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Nonlinear global modes and sound generation in hot jets.

Abstract : The objective of this thesis is twofold: firstly, to study the dynamics inherent in the near field of axisymmetric hot jets by direct numerical simulation, and describe the synchronized oscillations in terms of the theory of nonlinear global modes. On the other hand, characterize the aero-acoustic far field generated by the synchronized oscillations of the near field, also by direct numerical simulation, and identify the physical nature of the aero-acoustic sources in the jet. Thus it is established that the intrinsic synchronized oscillations are observed in the simulations have all the characteristics of a nonlinear global mode, governed by a stationary front located at the upstream edge of an absolutely unstable environment. A linear stability analysis reveals that a jet becomes absolutely unstable hot enough near the nozzle, through the destabilizing effect of the baroclinic torque. The emergence of self-sustained oscillations and their frequency, observed numerically, precisely follow the predictions derived from theoretical criteria. The numerical results are in good agreement with experiments from the literature. The global mode of a hot jet emits a dipole sound field and an analysis of the Lighthill equation shows that this radiation is due to fluctuations in entropy, in contrast to an isothermal jet forced, which sources linked quadrupole the Reynolds stresses are dominant.
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Submitted on : Friday, July 23, 2010 - 3:58:33 PM
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Lutz Lesshafft. Nonlinear global modes and sound generation in hot jets.. Engineering Sciences [physics]. Ecole Polytechnique X, 2006. English. ⟨pastel-00002958⟩

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