Book Description

"Turbulent jets are commonly encountered flows, which frequently transport a scalar and occur in both natural and man-made settings (e.g. pollutants being dispersed from a smokestack into the atmosphere; injection of reactants into a combustor). In the majority of such contexts, the jet issues into an environment that is turbulent – a factor that plays a key role in the jet’s subsequent evolution, yet one that has been the subject of little study. The objective of this work was therefore to quantify the (longitudinal and radial) velocity and scalar fields of an axisymmetric turbulent jet of heated air emitted into a turbulent background.To this end, turbulent coflows with three different levels of turbulence intensity were generated in a wind tunnel by means of passive and active grids and a flow-conditioning circuit was designed and built to generate the axisymmetric jet of heated air. Emphasis was placed on simultaneous measurements of the velocity and temperature fields, with the aim of both quantifying and further understanding the mixing of jets within a turbulent environment. Hot-wire anemometry and cold-wire thermometry were employed, using a combined hot- and cold-wire probe that was designed and built for this purpose. Simultaneous measurements of the jet’s velocity and temperature fields were made at various downstream positions and for four jet-to-coflow velocity ratios.Measurements of the velocity and temperature fields, and the combined velocity-scalar statistics, showed that the background turbulence accelerated the evolution of the jet, by increasing both the rate at which the velocity and scalar fields decay in magnitude and broaden in width with increasing downstream distance. Moreover, the effective “destruction” of the average jet was observed under certain conditions. Spectral analyses suggested that the entrainment mechanism occurs at intermediate scales for low levels of background turbulence, but that large scales of the background turbulence play a significant role when the intensity of the background turbulence increases (for the cases studied herein). Probability density functions demonstrated that (i) a larger range of instantaneous velocities throughout the jet, and (ii) the presence of cold air deeper into the jet were observed due to the background turbulence. Conditional expectations suggested that axial velocity fluctuations transport larger radial velocity (momentum) and temperature fluctuations, and that the radial transport of temperature fluctuations was enhanced when the intensity of the background turbulence increased"--