Scalar Mixing in a Turbulent Boundary Layer

We present results on the analysis of measurements on the dispersion of a contaminant in a turbulent boundary layer. The contaminant is a fluorescent dye (Rh.-B) that was introduced in a turbulent boundary layer (푅푅푒푒# = 3050) in a water channel. The velocity field was measured by means of tomographic PIV. Simultaneously, the concentration field was measured by means of laser-induced fluorescence (LIF). The LIF data clearly reveal uniform concentration zones with well-defined boundaries. These zones are extracted from the probability density function of the local concentration. The key question is what flow structures make up their boundaries, and how these uniform concentration zones are related to uniform momentum zones. We evaluate the Lyapunov exponents of the rate of separation of two close points while they are advected by the flow. Local maxima of the Lyapunov exponents are known to create temporary barriers for transport in two-dimensional flows. Alternatively, the boundaries of uniform concentration zones may also be formed by shear layers, which are local maxima of the shear vorticity. There is a striking concentration of both the Lyapunov exponent and the shear vorticity on real boundaries. The correlation with the Lyapunov field is the weakest of the two. However, the Lyapunov exponent is a Lagrangian average, extending over episodes where these boundaries were formed, whereas the shear vorticity is based on a planar cross section of the instantaneous velocity field