Harmonics of Jet Screech Tones

It is known that imperfectly expanded supersonic jets undergoing strong screech emit not only the fundamental screech tone but also multiple tones at the harmonics of the fundamental frequency. The dominant direction of radiation of the fundamental screech tone is in the upstream direction toward the nozzle exit. This is necessary to complete the feedback loop responsible for the generation of the screech tone. The harmonic tones, however, do not generally radiate in the same direction as the fundamental. For the mth harmonic screech tone, there are m distinct beams of radiation. Many of these beams are radiated in the sideline and downstream directions. The purpose of this study is to investigate the generation mechanism of the screech harmonics. It is proposed that the screech tone harmonics are produced by Mach wave radiation from supersonic traveling waves in the jet flow. These supersonic traveling waves are created by the interaction of the nonlinear instability wave of the jet flow and the shock-ce...