Determining the Onset of Hydrodynamic Erosion in Turbulent Flow

TL;DR

This study investigates the initiation of sediment transport in turbulent flow using particle tracking velocimetry, revealing that grain mobilization occurs gradually without a sharp transition, and highlights the need for advanced statistical methods to analyze grain motion.

Contribution

It introduces a mixture model approach to analyze grain velocities, providing new insights into the complex onset of sediment transport in turbulent conditions.

Findings

Fraction of transported grains increases with flow speed

Distribution of non-transported grain velocities broadens with heavy tails

Sharp transition in grain mobilization is not observed

Abstract

We revisit the longstanding question of the onset of sediment transport driven by a turbulent fluid flow via laboratory measurements. We use particle tracking velocimetry to quantify the fluid flow as well as the motion of individual grains. As we increase the flow speed above the transition to sediment transport, we observe that an increasing fraction of grains are transported downstream, although the average downstream velocity of the transported grains remains roughly constant. However, we find that the fraction of mobilized grains does not vanish sharply at a critical flow rate. Additionally, the distribution of the fluctuating velocities of non-transported grains becomes broader with heavier tails, meaning that unambiguously separating mobile and static grains is not possible. As an alternative approach, we quantify the statistics of grain velocities by using a mixture model consisting of two forms for the grain velocities: a decaying-exponential tail, which represents grains transported downstream, and a peaked distribution centered at zero velocity, which represents grains that fluctuate due to the turbulent flow but remain in place. Our results suggest that more sophisticated statistical measures may be required to quantify grain motion near the onset of sediment transport, particularly in the presence of turbulence.