Direct simulation of aerodynamic entrainment with inter-particle cohesions

TL;DR

This study uses direct numerical simulations to investigate aerodynamic entrainment of particles, revealing that particles leave the bed at smaller angles and higher velocities than previously thought, with cohesion affecting entrainment thresholds.

Contribution

It provides new insights into particle entrainment mechanics, showing the linear relationship with shear velocity and the impact of inter-particle cohesion on thresholds.

Findings

Particles leave the bed at smaller angles and higher velocities.

Entrainment rate depends nearly linearly on shear velocity.

Inter-particle cohesion raises the entrainment threshold without affecting the rate.

Abstract

Aerodynamic entrainment acts as the pioneer of saltation movement and is critical for understanding the development of aeolian phenomena. Here we performed direct numerical simulations on the aerodynamic lifting of surface particles on a random arranged sediment bed using the discrete element method, and provided evidence that particles do not leave the bed vertically with speeds equal to that needed to reach a height of one grain diameter, as people widely accepted, but with much smaller angles toward the bed surface and much larger velocities. The entrainment rate does not exhibit a quadratic dependence with the shear velocity, but a nearly linear one, because the velocity of entrained particle increases linearly with shear velocity. Moreover, inter-particle cohesion increases the fluid entrainment threshold significantly, but has no effect on the entrainment rate, as well as the take-off velocity and angle distributions of entrained particles, because the collisions between entrained and bed particles destroy the inter-particle bonds. The completely entrainment scheme is given, which provides a credible approach to explore the development and evolution of aeolian transports.