Turbulent Coherent Flow Structures to Predict the Behavior of Particles With Low to Intermediate Stokes Number Between Submerged Obstacles in Streams

Underwater obstacles are identified as local hotspots of various particulate matters in streams. As the transport of particles is dependent on surrounding flow, we expect that flow structures created in the vicinity of obstacles provide a favorable condition for high particle concentration. We quantify the particle behaviors at small to intermediate range of Stokes number as they move past two obstacles forming a gap of varying length, based on three indices: the ratio of particles entering the gap, the time span between entry and exit of particles, and the integral time scale of flow. We performed laboratory experiments in a recirculating racetrack flume using quantitative imaging, particle image velocimetry to obtain flow velocity fields, and particle tracking velocimetry to track full trajectory of particles. As the gap length increases, the interaction between inner and outer flows of the gap increases, which is followed by an increase of the ratio of particles entering the gap and a decrease of the time span of particles and the integral time scale. Noticeably, indices converged when the gap length reached a critical value and recirculating flow structures were fully developed in between the gap, which indicates that the transport of particles with small to intermediate Stokes number (St = 0.1-0.5) is related to the large-scale flow motions. Understanding these patterns allow us to select specific zones to sample and monitor both organic (seeds, eggs, and larvae of endemic aquatic species), and inorganic (sediment and microplastics) particles for better management of aquatic environments.