Turbulent molecular mixing in gaseous free shear flows

Previous investigations in free shear flows have shown that large-scale vortical structures dominate the entrainment of freestream fluid, and that a transition to small-scale turbulence within these structures results in a sudden increase in mixing. This turbulent mixing process is inherently difficult to study because of the large range of scales that must be resolved. In this investigation, a recently developed simultaneous nitric oxide (NO) and acetone planar laser-induced fluorescence (PLIF) technique is implemented for the nonintrusive, instantaneous measurement of macro- and micro-scale mixing quantities. Because NO PLIF is highly quenched when molecularly mixed with oxygen, it is used to mark the fraction of unmixed fluid within each measurement volume. By using simultaneous acetone PLIF to measure the total (mixed and unmixed) fluid fraction, instantaneous images of molecular mixing can be obtained for each laser pulse while capturing important macro-scale flow phenomena.; The flow regimes studied in this investigation span three main stages of shear layer development. Vortex mixing in the near field of an axisymmetric jet (Re_D = 2300) is studied by using an acoustic pulse to obtain repeatable vortex formation and merging events. Results indicate that the mixing process is initially slow within the laminar vortex rollers, but a dramatic increase in mixing is detected prior to and during vortex coalescence. In the next stage of research, non-pulsed axisymmetric jets from Re_D = 16,200 to 29,200 were used to study the transition to small-scale turbulence. This transition was found to take place near the average location of vortex merging, and resulted in a 20 to 25% drop in the mixed jet fluid volume fraction, a 30 to 35% drop in the preferred mixed jet fluid fraction, and a shift from stationary to hybrid radial probability density functions. In the final stage of research, fully-developed turbulence was studied in the far-field region of a planar shear layer with low- to high-speed velocity ratios of 0.25 to 0.44. Statistical analysis in this regime indicated that the physics of molecular mixing differs between the lowand high-speed fluids, and that molecularly mixed fluid quantities are not uniform across the shear layer.