Author(s): Volker Weitbrecht; Gerhard H. Jirka
Linked Author(s): Volker Weitbrecht
Keywords: Longitudinal dispersion; Recirculating flows; Particle-image-velocimetry
Abstract: This paper presents the results of an experimental work conducted in the framework of a research project that deals with the effect of dead zones (groyne fields, harbours) on the longitudinal dispersion in rivers. The objective is an improved prediction of travel time, maximum concentration and skewness of a tracer cloud than is possible with existing alarm models. The exchange processes between main stream and dead zones have major influence on the longitudinal stretching of a tracer cloud traveling down a river. The main physical process that governs the exchange of dissolved matter between the dead zone and the main stream are coherent two-dimensional structures that are generated at the head of a groyne. These structures are growing in horizontal direction during their travel between two groynes because the flow is very shallow. Planar velocity measurements with PIV (Particle-Image-Velocimetry) have been performed in a large shallow water table (5.5m×15m) at the Institute for Hydromechanics at the University of Karlsruhe to analyze the turbulent flow characteristics in a river in the presence of groynes. The mass exchange is related to the aspect ratio (width/length) of a dead zone and to the position of the groynes in relation to the main flow direction. With the aid of PIV it is possible to measure instantaneous velocity fields with a high spatial resolution so that the coherent structures which are mainly responsible for mass exchange can be determined by calculating vorticity fields. These measurements give detailed insight to the mean flow conditions and the turbulent flow characteristics in the dead zones. In a further step a dimensionless mass exchange parameter as has been defined by Valentine & Wood (1977) was extracted from the velocity data.
Year: 2001