The fluctuation energy balance in non-suspended fluid-mediated particle transport

TL;DR

This study compares particle fluctuation energy balances in saltation and bedload transport regimes, revealing that high-energy impacts in saltation lead to greater dissipation despite fewer collisions, with turbulence significantly influencing fluctuation energy.

Contribution

It provides new insights into the energy dissipation mechanisms in non-suspended particle transport, highlighting the dominant role of impact splashes in saltation.

Findings

Higher collisional dissipation in saltation despite fewer collisions.

Impact splashes are the main cause of energy dissipation in saltation.

Turbulent flow fluctuations significantly generate particle fluctuation energy in saltation.

Abstract

Here we compare two extreme regimes of non-suspended fluid-mediated particle transport, transport in light and heavy fluids ("saltation" and "bedload", respectively), regarding their particle fluctuation energy balance. From direct numerical simulations, we surprisingly find that the ratio between collisional and fluid drag dissipation of fluctuation energy is significantly larger in saltation than in bedload, even though the contribution of interparticle collisions to transport of momentum and energy is much smaller in saltation due to the low concentration of particles in the transport layer. We conclude that the much higher frequency of high-energy particle-bed impacts ("splash") in saltation is the cause for this counter-intuitive behavior. Moreover, from a comparison of these simulations to Particle Tracking Velocimetry measurements which we performed in a wind tunnel under steady transport of fine and coarse sand, we find that turbulent fluctuations of the flow produce particle fluctuation energy at an unexpectedly high rate in saltation even under conditions for which the effects of turbulence are usually believed to be small.