Beetham, SarahFoster, Emily SusanCooley, ChristopherGuessous, LailaRawashdeh, OsamahWang, Xia2026-09-282026-09-282026-01-01https://hdl.handle.net/10323/22225Gas-solid flows are present in many industrial and natural systems, including fluidized beds, energy systems, and pyroclastic density currents (PDCs). Predicting these flows remains challenging due to complex interactions between particles and the carrier fluid, particularly in polydisperse systems where particle sizes vary significantly. These challenges are especially important for PDCs, whose large temporal and spatial scales require coarse-grained multiphase flow solvers with closure models to represent unresolved particle-scale physics. This work investigates the settling and clustering dynamics of polydisperse gas-solid flows relevant to PDCs using high-fidelity Euler-Lagrange simulations and develops models and correlations for coarse-grained solvers. Particle size distribution effects were examined across multiple volume fractions and compared to equivalent monodisperse systems. Smaller particles were found to occupy voids within particle assemblies, increasing clustering intensity and settling velocity, while existing predictors based on volume fraction or isolated particle settling laws failed to represent this behavior. To address this, a new surface loading parameter was introduced and used to develop predictive models for particle-phase averaged fluid velocity and settling velocity. A proposed drag model accounting for clustering effects also provided improved agreement with the simulation data and accurately predicted settling behavior across particle sizes. Because existing scaling relationships developed for monodisperse systems performed poorly for polydisperse flows, a new logarithmic scaling relationship based on modal particle diameter was established to predict the characteristic cluster length and provide an a priori estimate of the domain size required to obtain domain-independent statistics, accurately capturing the observed domain-size dependence. Finally, filtering effects on particle-phase turbulent kinetic energy and granular temperature were examined. Particle size distributions were shown to significantly alter the smallest reliable filter size relative to monodisperse systems, and a new model was developed to predict the resulting unresolved granular temperature with good agreement to the simulation data. These results provide new insight into the behavior of polydisperse gas-solid flows and improve coarse-grained multiphase flow solvers for PDCs and other particle-laden systems.Gravity currentsMultiphase flowsParticle-laden flowsPolydisperse flows