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Breaking of rotational symmetry in a swirling jet experiment

Abstract : In this experimental investigation, the dynamics of symmetry-breaking instabilities in swirling jets is analyzed for swirl parameters S in the pre-breakdown range O=S=Sc, where Sc=1.3 is the critical swirl value for the appearance of vortex breakdown determined by Billant, Chomaz, and Huerre [J. Fluid Mech. 376, 183 (1998)]. As S is increased, three distinct dynamical regimes have been identified in the streamwise region extending to the end of the potential core. In the low swirl range S < 0.6, the evolution is governed by the same instability mechanisms as in a nonswirling jet. The shear in axial direction generates axisymmetric vortex rings at a Strouhal number independent of the swirl S. As S increases, the amplitude of the axisymmetric mode decreases in magnitude. Concurrently, co-rotating streamwise vortices form in the braids connecting the rings due to a secondary instability mechanism. The advection by the mean rotation of these secondary structures generates an azimuthal wave propagating cyclonically when compared to the imposed rotation, at a phase velocity proportional to swirl. When swirl reaches the transitional swirl level S ~0.6, no azimuthal or standing wave is observed, and the swirling jet is completely dominated by the development of the axisymmetric mode into ring vortices. In the intermediate swirl range 0.6 < S =1, vortex rings form concurrently with several interacting helical cyclonic waves of azimuthal wave number 2. The mean phase velocity of the resulting propagating wave increases at a constant rate with swirl, and much more rapidly than in the low swirl regime. In this swirl range, azimuthal and axisymmetric deformations are of comparable high levels. In the high swirl range 1 < S < 1.3, another step toward complexity is reached, and there is a strong interaction between the azimuthal waves and the ringlike structures. The most striking feature of this flow regime is the emergence of a bending mode m=1 propagating with a high negative phase velocity. © 2003 American Institute of Physics.
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Thomas Loiseleux, Jean-Marc Chomaz. Breaking of rotational symmetry in a swirling jet experiment. Physics of Fluids, American Institute of Physics, 2003, 15 (2), pp.511-523. ⟨10.1063/1.1533068⟩. ⟨hal-01024930⟩

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