Morphology and displacement of dunes in a closed-conduit flow

TL;DR

This study experimentally investigates the morphology and movement of isolated dunes formed by particles in a closed conduit flow, revealing shape deformation, self-similarity, and velocity scaling related to fluid shear conditions.

Contribution

It provides new experimental insights into dune dynamics, morphology, and velocity scaling in a controlled closed-conduit environment, extending understanding beyond natural desert dunes.

Findings

Dunes rapidly deform into croissant shapes under flow.

Dune size and velocity exhibit self-similarity.

Dune velocity scales inversely with size and relates to shear velocity.

Abstract

The transport of solid particles entrained by a fluid flow is frequently found in industrial applications. A better knowledge of it, is of importance to improve particle related industrial processes. When shear stresses exerted by the fluid on the bed of particles are bounded to some limits, a mobile layer of particles known as bed-load takes place in which the particles stay in contact with the fixed bed. If it takes place over a non-erodible ground, and if the particle flow rate is small enough, an initial thin continuous layer of particles becomes discontinuous and composed of isolated dunes. We present here an experimental study to understand some features of the dynamics of isolated dunes under a fluid flow using a closed-conduit experimental loop made of transparent material. Acquired data concerns mainly dune morphology and displacement velocity under different conditions: different types of beads (diameters and densities) and different water flow conditions. We observed that an initial pile of beads placed in the conduit is rapidly deformed by the water flow, adopting a "croissant" shape, like barchan dunes found in deserts at a much larger scale. We observed also self-similarity in dunes dimensions and that dune displacement velocity scales with the inverse of their dimensions. The variation of the dune displacement velocity with the fluid shear velocity is discussed here.