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Global Modes and Aerodynamic Sound Generation In Self-Excited Hot Jets

Lutz Lesshafft Postdoctoral Fellow University of California at Santa Barbara Santa Barbara, CA One of the most remarkable phenomena in the field of aerodynamic instability is the spontaneous bifurcation of a steady flow towards a selforganized state of intrinsic oscillations. Similar to the von Kármán vortex street in cylinder wakes, hot jets constitute another class of such globally unstable flows: whereas an isothermal jet behaves as an amplifier of external perturbations, sufficiently heated jets display intrinsic oscillations in the form of regularly spaced ring vortices. Comparison of direct numerical simulation results to theoretical predictions, derived from Ginzburg-Landau model equations, demonstrates that these self-sustained oscillations in subsonic hot jets are dominated by the dynamics of a nonlinear wave front, which separates an oscillating flow region from the upstream steady flow. The bifurcation towards a state of self-sustained synchronized oscillations ('nonlinear global mode') is due to the existence of an absolutely unstable region in the underlying base flow. A linear stability analysis allows us to predict the naturally selected frequency, as well as the critical temperature ratio for the onset of global instability. Both the near- and the far-field of the jet are resolved via DNS: the acoustic field generated by such a synchronized vortex street is found to be that of a compact dipole, with maximum acoustic intensity in the axial direction of the jet. A numerical analysis of the Lighthill equation reveals that this radiation pattern is due to strong entropy fluctuations within the jet.

Location: Stauffer Science Lecture Hall, Rm 102

Audiences: Everyone Is Invited

Contact: April Mundy