Turbulent entrainment in flows induced by distributed buoyancy sources

Free shear and wall-bounded buoyancy-driven turbulent flows occur in both natural environments and industrial situations. In this thesis, to better understand the entrainment process within these flows, experiments and theory have been used to investigate point and distributed buoyancy sources and, in particular, the effect of a bounding vertical wall on these flows. A free shear flow was first investigated by performing velocity and scalar edge measurements on an axisymmetric plume created by a continuous point source release of buoyancy. By conditionally sampling the velocity measurements based on the presence of both eddies and plume fluid, engulfment, whereby large pockets of ambient are engulfed in to the plume, was shown to be a dominant turbulent entrainment process. To isolate the effect of a wall on a turbulent buoyancy-driven flow, a line plume distant from all vertical boundaries and a wall plume, adjacent to a vertical wall were also studied. Simultaneous velocity and buoyancy field measurements were performed and a reduction in the net entrainment, and entrainment coefficient, for a wall plume were found. This reduction was investigated by considering an energy decomposition of the entrainment coefficient where the relative contributions of turbulent production, buoyancy and viscous terms were calculated. The reduced entrainment was also investigated by considering the statistics of the turbulent interface. Finally, simultaneous velocity and buoyancy field measurements on a vertically distributed buoyant plume were performed by forcing relatively dense fluid through a very low porosity plate. A reduced entrainment coefficient, compared to that of a wall plume, was observed. In order to model the ventilation of a room with a heated or cooled wall the flow was then enclosed within a mechanically ventilated model room. The evolving and steady-state ambient stratification was measured using dye-attenuation with an LED-light bank for varying buoyancy fluxes and ventilation flow rates.PhD sponsored and partly funded by Aru