Analysis of the Effects of Coherent Flow Structures on the Transport of Particles Around Submerged Obstacles in Streams
Advisor: Professor Rafael O. Tinoco
Abstract
Freshwaters transport various types of particulate matter, from plant seeds, fish eggs, and drifting invertebrates, to particles that can be harmful for aquatic biota, such as plastic debris, including macro-, meso-, and microplastics. Common obstacles in freshwater such as branches, logs and hydraulic structures are identified as local hotspots of various types of particles. Hydrodynamic analysis of these locations will provide a more efficient approach to capture and redirect particles in freshwater ecosystems by predicting particle trajectories under different flow conditions.
In this research, we identify flow characteristics that create hotspots of particles, and study the interaction between flow and particles with various obstacle configurations and particle characteristics in a laboratory setting. The transport mechanisms of particles are analyzed as a function of spacing between neighboring obstacles, submergence ratio and porosity of obstacles, as well as particle diameter and particle density. Laboratory experiments were conducted on a closed-loop racetrack flume, using Lagrangian Particle Tracking (LPT) to track the transport of particles and Particle Image Velocimetry (PIV) to identify specific mean and turbulent conditions that determine particle retention or redirection.
The study yields 4 main findings: (1) Coherent flow structures are developed at a threshold gap length and at a threshold submergence ratio, beyond which capture of particles within the gap increases. (2) Particles follow coherent eddies to enter the gap, while enhanced turbulence increases particles deviation from mean flow and deter particle entry. (3) Increased particle size and density retards the particle response to flow, creating high particle concentrations at low-velocity regions. (4) Porous obstacles create flow structures across various scales ranging from individual pore size to entire obstacle dimension, with increased pore sizes delaying the onset of recirculation, which affects the location of high particle concentration zones depending on the particle response time.
Through a comprehensive understanding of flow-particle-obstacle interaction, we expect that our study will offer valuable insights for the efficient management of freshwater ecosystems by: 1) predicting particle accumulation zones, and 2) suggesting effective design of traps to capture specific organisms and particulate matter through assessment of the capture ratio and retention time of particles within gaps between submerged in-stream obstructions.
