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GEOMETRIC CHARACTERISTICS OF PASSIVE SCALAR INTERFACES FOR PLUMES IN TURBULENT BOUNDARY LAYERS

L. P. Dasi
School of Civil & Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

Donald R. Webster
School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332-0355 USA

Abstract

The fractal geometry is quantified for concentration iso-surfaces of a high Schmidt number passive scalar field produced by an iso-kinetic source with an initial finite characteristic length scale released into the inertial layer of fully developed open channel flow turbulent boundary layers. The fractal dimension and other fractal measures of two-dimensional transects of the passive scalar iso-surfaces are found to be scale dependent. The fractal dimension is around 1.0 at the order of the Batchelor length scale and increases in a universal manner to reach a local maximum at a length scale around the Kolmogorov microscale. We introduce a new parameter called the coverage length underestimate, which reflects universal behavior in the viscous-convective regime and hence is a useful practical tool for many mixing applications. At larger scales (in the inertial-convective regime), the fractal measures are dependent on Reynolds number and concentration threshold of the iso-surfaces.