O'Gorman, P. A. and Pullin, D. I. (2003) The velocityscalar cross spectrum of stretched spiral vortices. Physics of Fluids, 15 (2). pp. 280291. ISSN 10706631. http://resolver.caltech.edu/CaltechAUTHORS:OGOpof03

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Abstract
The stretchedspiral vortex model is used to calculate the velocityscalar cross spectrum for homogeneous, isotropic turbulence in the presence of a mean scalar gradient. The only nonzero component of the cospectrum is that contributed by the velocity component in the direction of the imposed scalar gradient while the quadrature spectrum is identically zero, in agreement with experiment. For the velocity field provided by the stretchedspiral vortex, the velocityscalar spectrum can be divided into two additive components contributed by the velocity components along the vortex axis, and in the plane normal to this axis, respectively. For the axial velocity field, a new exact solution of the scalar convectiondiffusion equation is found exhibiting scalar variation in the direction of the vortex tube axis. An asymptotic expression was found for the cospectrum contributed by this solution and the axial velocity, with the leading order term showing a k–5/3 range. This term is produced by the winding of the initial axial velocity field by the axisymmetric vortex core. The next order term gives a k–7/3 range, and arises from the lowest order effect of the nonaxisymmetric vorticity on the evolution of the axial velocity. Its coefficient can be of either sign or zero depending on the initial conditions. The contribution to the cospectrum from the velocity in the plane of the vortex is also calculated, but no universal high wave number asymptotic form is found. The integrals are evaluated numerically and it is found that the resulting cospectrum does not remain of one sign. Its form depends on the choice of the vortex core velocity profile and time cutoff in the spectral integrals. The onedimensional cospectrum contributed by the axial velocity is compared with the experimental data of Mydlarski and Warhaft [J. Fluid Mech. 358, 135–175 (1998)].
Item Type:  Article 

Additional Information:  ©2003 American Institute of Physics. (Received 2 July 2002; accepted 17 October 2002; published online 19 December 2002) We wish to thank L. Mydlardski for providing experimental data, and also P. Dimotakis for useful suggestions. P.A.O’G. and D.I.P. were supported in part by the National Science Foundation under Grant No. CTS9978551. 
Subject Keywords:  vortices; turbulence 
Record Number:  CaltechAUTHORS:OGOpof03 
Persistent URL:  http://resolver.caltech.edu/CaltechAUTHORS:OGOpof03 
Alternative URL:  http://dx.doi.org/10.1063/1.1527916 
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ID Code:  2785 
Collection:  CaltechAUTHORS 
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Deposited On:  27 Apr 2006 
Last Modified:  26 Dec 2012 08:50 
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