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TURBULENT DISPERSION IN STABLY STRATIFIED SHEAR FLOW

Frank G. Jacobitz
Department of Mechanical Engineering, University of California, Riverside, Riverside, California 92521-0425, USA

Abstract

Direct numerical simulations are performed in order to investigate turbulent dispersion of concentration fields in stably stratified shear flow. The Richardson number is varied from Ri = 0, corresponding to unstratified shear flow, to Ri = 0.4, corresponding to strongly stratified shear flow. The total energy is found to grow for weakly stratified cases with Ri ≤ 0.1 and to decay for strongly stratified cases with Ri > 0.1. The kinetic energy is distributed unevenly over the three velocity components with downstream > span wise > vertical.
Turbulent dispersion of two species cz and cy with initially Gaussian mean concentration variations in the vertical z and spanwise y directions is investigated. At a given Richardson number Ri, a slower spreading of cz in the vertical direction is observed compared to the spreading of cy in the spanwise direction. This observation is consistent with a lower turbulent fluctuation level in the vertical velocity component compared to the spanwise velocity component. With increasing Richardson number, dispersion in both the vertical and spanwise direction is decreased due to decreased turbulent velocity fluctuations. A variation of the initial widths of the mean concentration profiles results in nearly identical values of the widths toward the end of the simulations.