Session J23: Particle-Laden Flows: Particle-Turbulence Interactions in the Environment and Beynod

Settling dynamics of Kolmogorov-scale sediment particles in homogeneous isotropic turbulence using two-way coupled point--particle model

Settling dynamics of slightly heavier-than-fluid particles in turbulence is of critical importance for suspended sediment transport in coastal environments. These interactions typically involve particle sizes on the order of or larger than the Kolmogorov scale, moderate-to-dense volume loadings, and small particle-to-fluid density ratios.  Of particular interest is the mean particle settling speed in turbulence, which can be influenced by fast tracking, vortex trapping, and loitering mechanisms. A two-way coupled point-particle model based on the complete Maxey-Riley equation is used to investigate settling dynamics of particles in a two-way coupled, forced, homogeneous isotropic turbulence by varying the turbulence intensity relative to the settling speed in quiescent flow for multiple Stokes numbers. To correctly capture the motion of Kolmogorov-scale particles, a self-disturbance corrected fluid velocity, obtained using the advection-diffusion-reaction (ADR) model (Keane et al., IJMF 2023) is used. For small-to-moderate Stokes numbers, increased settling speed is observed with increasing turbulence intensity. Preferential sweeping is found to be the main mechanism of increase in settling speed. However, for larger Stokes numbers, the settling speed is decreased even with increasing turbulence intensity. Larger particles with higher Stokes numbers tend to sample upward moving fluid more frequently resembling loitering mechanism resulting in decreased settling speed. A multiscale wavelet analysis of particle curvature trajectories as well as the vertical fluid velocity sampled by the particles at different scales is conducted to further elucidate the different mechanisms affecting settling dynamics.   

Particle residence time between porous obstacles: application to management strategies of invasive aquatic species.

Freshwaters transport a wide range of particulate matter, such as plant seeds, fish eggs, drifting invertebrates and plastic debris. Such particles experience complicated flow structures across a wide range of scales generated by in-stream obstacles and characterized by the obstacles’ pore sizes and densities. We analyze the residence time of particles in the gap between two porous obstacles focusing on the relevant flow scales that determine particle transport. Laboratory experiments were conducted on a closed-loop racetrack flume using six types of porous obstacles and two particle types analogous to aquatic invasive species in their early life stages: 1) neutrally buoyant with 1 mm diameter and 2) negatively buoyant with 4.8 mm diameter. We used particle tracking velocimetry (PTV) to identify particle trajectories and preferred locations relative to the obstructions with longer local residence time, and particle image velocimetry (PIV) to characterize mean and turbulent conditions driving particle retention and redirection within and past the gap. Analysis on the interaction between particles and flow structures of different scales provide guidelines for efficient management strategies of aquatic invasive species based on: 1) accurate prediction of accumulation zones; and 2) physics-based guidelines for effective design of in-stream traps.   

Particle transport under a wind-driven boundary layer

Microplastic pollution accumulates at the surface of the ocean in the surface boundary layer. Ocean surface dynamics, in particular wind-generated turbulence and waves, can influence how these particles disperse below the water surface. Typically, the vertical concentration of particles is modeled using a parameterized turbulent diffusivity profile and a particle rise velocity. These models often assume a constant rise velocity, despite literature showing that the rise velocity is affected by the turbulence and waves present in the flow around the particles. In addition, the turbulent Schmidt number is still uncertain in these flows, especially for buoyant particles. To test these models, we use both Lagrangian data (i.e., particle trajectories from large-scale shadow tracking) and Eulerian data (i.e., velocity fields from particle image velocimetry) to relate the vertical transport of particles to turbulent flow and particle characteristics. This will allow for better estimations of particle fate and transport in wind-driven flows like the ocean surface.   

Investigation of aeolian streamers in an atmospheric wind tunnel

Aeolian saltation critically affects geo-morphological processes and Earth's climate, but our understanding of it is incomplete. An apparent but elusive phenomenon is represented by aeolian streamers, elongated regions of seemingly large concentration of dust and sand that are believed to significantly modulate the transport. Their systematic investigation in the field is difficult as the conditions are not well controlled, thus any connection between the streamers and flow structures in the turbulent boundary layer remains unclear. In this study, we conduct experiments in a large atmospheric wind tunnel using monodisperse microspheres. Using high-speed laser imaging, the concentration and velocity of the saltating particles are measured by particle tracking velocimetry along a wall-parallel plane, while the airflow is simultaneously characterized by time-resolved particle image velocimetry along a spanwise wall-normal plane. This extensive dataset allows us to characterize the spatio-temporal distribution of particle concentration and velocity in a cross-section of the saltation layer, and gives us insight in the correlation between the particle motion in the saltation layer and the airflow above. We investigate the length and time scale of particle streamers in the saltation layer, and gain further understanding of how this mode of particle transport is coupled with the dynamics of the turbulent boundary layer.   

Turn-over times of microplastic fibers in wall-bounded turbulence

The spinning and tumbling turn-over times of microplastic fibers have been measured in wall-bounded turbulence. 

The experiments were performed in the TU Wien Turbulent Water Channel at a Shear Reynolds number of 720. The used Polyamide fibers are 1.2 mm long and 0.01 mm in diameter (aspect ratio 120). 

Their tip-to-tip length ranges between 3 and 10 times the Kolmogorov length scale and they are inertial-less, neutrally buoyant, and rigid. 

Their motion in the near-wall region and channel center is observed with six high-speed cameras. 

An established methodology, which involves the time-resolved tomographic reconstruction of each fibre and their subsequent tracking, is employed and improved upon.

Their curved shape is leveraged to uniquely identify the temporal evolution of their orientation, enabling measurements of spinning and tumbling turn-over times.

The analysed converged statistics reveal that on average turn-over times close to the wall are shorter than those at the channel center, which can be explained by a decreasing turbulent kinetic energy dissipation rate with increasing wall-normal position.

These results are original, as previous studies are restricted to measuring the effect of fiber length on turn-over times in homogeneous isotropic turbulence only.