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15 mins
Dispersion in unsteady jets and plumes
John Craske, Maarten van Reeuwijk
Session: Jets and Wakes 2
Session starts: Tuesday 25 August, 15:00
Presentation starts: 16:30
Room: Room E

John Craske (Imperial College London)
Maarten van Reeuwijk (Imperial College London)

We investigate the transport of both passive and active scalars in fully developed turbulent axisymmetric jets and plumes using data from direct numerical simulation. In both cases we simulate the response of the flow to an instantaneous increase in the scalar flux at the source and our focus is on the determination of the rate at which the resulting disturbance propagates and spreads in the longitudinal direction. We apply Taylor's theory of shear-flow dispersion to free-shear flows and therefore model the way in which departures from self-similarity result in the longitudinal mixing of integral quantities. The resulting integral models exhibit a good agreement with the simulation data and, in the case of passive scalar transport, admit an analytical similarity solution. For the case of active scalar transport we examine the buoyancy flux in an unsteady plume and show that the momentum-energy framework, rather than the classical volume-momentum framework, provides the natural setting from which to view the effects of dispersion. Consequently, we demonstrate the effect that dispersion has on turbulent entrainment and the way in which a plume responds to source perturbations in its buoyancy flux.