Formation of granular jets

¹ Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei-city, Tokyo, Japan
² Department of Mechanical Engineering, Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro-machi, Minamisaitama-gun, Saitama, Japan

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Published online: 1 December 2025

Abstract

Based on discrete element method (DEM) simulations, this study investigates the formation of granular jets triggered by the sudden unloading of a compressed granular column. After unloading, a jet can be generated from a granular column with a meniscus-shaped free surface. The evolution of jet velocity can be divided into two stages: the acceleration stage and the gravity-governed stage. In the gravity-governed stage, a grain stream can exhibit different regimes, including dispersed atomized grains, a sharp tip, and an elongated root. The effects of the key physical properties of grains on the jet velocity in the early stage are investigated. We find that a higher Young’s Modulus is beneficial for generating a granular jet. The sliding friction coefficient and the coefficient of restitution have limited effects, which implies that the jet velocity is insensitive to the damping and tangential forces of the grains. Mass density significantly influences the velocity of granular jets. It is found that the jet velocity is proportional to the natural frequency of the grain-spring system. An increase in cohesion energy density leads to a reduction in jet velocity.