Direct measurement of the inertial drag and lift forces on entrained coarse particles at various protrusion heights

Abstract

AbstractLaboratory experiments were performed to study the impact of relative particle protrusion P/D (P is the protrusion height and D is the diameter of the spherical target particle) on the entrainment of sediment particles from a rough bed of spherical grains. The target particle to be entrained was instrumented with electronic sensors to measure the triaxial linear acceleration, and consequently the particle inertial forces, during the entrainment process. The velocity field around the particle was obtained using two‐dimensional particle tracking velocimetry (PTV) technique and the velocity data were synchronised with the force data relative to the moment of entrainment. Experimental results show that the inertial drag and lift forces systematically decreased as relative particle protrusion increased. The ratio of inertial lift force to drag force revealed that drag force slightly dominated the entrainment process at P/D > 0.7, while lift force slightly dominated at P/D < 0.62. Inertial drag coefficient was found to be independent of particle protrusion for low P/D, but rapidly declined for P/D > 0.62–0.7. Differently, inertial lift coefficient remained relatively constant within the tested protrusion ranges. Additionally, the critical Shields number was linearly related to the normalised inertial forces over the present particle protrusion range. In summary, the particle protrusion demonstrates a significant impact on the inertial forces and, further, the entrainment process.