Differential diffusion in a turbulent jet

Differential diffusion in a turbulent jet was investigated experimentally. The two primary objectives associated with this work were to systematically study: (i) the Reynolds number dependence, and (ii) the radial distribution of differential diffusion in the jet. These goals were simultaneously achieved, along with several secondary objectives, such as determining the scales at which differential diffusion is manifested and examining the effect of background turbulence on differential diffusion.Experiments were performed in a momentum-driven, axisymmetric turbulent water jet, containing two passive scalars (fluorescent dyes) with differing molecular diffusivities (D). The jet issued into either a quiescent background or an approximately homogeneous, isotropic turbulent background. To determine the Reynolds number dependence, tests were conducted at five jet Reynolds numbers, ranging from Re = 900 to Re = 10600. (Re = u 0d/v, where u0 is the jet exit velocity, d is the jet exit diameter, and v is the kinematic viscosity.)Laser-induced fluorescence (LIF) was employed to obtain instantaneous individual species concentrations at a downstream location, located in the self-similar region. At each Reynolds number, measurements were acquired at radial positions extending from the jet centreline to its edges. The normalized concentration difference, Z, was defined to elucidate differential diffusion effects. Statistics of this parameter revealed the magnitude of differential diffusion at each Reynolds number and radial location. In particular, the root-mean square of the normalized concentration difference, Z rms, gave an indication of the intensity of these effects.Centreline results showed that differential diffusion effects slowly decay with increasing Reynolds number, scaling as Zrms ∝ Re-0.09. Even though the effects were decreasing, they were still non-zero at the highest Reynolds number studied (Zrms = 0.064 at the centreline at Re = 10600). Differential diffusion effects were also shown to increase (i) with increased radial position from the centreline, and (ii) in the presence of a turbulent background. These two instances, along with the increased differential diffusion at lower Reynolds numbers, support the hypothesis of an increase of differential diffusion effects as the occurrences of interfaces between dyed jet fluid and entrained ambient fluid increases.Also presented are the first experimental measurements of the spectrum of the normalized concentration difference, EZ(K1 eta). These revealed the scales at which differential diffusion manifests itself. In all instances, these spectra were observed to decrease with wavenumber over all wavenumbers, consistent with a flow dominated by the decay of its scalar field. Although differential diffusion effects are molecular in origin, they persist at scales larger than the Kolmogorov scale