Experimental study of the fine-scale structure of conserved scalar mixing in turbulent shear flows. Part 2. Sc≈1

Abstract

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 Re_δ≡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 λ_D/δ ≡ΛRe^−3/4_δSc^−1/2, with the average thickness found to be 〈Λ〉=11.2, with both the largest and smallest layer thicknesses following Kolmogorov Re^−3/4_δ) scaling.