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A planar concentration analysis (PCA) system is used for observing the transport and mixing of a tracer mass in a shallow turbulent free-surface wake flow of a large cylindrical obstacle. The nonintrusive, fieldwise PCA measuring technique is applied to evaluate depth-averaged mass concentrations by making use of light attenuation due to absorption and scattering processes related to a dissolved tracer mass. The scalar fields are decomposed into a low-frequency quasiperiodic part, the coherent flow, and a randomly fluctuating part. From accompanying near-surface velocity measurements, large-scale coherent structures are identified and related to the coherent mass fields. This allows one to assess the role of the large-scale vortices for advection and diffusion in shallow wake flows. The time–mean wake flow displays a self-similar spanwise distribution both for mass and velocity. The longitudinal development of shallow wakes initially shows the growth of unbounded wakes; in the wake far field an attenuated...

For the experimental investigation of the transfer of mass concentrations in shallow turbulent flows an time/cost efficient and easy-to-use measuring technique will be presented. The Planar Concentration Analysis (PCA) technique allows to evaluate the depth-averaged concentra- tion of a soluble conservative tracer, namely food coloring. Since PCA is based on video image analysis, the spatial and temporal resolution is high, depending on the optical components and on the digitalisation and storage capacity. Details will be given to the algorithm for the conversion of light intensity to concentration , which also accounts for the non-linear relation. This extension significantly increases the measurement range for mass concentrations. Finally the PCA system will be applied to the spreading of a tracer plume for a continuous point- source in a shallow turbulent laterally unbounded flow.