_ Vol. 26 (2) (2000) pp. 105-117 _
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Fluid Dynamics Research, Vol. 26 (2) (2000) pp. 105-117
© 1999 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All rights reserved.
PII: S0169-5983(99)00017-9

Coherent structures in numerically simulated jets with and without off-source heating

S.S. Siddhartha a, R. Narasimha b * roddam@caos.iisc.ernet.in, A.J. Basu b and S.V. Kailas c

a Indian Institute of Technology Madras, Chennai 600 036, India
b Fluid Dynamics Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India
c Centre for Civil Aircraft Design and Development, National Aerospace Laboratories, Bangalore 560 012, India

Received 18 January 1999; accepted 25 March 1999

Abstract

Direct numerical solutions of the incompressible Navier-Stokes equations, under the Boussinesq approximation, for the temporal evolution of a jet-like flow have been analyzed to educe coherent structures. The eduction procedure involved both conventional image processing and the application of the wavelet transform -- here used as a spatially delimited filter to smooth out fine scale discontinuities and reveal the underlying order. Attention has been focussed on the vorticity and its components (azimuthal, radial and streamwise). It is found that the nature of the coherent motion is most strongly evident in the azimuthal component of the vorticity, and is revealed to consist of a toroidal base supporting a thin conical sheath; the interior of the structure is nearly devoid of azimuthal vorticity. There is some evidence of a secondary structure in the radial and streamwise components of the vorticity, which show strips of opposite sign close to each other, suggesting vortex pairs, possibly helically organized. With addition of volumetric heat after the (unheated) jet has achieved self-similarity, the structures tend to telescope into each other because of the acceleration produced by the heating, and the coherence present in the unheated jet is severely disrupted.

Keywords: Turbulent jet; Direct numerical solutions; Coherent structures; Off-source heating; Wavelet transform

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