TURBULENT STRUCTURE AND ENTRAINMENT IN HEATED JETS - THE EFFECT OF INITIAL CONDITIONS

The turbulent structure in the near field of heated jets was investiga ted at a Reynolds number of 10 000 for density ratios between 1.0 and 0.5; the corresponding mixing and entrainment of these low density jet s was examined. The exit conditions of the jet were carefully controll ed using extension tubes to alter the nozzle-exit boundary layer thick ness, as well as using screens to generate turbulent conditions while keeping the boundary layer thickness approximately constant. Heated je ts with initially laminar exit conditions were dominated by the format ion and pairing of vortex structures. Increasing the boundary layer th ickness produced longer wavelength structures which saturated and pair ed at farther downstream distances; under these conditions jet momentu m mixing was reduced, giving rise to an increase in the jet potential core length. The shear layer structures also became more organized as the jet density was reduced relative to the density of the ambient flu id, resulting in increased momentum mixing and a dramatic visual sprea ding of the jet. Turbulent exit conditions disrupted the formation of these large-scale vortex structures for all of the density ratios inve stigated, producing smaller spreading rates and significantly lower mi xing and entrainment.