Session T46: Particle-Laden Flows: Clustering I

Unraveling the persistence and dynamics of high-concentrated regions of inertial particles in turbulent flows

Inertial particles in turbulent flows preferentially concentrate forming regions of high particle concentrations. The persistence of these high-concentrated regions over time and the associated dynamical behavior of the particles confined in these regions are investigated in this study. Time-resolved data of sub-Kolmogorov solid particles is collected at the central plane of a vertical channel for varying Reynolds number and particle volume fraction. Local regions of high concentration are identified based on the particle number density surrounding a particle in its neighborhood. To quantify the persistence of these high-concentrated regions, temporal autocorrelation of local concentration fluctuations along the particle-tracks is studied conditioning on the initial local concentration. The timescale for which the autocorrelation function remains above 0.5, is defined as the coherence time of the local concentrated region. The variation of the coherence time with the initial local concentration is reported. The effect of Stokes number on the coherence time for a particular value of initial local concentration is also examined. The kinematics of the particles confined in these concentrated regions are analyzed to gain insights into the influence of collective inertia of high-concentrated regions on particle behavior.   

Modification of Preferential Concentration by Strong Radiative Heating

Particle clustering affects macroscopic heat transfer rates in systems such as spray burners, pulverized coal furnaces, biomass reactors, and particle-solar receivers. This process is often conceptualized as unidirectional: turbulent eddies determine the instantaneous particle distribution which subsequently impacts local heat transfer rates to the fluid. This work explores strong coupling regimes where heat transfer due to particles has a feedback effect on the underlying turbulent flow. Experiments are performed on a particle-laden jet exposed to high intensity radiation. A square duct at a Reynolds number of 10,000 laden with small Nickel particles at mass loading ratios up to 200 percent issues into an isokinetic co-flow and is exposed to 5 kW of near infrared radiation using a laser diode array. Mean gas temperature measurements within the jet show an average increase of 200 ^oC at the highest loading conditions. Instantaneous temperatures within particle clusters are expected to be higher. Particle positions and velocities are measured using high resolution imaging and particle image velocimetry. When subjected to radiative heating, both particle velocity fluctuations and the degree of preferential concentration decrease. The roles of variable property effects, dilatation of the gas within clusters, and buoyancy mechanisms are investigated using scaling analyses and directional measures of preferential concentration.   

Experimental investigation of eelgrass seed settling and clustering for ecosystem restoration

Eelgrass is a type of aquatic vegetation that is an essential part of the ecosystem in the Great Bay Estuary in New Hampshire, USA. This vegetation performs many ecosystem functions such as providing habitat for marine organisms, attenuating waves and currents, and reducing coastal erosion. Unfortunately, the eelgrass population in the estuary has been in decline over the past several decades. A proposed method to increase the eelgrass population and help with reproduction is to harvest eelgrass seeds from nearby locations and introduce these seeds into the estuary. So far, this technique has not produced a significant increase in eelgrass population due to the seeds not settling into the sediment and germinating. The work presented herein will provide preliminary results discussing which water conditions are the most suitable for eelgrass seed dissemination, settling, and germination. 

The laboratory tests are performed in a 10m long oscillating water tunnel with a vegetative meadow made from eelgrass mimics. The water tunnel can be set-up with either a sediment bed or a vegetative canopy and the water can be run as unidirectional or oscillatory. The results presented here are for both a sediment bed and a vegetative canopy in still and unidirectional flow. We present statistics for where cylidrical model eelgrass seed particles are most likely to settle given various vegetation patterns, seed release points, and water conditions.   

Particle clustering in renewable energy systems: a spatiotemporal characterisation with Voronoi tessellations and lacunarity

Wind-thrust debris in wind farms and solar photovoltaic (PV) plants induces degradation and threatens production. Turbines suffer blade erosion and forcing due to particle impact. Deposition on PV surfaces blocks light and creates surface-degrading hotspots. Mitigating these effects is difficult, as turbulent structures capture or carry particles depending on inertial effects. Recent studies discuss effects of inertial particles in PV and turbine wakes, including particle path simulations. Yet, little is known about clustering in array wakes, an expected influence on soiling and erosion for PV and wind arrays. Here, particle-laden wind tunnel studies observe clustering patterns in the wake of a model turbine and, separately, a model PV array. Voronoi cell distribution is introduced to identify clusters and voids, and lacunarity analysis quantifies wake-dependent cluster heterogeneity. Varying inflow velocity and particle volume fraction (φ_v), turbine wake cluster persistence (>10D) shows implications for wind farm spacing. For a model PV array, panel wake modification dictates particle distribution for a panel downstream. Together, these studies present novel analysis informing on particle-laden wake physics and highlight the importance of clustering in renewable energy systems.