Competitive dynamics of two erosion patterns around a cylinder

TL;DR

This study experimentally examines erosion patterns around a fixed cylinder in a granular bed, identifying the onset of erosion linked to flow effects and discovering a new downstream erosion pattern of elongated holes.

Contribution

It introduces a new downstream erosion pattern and explains the flow contraction effect responsible for the initial erosion at the cylinder base.

Findings

Erosion begins at the cylinder base due to flow contraction effects.

A new pattern of elongated holes forms downstream near the horseshoe scour threshold.

Flow regimes close to horseshoe scour onset inhibit the growth of the new pattern.

Abstract

We investigate experimentally the local erosion of a granular bed near a fixed vertical cylinder that emerges from the bed. The onset of erosion arising at the base of the cylinder and usually ascribed to the wrapping horseshoe vortex is determined and rationalized by a flow contraction effect. We report a new erosion pattern visible downstream of the cylinder that consists of two side-by-side elongated holes. This pattern is observed for flow regimes close to the horseshoe scour onset, whose growth usually inhibits its spatiotemporal development.