Plane turbulent buoyant jets. Part 2. Turbulence structure

Abstract

The turbulence structure of a plane vertical buoyant jet in the transition state from jet-like to plume-like growth is the object of this investigation. The ambient fluid is of uniform density and motionless except for the flow induced by the jet. An analysis of the turbulence energy equation reveals that the production of turbulent energy by the buoyancy forces relative to the production by the shear stress increases as the jet Richardson number increases, and becomes constant for a plume-like buoyant jet. A systematic set of experiments was carried out to examine the turbulence structure for a wide range of initial Richardson numbers, extending from a value appropriate to a jet-like flow (very close to zero) to that appropriate for a plume-like flow (approximately 0·6). Fast-response thermistors and a laser-Doppler velocimeter were used to measure the buoyant jet's temperature and velocity respectively. The temperature and velocity data were recorded magnetically in digital form and subsequently processed to extract both mean and fluctuating values. The turbulence intensity and the probability density distribution of the temperature and velocity fluctuations, the maximum and minimum temperature, the intermittency, and the frequency of crossing of the hot/cold and the cold/hot interface of a buoyant jet were investigated. It was determined that the intensity of temperature and velocity fluctuations increases with increasing Richardson number. An explanation is suggested for the large-scale vortices observed in a plume.