Author(s): Hyoungchul Park; Jinhwan Hwang

Linked Author(s): Hyoungchul Park

Keywords: Bedload transport; Particle Image Velocimetry; Particle Tracking Velocimetry; Quadrant analysis; Hairpin vortex

Abstract: Bedload transport is the most fundamental morphodynamics phenomenon. Therefore, many previous researchers in fluvial geomorphology have performed empirical and experimental studies to investigate how the sediment particles interacted with the flow motions and why they started to move. Most of them have defined the initial motion of the particle as incipient motion and revealed that the particles started to move by receiving momentum from the flow within the boundary layer. In particular, they have focused on the turbulent vortex motions within the boundary layer since various turbulent coherent structures were generated near the bottom and were in charge of producing turbulence and transferring the momentum toward the bed. In order to find the turbulent event closely related to the bedload transport, quadrant analysis was mainly applied to the measured velocity. Even though many studies have examined the cause of bedload transport based on the quadrant analysis, the dominant event is still controversial depending on the studies due to the limitation of measurement technique. Accordingly, the corresponding work investigated the effect of vortex motion on bedload transport based on the advanced optical measurement technique. Laboratory experiments were performed under the various conditions with three-dimensional Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). These techniques allowed to acquire the three-dimensional velocity field and sediment trajectory with the high spatiotemporal resolution simultaneously. We extracted the large-scale vortex motions such as hairpin vortex and vortex packet by applying proper orthogonal decomposition (POD) to the measured velocity field. According to result, the dominant turbulent event was varied depending on the flow conditions. At the lower velocity conditions, the outward interaction and sweep were the most related to the bedload transport even though the ejection occurred most frequently. However, at higher velocity conditions, ejection and sweep became the most dominant event related to the bedload transport. Also, this study revealed that additional parameters should be required other than turbulent kinetic energy to judge the occurrence of sediment movement, especially in the low velocity condition.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022120

Year: 2022