EXPERIMENTAL-STUDY OF THE FINE-SCALE STRUCTURE OF CONSERVED SCALAR MIXING IN TURBULENT SHEAR FLOWS - PART 2 - SC-APPROXIMATE-TO-1

Results are presented from an experimental study into the fine-scale s tructure of generic, Sc approximate to 1, dynamically passive, conserv ed scalar fields in turbulent shear flows. The investigation was based on highly resolved, two-dimensional imaging of laser Rayleigh scatter ing, with measurements obtained in the self-similar far field of an ax isymmetric coflowing turbulent jet of propane issuing into air at loca l outer-scale Reynolds numbers Re-delta = u delta/v of 11000 and 14000 . The resolution and signal quality of these measurements allowed dire ct differentiation of the scalar field data zeta(x, t) to determine th e instantaneous scalar energy dissipation rate field (Re Sc)(-1)del ze ta . del zeta(x, t). Results show that, as for large-Sc scalars (Buch & Dahm 1996), the scalar dissipation rate field consists entirely of s trained, laminar, sheet-like diffusion layers, despite the fact that a t Sc approximate to 1 the scale on which these layers are folded by vo rticity 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 dis tribution of dissipation layer thicknesses shows that the ratio of max imum to minimum thicknesses is only 3 at these conditions. The local d issipation layer thickness is related to the local outer scale as lamb da(D)/delta = Lambda Re-delta(-3/4) Sc-1/2, With the average thickness found to be [Lambda] = 11.2, with both the largest and smallest layer thicknesses following Kolmogorov (Re-delta(-3/4)) scaling.