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Dispersion in steady and unsteady turbulent jets.

Dr. Maarten van Reeuwijk

Department of Civil and Environmental Engineering, Imperial College

Shear-flow dispersion is a well-known phenomenon that arises in the formulation of integral models when the (mean) velocity fields contains gradients. The classical theory of shear-flow dispersion as developed by Taylor considers problems with well-defined boundaries, such as pipes and channels. Free-shear flows such as turbulent jets, however, have no well-defined boundaries. In this presentation I present data obtained from direct numerical simulation of a turbulent jet, which demonstrates that shear-flow dispersion is the primary longitudinal mixing mechanism. We develop a dispersion closure for the total concentration flux which, when integrated in an integral model, is in good agreement with the DNS data. We then proceed to study unsteady turbulent jets with DNS and demonstrate that the dispersion closure models the jet behaviour excellently if applied to the flux of mean kinetic energy.