Role of low-order proper orthogonal decomposition modes and large-scale coherent structures on sediment particle entrainment

Abstract

This study presents experimental investigations of entrainment events of a single sediment particle resting in a small pocket on a smooth bed in an open channel flow. The data were acquired with a tomographic particle tracking velocimetry system. The shake the box algorithm, a spatially high resolved Lagrangian tracking method, was applied to determine time-resolved and three-dimensional flow velocities in a volume during particle entrainment. The proper orthogonal decomposition (POD) method was applied to identify motions in the flow field carrying the most turbulent kinetic energy (TKE). Based on the POD method, the most energetic flow structures were linked with the occurring quadrant events at particle entrainment. It was shown that particle entrainment follows TKE peaks based on low-order POD modes caused by large sweeps. Two streamwise elongated counter rotating vortices with emerging sweeps in between were observed during particle entrainment. Here, most of the TKE exists in the first POD mode. This signature is suggested to be part of very large-scale coherent motions.

Plunging Flow and Coherent Turbulent Structures in a Straight Pool‐Riffle

“…Perhaps the most important implication from the current results is that turbulence is likely necessary for a comprehensive model of sediment mobility and transport in pools. An instantaneous view of the flow in skimming and plunging regimes emphasizes that the flow remains highly turbulent in all flow conditions (Figure 13), and that the exchange of slow and faster moving fluids leads to time varying stress and pressure dynamics (Figures 8-10), which have been shown in other studies to be important for modeling sediment dynamics (Schobesberger et al, 2022;Shih et al, 2017). The smooth bed assumption and geometrical differences mean that the turbulent structures and pressure fluctuations will not match precisely with what observed in natural channels, but the observations in the current study are consistent with field observations (Clifford & Richards, 1992 Rennie, 2012;Najafabadi et al, 2017).…”

“…The latter effect aligns with fluid mechanics research on the effects of flow deceleration (Krogstad & Skåre, 1995; Monty et al., 2011; Simpson et al., 1981; Wu et al., 2006) and descriptions of high turbulent kinetic energy in a 3D large eddy simulation (LES) of flow through a meandering pool‐riffle (Kang & Sotiropoulos, 2011). The role of turbulence in sediment transport is uncertain, in part due to the difficulty of characterizing the size and strength of the relevant events and limited field data, but the role of turbulent velocity and pressure fluctuations on sediment entrainment is increasingly well documented (Schobesberger et al., 2022; Shih et al., 2017). Additional high‐resolution numerical modeling of flow through pools with turbulence resolving capabilities would be useful to assess the strength and coherence of turbulent structures in pools and ascertain their role in pool formation processes.…”

Bewegung eines Einzelkorns unter dem Einfluss kohärenter Strukturen

“…Eine detaillierte Beschreibung dieser Methode und der zum Einsatz kommenden Geräte findet sich in Schobesberger et al. ( 2020 , 2021 ) sowie im Beitrag von Gold et al. ( 2023 ; in diesem Heft).…”

Lagrangian particle tracking velocimetry investigation of vortex shedding topology for oscillating heavy spherical pendulums underwater

“…As a result, approaches based on Lagrangian particle tracking, commonly referred to as particle tracking velocimetry (PTV), have become more popular (Raffel et al 2018). Nowadays, time-resolved 3-D PTV (tr-3-D-PTV) constitutes one of the most advanced approaches in 3-D flow measurements, and has proven its utility for identifying and visualising coherent flow structures and vortex dynamics (Schobesberger et al 2022).…”

“…Nowadays, time-resolved 3-D PTV (tr-3-D-PTV) constitutes one of the most advanced approaches in 3-D flow measurements, and has proven its utility for identifying and visualising coherent flow structures and vortex dynamics (Schobesberger et al. 2022).…”