Search

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...

In order to examine the instability mechanisms in shallow turbulent shear flow, a large and well equipped shallow water test facility has been constructed at the Institute for Hydromechanics, University of Karlsruhe. In this 5.5 m wide and 13.5 m long basin - operated at water depths of only a few centimeters - a plane turbulent equilibrium shear flow is produced. In the present study a disturbance is induced to the base flow by forcing it to flow around a single cylindrical body with a diameter D, which is clearly larger than the water depth h. This destabilization results in a vortex street-like shallow turbulent wake flow. A Particle Image Velocimetry (PIV) measuring technique using particles floating at the free surface is employed to obtain horizontal velocity fields, and a Planar Concentration Analysis (PCA) method processing digitized video frames provides depth-averaged concentration fields. A phase- resolved averaging procedure based on individually variable cycle periods is applied to extract the periodic 'coherent' flow. Large-scale coherent vortical structures can directly be identified from the coherent motion using an eduction scheme, which distinguishes between vorticity- and strain-dominated flow regions. After adjustment and cross-correlation the phase-resolved averaged fields of velocity and tracer mass from the PIV and PCA measurements reveal the transport capacity of these large-scale structures for momentum and mass.

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.

Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinde...