Velocity fields of a bed-load layer under a turbulent liquid flow

TL;DR

This paper experimentally investigates grain velocities and transport rates in a turbulent water flow over a granular bed near incipient bed load, providing new detailed data on grain motion in such conditions.

Contribution

It offers novel experimental measurements of grain velocities and transport rates near incipient bed load under fully developed turbulent flow conditions.

Findings

Grain velocities increase with flow rate.

Transport rates correlate with flow conditions.

Distinct velocity profiles observed near the bed.

Abstract

The transport of sediments by a fluid flow is commonly found in nature and in industry. In nature, it is found in rivers, oceans, deserts, and other environments. In industry, it is found in petroleum pipelines conveying grains, in sewer systems, and in dredging lines, for example. This study investigates experimentally the transport of the grains of a granular bed sheared by a turbulent liquid flow. In our experiments, fully developed turbulent water flows were imposed over a flat granular bed of known granulometry. Under the tested conditions, the grains were transported as bed load, i.e., they became entrained by rolling and sliding over each other, forming a moving granular layer. The present experiments were performed close to incipient bed load, a case for which experimental data on grains velocities are scarce. Distinct from previous experiments, an entrance length assured that the water stream over the loose bed was fully developed. At different water flow rates, the moving layer was filmed using a high-speed camera, and the grains' displacements and velocities were determined by post-processing the images with a numerical code developed in the course of this study. The bed-load transport rate was estimated and correlated to the water flow conditions.