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

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.