Hydrodynamic Behavior of Non-spherical Particles in Confined Vertical Flows: A Resolved CFD-DEM Study

TL;DR

This work uses a fully resolved CFD-DEM framework with immersed boundary coupling and multisphere particle representations to study the hydrodynamics of irregular polymetallic nodules (PMNs) in confined vertical flows relevant to deep-sea mining. By comparing non-spherical PMNs to volume-equivalent spheres under identical conditions, the study quantifies shape-induced drag enhancements, reduced terminal velocities, and altered residence-time and drag-force statistics. The results show a consistent 2.0–2.3× drag increase and 29–33% slower settling for PMNs, driven by 50% larger frontal areas and wake asymmetry, while ensemble transport progresses similarly to spheres; this provides bounds on when spherical models are adequate and highlights the need for shape-aware corrections in reduced-order models. Overall, the resolved framework demonstrates the importance of particle morphology and confinement in predicting hydraulic transport efficiency and informs design and optimization of riser systems for deep-sea mining.

Abstract

We investigate the sedimentation and vertical hydraulic transport of irregular polymetallic nodules (PMNs) using resolved CFD-DEM with multisphere particles spanning $100 < Re_p < 3000$. Shape effects induce 2.0-2.3 times drag enhancement relative to volume-equivalent spheres, arising from 50\% larger frontal areas and wake asymmetry, reducing terminal velocities by 29-33\%. Vertical transport exhibits velocity-driven transitions from intermittent settling to stable convection, as demonstrated by residence-time and drag-force statistics. While PMNs exhibit enhanced rotational-translational coupling and broader force fluctuations, the regime progression qualitatively resembles that of volume-equivalent spherical particles. Drag variance evolution reveals contrasting behavior: small particles $(d/D=0.082)$ show narrow distributions and wake suppression at higher velocities, while large particles $(d/D=0.22)$ exhibit non-monotonic variance. These findings elucidate shape-confinement interactions in vertical transport and establish bounds on the applicability of volume-equivalent spherical particles in reduced-order models.

Figures (16)

• Figure 1: Multisphere representation of non-spherical particles in the resolved CFD-DEM framework. (a) Example particle geometry constructed from 14 overlapping spherical sub-particles, demonstrating the multisphere approach for capturing irregular surface features. (b) Corresponding void fraction field distribution in the computational domain, where white regions indicate solid particle volume and blue regions represent pure fluid. The void fraction field resolves the fluid-solid interface within the Eulerian CFD grid, enabling accurate computation of hydrodynamic forces via the IB method without requiring empirical drag correlations.
• Figure 2: Temporal evolution of settling velocity for verification cases comparing CFD-DEM predictions (solid lines) with experimental measurements from ten2002particle (symbols). The simulations capture both the acceleration phase and the approach to terminal velocity, with an accuracy of 10%, verifying the IB method for hydrodynamic force calculation across the transitional Reynolds number regime.
• Figure 3: Velocity contours and streamlines for $Re_\text{p} = 1.3$ (case 1) showing symmetric flow patterns characteristic of the viscous regime. (a) Initial acceleration phase with developing boundary layer. (b) Approach to terminal velocity exhibiting symmetric streamlines that, consistent with Stokes flow past a sphere. (c) Particle at $0.5d$ from the domain bottom showing a fully developed symmetric flow field with smooth velocity decay to far-field values.
• Figure 4: Velocity contours and streamlines for $Re_\text{p} = 29.8$ (case 2) showing wake formation and flow separation characteristic of the inertial regime. (a) Initial acceleration phase. (b) Approach to terminal velocity with clear flow separation and wake region extending approximately $1.0d$ downstream, indicating transition from viscous-dominated to inertia-dominated settling. (c) Particle at $0.5d$ from the domain bottom, showing an established recirculating wake structure and an asymmetric pressure distribution that contributes to enhanced form drag.
• Figure 5: Comparison of settling velocity evolution for the two-particle drafting-kissing-tumbling benchmark. CFD-DEM predictions (solid lines) show good agreement with DNS results from sharma2005fast (dashed lines). The three characteristic phases are evident: drafting $(t < 0.14s)$ with independent settling, kissing $(0.14s < t < 0.35s)$ with wake entrainment causing trailing particle acceleration and gap closure, and tumbling $(t > 0.35s)$ following collision with momentum exchange. The maximum velocity deviation remains below 5% throughout the drafting and kissing phases, verifying the IB method for multi-particle hydrodynamic interactions.