COHERENT STRUCTURES DURING PARTICLE ENTRAINMENT

Abstract

Turbulence and coherent structures are often referred to as the decisive conditions concerning single particle dislodgement and sediment entrainment. Since these structures are three dimensional, even in straight channel flows, it is, therefore, necessary to consider all three velocity components for detailed investigations and to improve the process understanding. The objectives of this paper are (i) to visualize and (ii) to describe the threedimensional flow structures such as coherent structures under threshold conditions during particle entrainment. For this purpose, experiments in a straight hydraulic flume were carried out under uniform and subcritical flow conditions in the Hydraulic laboratory of the University of Natural Resources and Life Sciences. A single particle (arbitrarily shaped stone) was placed in a pocket centered on the flume bottom. The hydraulic threshold conditions for particle entrainment were evaluated in pre-experiments to achieve a precise hydraulic setup where stone dislodgement occurred frequently during a time-period of 10 minutes. The time resolved tomographic particle tracking velocimetry system (Tomo-PTV) from LaVision was implemented and the novel Lagrangian particle tracking algorithm “Shake the Box”, with a high spatial resolution was used to achieve threedimensional velocity data in a volume. The coherent structures during particle entrainment were analyzed with the quadrant analysis and spatially visualized. The analysis of the measurements resulted in dominant sweeps and outward interactions during particle entrainment. It can be stated that the Tomo-PTV method is well suited to shed new light on the interactions between coherent structures and particle as well as sediment entrainment.