Beyond Idealized Geometry: Settling Dynamics of Natural Irregularly‐Shaped Sediment Particles

Abstract Fundamental to Earth surface dynamics and engineering is the understanding of sediment particle settling. To date, however, the settling of natural irregularly‐shaped sediment particles has remained poorly understood. Here, free settling of individual sediment particles of irregular geometry in still water is investigated using particle‐resolved direct numerical simulation. Natural sediment particles from the source region of the Yangtze River, China, are selected, generally comprising compact, oblate, and prolate forms with nine characteristic sizes covering the full range of sands and gravels. The particle nominal diameter ranges from 0.07 to 16.6 mm, with the Galileo number Ga between 1.8 and 6665.7. In addition to rectilinear, zigzag, helical, and chaotic motions, a yaw motion is newly identified within the Ga range from 10 2 to 10 3 , where a compact/oblate particle experiences a direction‐changing lateral drift in line with vertical autorotation. As for intermediate settling between rectilinear and chaotic regimes, oblate particles adopt yaw or zigzag motions, whereas prolate particles favor helical motions. Sediment particles attain lower terminal velocities than their equivalent spheres or equivalent ellipsoids because shape irregularities lead to enhanced fluid drag. At large particle Reynolds numbers, a particle may follow distinct settling motion patterns, depending on the initial orientation. This indicates multi‐stable behavior and reveals the complex role of initial orientation in particle settling. The complex settling dynamics of irregularly‐shaped sediment particles implies that existing continuum‐based mathematical models for morphodynamic processes need to be modified to better inform applications.