions obtained with existing nonlinear spectral evolution methods. Even though the prediction results compare poorly with the actual measurements, this is attributable to limitations of the spectral evolution methods themselves. I. Introduction HIS paper contains a preliminary analysis of F/A-18E/F Super Hornet static engine run-up noise measurements made at NAVAIR Lakehurst, NJ on 15 April 2003. Results of the analysis show evidence of nonlinear acoustic propagation effects. These effects are typified by a spectral broadening in which energy is transferred from midfrequencies to the ends of the spectrum. The evolution of a finite-amplitude noise spectrum may be explained in terms of two time-domain phenomena. Waveform steeping is responsible for the transfer of energy from mid to high frequencies, whereas shock coalescence and a corresponding increase in time scale accounts for a relative increase of energy at low frequencies 1 . In this paper, the measurement setup and apparatus as well as analysis procedure are first described. Next, the probability density function (PDF) and related statistical quantities are calculated. The measured spectra are compared with linear predictions, which take into account spherical spreading and atmospheric absorption, as well as nonlinear predictions from two existing jet noise prediction schemes 2-4 . The assertion that the disparity between linear predictions and measurements are caused by nonlinear propagation is supported by calculation of a quantity derived by Howell and Morfey as a useful nonlinearity indicator 3 .