Direct numerical simulation of passive scalar mixing in spatially evolving turbulent round jets

Direct numerical simulation of passive scalar transport in a spatially evolving turbulent jet is performed at Reynolds number of 2400 and Schmidt number of unity. The computational domain extends upstream of the jet exit plane to allow for entrainment near the exit plane. Good comparison with experimental data is obtained for the mean velocity, mean scalar concentration, and fluctuations of velocity and scalar. The instantaneous radial profiles of velocity and passive scalar are examined. The velocity and scalar profiles are shown to be qualitatively similar, and alternate between 'top-hat' and 'triangle' shapes. The profiles are found to be determined by entrainment from the free-stream into the jet and this behavior yields a Gaussian mean. The role of diffusion in scalar transport is studied for its relevance to mixing transition. The instantaneous convective and diffusive terms in the passive scalar transport equation are analyzed, and diffusion-dominated regions are identified. Diffusion-dominated regions occur in very thin regions closer to the jet center. However, diffusion is important over fairly thick, 'brush-like' regions near the jet edge. Higher residence times near the jet edge are thought responsible for this behavior. It is speculated that these brush-like regions determine the overall effect of Reynolds number on the scalar fluctuations. The width of these outer regions is expected to decrease with increase in Reynolds number, and the scalar fluctuations are hypothesized to undergo mixing transition once these outer regions are thin compared to the jet diameter.