Similarity spectra and the effect of propagation on F-22 spectra at afterburner

Abstract

The afterburner spectra of a static F-22 Raptor have been compared to the similarity spectra from the two-source theory for jet noise. In the two-source theory, fine-scale turbulent structure (FSS) account for sideline radiation and large-scale structures (LSS) dominate the radiation for inlet angles aft of 125 deg for model-scale jets [Tam and Zaman, AIAA J. 38(4), 592–599 (2000)]. A comparison of the similarity spectra with F-22 afterburner data, recorded at ground-based microphones along the 40-ft foul line, shows good agreement between the FSS spectrum and the sideline radiation and the LSS spectrum and the spectrum measured at 150 deg. However, discrepancies exist between the measured spectrum in the direction of maximum radiation and the LSS spectrum. Most significantly, the high frequency roll-off in the data is shallower (approx. 20 dB/decade) than predicted by the LSS (29.8 dB/decade). Most likely this difference comes from nonlinear propagation of the noise, which could then cause the agreement with the similarity spectra to be dependent on distance. Results from the ongoing investigation into how the measured spectra evolve relative to the FSS and LSS spectra as the noise propagates are presented. [Work sponsored by the Air Force Research Laboratory.]