Self-Preserving Properties of Steady Round Nonbuoyant Turbulent Jets in Uniform Crossflows

Abstract

The properties of steady round nonbuoyant turbulent jets in uniform crossflows were studied, motivated by applications to the dispersion of heat and potentially harmful substances from steady exhaust flows. Emphasis was placed on self-preserving conditions far from the source where source disturbances have been lost and where jet properties are largely controlled by the conserved properties of the flow. The experiments involved steady round nonbuoyant fresh water jet sources injected into uniform and steady fresh water crossflows within the windowed test section of a water channel facility. Flow visualization was carried out by photographing dye-containing source jets. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using Planar Laser-Induced Fluorescence (PLIF). The self-preserving properties of the flow were correlated successfully based on scaling analysis due to Fischer et al. (1979) which involves assumptions of no-slip convection in the cross stream direction (parallel to the crossflow) and a self-preserving nonbuoyant turbulent line puff having a conserved momentum force per unit length that moves in the streamwise direction (parallel to the initial source flow). The flow structure consisted of two counterrotating vortices, with their axes nearly aligned with the crossflow direction, that move away from the source in the streamwise direction due to the action of source momentum. Present measurements extended up to 160 source diameters from the source in the streamwise direction and yielded the following results: jet motion in the cross stream direction satisfied the no-slip convection approximation; geometrical features, such as the penetration of flow boundaries and the trajectories of the axes of the counter-rotating vortices, reached self-preserving behavior at streamwise distances greater than 40–50 source diameters from the source; parameters associated with the structure of the flow, e.g., contours and profiles of mean and fluctuating concentrations of source fluid, however, did not reach self-preserving behavior prior to reaching streamwise (vertical) distances greater than 70–80 source diameters from the source.