When
Thursday, January 22, 2026, at 4:00 p.m.
Christopher Tam
Robert O. Lawton Distinguished Professor
Department of Mathematics
Florida State University
"The Kinematics and Dynamics of Amplitude Modulation Phenomenon of Screech Tones"
AME Lecture Hall, Room S202 | Zoom link
Abstract: It is well-known that imperfectly expanded supersonic jets emit screech tones. The amplitudes of the tones remain nearly constant over time. However, in a recent experiment involving large aspect ratio rectangular jets, Malla and his coworkers reported the observation of screech tones with amplitudes that vary periodically in time. The phenomenon does not seem to have been reported in the literature before. They referred to the phenomenon as amplitude modulation. The present investigation studies the kinematics and dynamics of the phenomenon. Kinematically, it is found that amplitude modulation is akin to beats of two tones in vibration theory. However, the observed phenomenon is not simple harmonic. Strong higher harmonics are involved. Dynamically, amplitude modulation is driven by a pair of screech tones with a slight difference in frequencies. The nozzle used in Malla’s experiment has a non-symmetric design. This results in slightly non-uniform jet flow. Such a jet supports a pair of Kelvin-Helmholtz instabilities with a slight difference in frequency. They form the energy sources of the screech tones. In this investigation, a two-layer vortex-sheet jet model is used to account for the slightly non-uniform jet flow. It is well known that screech tone is generated by a feedback loop. The feedback loop consists of three important elements. They are the downstream propagating Kelvin-Helmholtz (K-H) instability wave, the upstream propagating feedback wave (a special wave which can propagate upstream against the flow of a supersonic jet and an unusual interaction between the K-H instability wave and the shock-cell structure as the former flows pass the latter. This interaction generates the tone, the upstream propagating feedback wave and a downstream propagating evanescent wave. The screech tone frequencies predicted by the model are found to be in good agreement with experiment.
Bio: Christopher Tam is the Robert O. Lawton Distinguished Professor of Mathematics at the Florida State University. He is a fellow of AIAA, the American Physical Society and the Acoustical Society of America. He received the Aeroacoustics Award and the Pendray Aerospace Literature Award from AIAA.