Modeling Vertical Size Sorting in Geophysical Granular Flows: Role of Particle Size Compositions

Abstract

Vertical size sorting in geophysical mass flows, arising from the competition of size segregation and diffusive remixing, is controlled by particle size composition and the properties of the interstitial fluid. However, size‐sorting models for both dry and saturated conditions are typically formulated for bidisperse mixtures with fixed size ratios, and their applicability to broader size compositions—varying in size ratio, concentration, and polydispersity—remains unclear. Using particle–fluid simulations, we first quantify the effects of particle size ratios and mixture concentration on segregation and diffusion in sheared bidisperse granular flows saturated in fluids of varying viscosity and density. The segregation flux and diffusion are reduced but only when viscous drag dominates particle inertia; a trend consistent across all size ratios. Small particles descend faster than large particles rise, and this asymmetry depends primarily on size ratio and is insensitive to fluid properties. We then develop equations describing pairwise segregation and diffusion and use them to extend existing bidisperse and polydisperse continuum models. While these closures accurately reproduce segregation dynamics in saturated bidisperse flows, the extended polydisperse model systematically overpredicts segregation rates and final stratification with increasing fluid viscosity and density. This demonstrates that in continuum modeling, pairwise bidisperse segregation closures do not extend to fluid‐saturated polydisperse mixtures, indicating a breakdown of the superposition assumption under fluid–particle interactions and motivating improved models for broadly graded geophysical flows.