Coherent Structures during Particle Entrainment

In the field of sediment research, a detailed description of flow structures and their influences on incipient sediment entrainment is still a key challenge. Under threshold conditions, turbulence and coherent structures, i.e. hairpin vortices or rather burst events, 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 to improve the process understanding. The objectives of this paper are (i) to visualize and (ii) to describe the three-dimensional flow structures such as large-scale coherent structures under threshold conditions during particle entrainment. For this purpose, experiments in a straight hydraulic flume under uniform and subcritical flow conditions were carried out at the hydraulic laboratory of the University of Natural Resources and Life Sciences (BOKU). A single particle (arbitrary shaped stone) was placed in a pocket centered in the flume. The hydraulic threshold conditions for particle entrainment were evaluated in more than 55 pre-experiments to achieve a precise hydraulic setup where stone dislodgement occurred frequently during a time-period of 0.5 and 10 minutes. Time resolved tomographic particle tracking velocimetry (4D-PTV) was implemented as the measurement system. There a laser, expanded to a volume, illuminated the area in front and around the particle. Ten experiments measured with 4D-PTV were carried out under the same flow conditions like in the pre-experiments. To obtain the addressed time resolved three-dimensional velocity data in the volume of interest, the novel Lagrangian particle tracking algorithm “Shake the Box”, with a spatial resolution of 1 x 1 x 1 mm was used. The (large scale) coherent structures during particle entrainment were visualized by iso-surfaces of the Q-criterion. Based on the results it can be stated that the 4D-PTV method is well suited to shed new light on the interactions between coherent structures and particle as well as sediment entrainment. In contrast to other studies, where coherent structures were investigated with two-dimensional measurement devices like LDA or Planar Particle Image Velocimetry (2D PIV) this study results in time resolved three-dimensional velocity data in a volume. The analysis of the measurements resulted in dominant coherent structures during particle entrainment.