ROLE OF LARGE-SCALE STRUCTURE IN A NONREACTING TURBULENT CH4 JET

The existence of organized large-scale structure in jet flows has been studied using planar imaging of the CH4 concentration in a turbulent CH4 jet. The measurements show the development of large-scale vortical structures in the mixing region adjacent to the central jet. These structures result in instantaneous radial profiles that differ considerably from the classical time-averaged Gaussian profiles measured previously using single-point measurements. The profiles typically consist of one or two plateau regions of relatively uniform concentration and are consistent with planar measurements obtained by other investigators in constant density and reacting flows. The results suggest a mixing model in which the dominant mechanism for entrainment is large-scale engulfment of surrounding air by the vortical structures, followed by rapid mixing with jet fluid. Conventional time-averaged statistics (mean, RMS fluctuations, and probability distributions) can be interpreted in terms of this large-scale structure.