Instantaneous, quantitative measurements of molecular mixing in the axisymmetric jet near field

Results from a dual-tracer planar laser-induced fluorescence (PLIF) techniq ue for making instantaneous, quantitative measurements of molecularly mixed fluid fraction are presented for an axisymmetric jet in a slow co-flow. Th e two-camera, two-laser technique uses PLIF of nitric oxide seeded in a nit rogen jet to mark the unmixed jet fluid fraction, while PLIF of acetone see ded into the low velocity air co-flow marks the total co-flow fluid fractio n. By combining data from these two simultaneous images, quantitative measu rements of molecularly mixed jet fluid fraction can be made on a pixel-by-p ixel basis, while simultaneously allowing visualizations of large-structure behavior and regions of subresolution stirring. Instantaneous images of mo lecularly mixed jet fluid fraction and jet fluid mixing efficiency, probabi lity density functions (PDFs) of mixed jet fluid fraction, and associated s tatistics are presented for Re-jet = 1000, 5000, 10,000, 50,000, and 100,00 0. For fully turbulent conditions (Re-jet greater than or equal to 30,000), stirring at subresolution scales is detected primarily on the jet side of the mixing layer. This creates a hybrid PDF behavior (stationary on the jet side of the mixing layer, marching on the co-flow side) that is not shown by passive scalar methods at equivalent image resolution. (C) 1999 American Institute of Physics. [S1070-6631(99)01802-4].