Conditionally filtered scalar dissipation, scalar diffusion, and velocity in a turbulent jet

Conditionally filtered conserved scalar (temperature) dissipation (CFD), diffusion, and conditionally filtered velocity are studied experimentally in the fully developed region of a turbulent jet with a jet Reynolds number Rej=UjDj/ν of 40 000. These variables are the unclosed terms in the transport equation of the conserved scalar filtered density function. One-dimensional box filters of widths Δ ranging from 30 to 496 scalar dissipation scales (ηφ) as well as a two-dimensional box filter (Δ/ηφ=90) which consists of three discrete sensors are used to obtain filtered variables. Taylor’s hypothesis is used to perform the streamwise filtering. The means of these conditionally filtered variables conditional on the resolvable-scale scalar fluctuations 〈φ〉L and the subgrid-scale (SGS) variance 〈φ″2〉L indicate two regimes of the SGS scalar mixing. For large SGS variance (〈φ″2〉L/〈φ″2〉>1) the SGS scalar exhibits similarities to scalars in initially binary mixing at early times, suggesting existence of diffusion-layer-like structure and nonequilibrium SGS scalar. The nonequilibrium is further evidenced by the observed large SGS mechanical-to-scalar time scale ratio and large SGS variance production-to-dissipation ratio. These characteristics have not been previously observed in fully developed flows. For small SGS variance the conditionally filtered variables indicate well-mixed SGS scalar. The results also show that the SGS advection is only significant at large SGS variance. Our measurements also suggest that the dependence of the scalar dissipation (and the filtered dissipation) on large-scale scalar fluctuations is caused primarily by the fluctuations in the spectral transfer and that the conditional CFD is directly affected mostly by scales near the filter scale. The present study suggests that the nonequilibrium effects related to sheet-like structures are important for modeling the conditionally filtered variables.