Single buoyant jets in a crossflow and the advected line thermal

Abstract

The behaviour of a single buoyant jet in a stationary and in a flowing ambient has been investigated. The entrainment velocities into the jets were studied using a particle image velocimetry technique, which resulted in velocity vector maps of the entrainment at a vertical centreline section and at a cross-section. The results indicated that the cross flow affected the entrainment into the buoyant jet and supported the hypothesis of superposition of the crossflow and entrainment velocities in the irrotational flow region outside the jet. This was incorporated into the analysis of a buoyant jet having Gaussian distributions of excess velocity and buoyancy in a crossflow and resulted in equations showing how, as the crossflow increased, the normal entrainment changed naturally into a forced entrainment formulation, in which the entrainment flux is approximately equal to the projected area of the jet. The theoretical predictions were in good agreement with the experimental results. The mean properties of the advected line thermal were studied using planar laser induced fluorescence. The results compared well to the existing theory and previous research. Turbulence in the ambient was determined to disrupt the flow structure and result in greater dilutions. The experiments revealed the advected thermals as a complex and irregular three dimensional flow characterized by subthermal structures. The unsteady behaviour and the three dimensional structure were studied using statistical studies of the concentration fluctuations and the temporal intermittency of the flow. These results demonstrated the importance of the sub thermal structures in determining the behaviour of the flow. The subthermal structures were further studied to determine the periodicity of the subthermal formation.