Results are presented from an experimental study into the fine-scale structure of generic, Sc≈1, dynamically passive, conserved scalar fields in turbulent shear flows. The investigation was based on highly resolved, two-dimensional imaging of laser Rayleigh scattering, with measurements obtained in the self-similar far field of an axisymmetric coflowing turbulent jet of propane issuing into air at local outer-scale Reynolds numbers Reδ≡uδ/v of 11000 and 14000. The resolution and signal quality of these measurements allowed direct differentiation of the scalar field data ζ(x, t) to determine the instantaneous scalar energy dissipation rate field (Re Sc)−1∇ζ·∇ζ(x, t). Results show that, as for large-Sc scalars (Buch & Dahm 1996), the scalar dissipation rate field consists entirely of strained, laminar, sheet-like diffusion layers, despite the fact that at Sc≈1 the scale on which these layers are folded by vorticity gradients is comparable to the layer thickness. Good agreement is found between the measured internal structure of these layers and the self-similar local solution of the scalar transport equation for a spatially uniform but time-varying strain field. The self-similar distribution of dissipation layer thicknesses shows that the ratio of maximum to minimum thicknesses is only 3 at these conditions. The local dissipation layer thickness is related to the local outer scale as λD/δ ≡ΛRe−3/4δSc−1/2, with the average thickness found to be 〈Λ〉=11.2, with both the largest and smallest layer thicknesses following Kolmogorov Re−3/4δ) scaling.

We present results from an experimental investigation into the fine-scale structure associated with the mixing of a dynamically passive conserved scalar quantity on the inner scales of turbulent shear flows. The present study was based on highly resolved two- and three-dimensional spatio-temporal imaging measurements. For the conditions studied, the Schmidt number (Sc ≡ v/D) was approximately 2000 and the local outerscale Reynolds number (Reσ≡ uσ/v) ranged from 2000 to 10000. The resolution and signal quality allow direct differentiation of the measured scalar field ζ(x, t) to give the instantaneous scalar energy dissipation rate field (Re Sc)−1 ∇ζċ∇ζ(x, t). The results show that the fine-scale structure of the scalar dissipation field, when viewed on the inner-flow scales for Sc ≡ 1, consists entirely of thin strained laminar sheet-like diffusion layers. The internal structure of these scalar dissipation sheets agrees with the one-dimensional self-similar solution for the local strain–diffusion competition in the presence of a spatially uniform but time-varying strain rate field. This similarity solution also shows that line-like structures in the scalar dissipation field decay exponentially in time, while in the vorticity field both line-like and sheet-like structures can be sustained. This sheet-like structure produces a high level of intermittency in the scalar dissipation field – at these conditions approximately 4% of the flow volume accounts for nearly 25% of the total mixing achieved. The scalar gradient vector field ∇ζ(x, t) for large Sc is found to be nearly isotropic, with a weak tendency for the dissipation sheets to align with the principal axes of the mean flow strain rate tensor. Joint probability densities of the conserved scalar and scalar dissipation rate have a shape consistent with this canonical layer-like fine-scale structure. Statistics of the conserved scalar and scalar dissipation rate fields are found to demonstrate similarity on inner-scale variables even at the relatively low Reynolds numbers investigated.

Results from highly resolved, four-dimensional measurements of the fine structure of the fully space-and time-varying SC, 1 conserved scalar field and the associated scalar energy dissipation rate field in a turbulent flow are presented. The resolution achieved in all three spatial dimensions and in time reaches down to the local strain-limited molecular diffusion scale in the flow, allowing all three components of the instantaneous scalar gradient vector field VlJ x,t) and their time evolution at every point in the data space to be directly evaluated. Results are presented in the form of fine structure maps of the instantaneous dissipation field log, VQV<(x,t) in several spatially adjacent data planes within an individual threedimensional spatial data volume, as well as in several temporally successive data planes from a sequence of such three-dimensional data volumes. The degree of anisotopy in the underlying scalar gradient field is characterized in terms of the joint distribution /?(8,~) of spherical orientation angles. The probability density of true scalar energy dissipation rates is presented and compared with the distributions that would result from lower-dimensional measurements of the scalar gradient vector. From this the "spottiness" of the scalar dissipation field is directly quantified by determining the true fraction of the total dissipation that occurs in any given volume fraction of the flow.

It is shown that measurements of lower-dimensional projections of the scalar gradient in an isotropic conserved scalar field ζ(x,t) are sufficient to construct the probability density function (pdf) for the true scalar dissipation χ. With this general method, the true scalar dissipation pdf is obtained from the result reported recently for the approximate pdf based on ‖∇ζ‖2≈3⋅(∂ζ/∂x)2. Results show that (i) the pdf of the true scalar dissipation is virtually lognormal and (ii) the most probable value of χ increases and the rms fluctuations of χ decrease in comparison with their one-dimensional estimates.