Erosion patterns on a granular bed around a vertical cylinder

Florent Lachaussée,Yann Bertho,Cyprien Morize,Philippe Gondret,Alban Sauret,F Radjai,S Luding,S Nezamabadi,J.Y Delenne

doi:10.1051/epjconf/201714009008

Abstract

We report on two different patterns that can be observed at the bed surface close to a vertical cylinder when submitted to a strong enough steady water flow. The classical scour pattern observed at the cylinder foot and due to the “horseshoe” vortex around occurs at a critical velocity U c 1 below the critical velocity U c 0 for erosion without any cylinder, thus under clear-water conditions. But we observe also another pattern downstream the cylinder which consists of two symmetrical ovoid holes that look like “bunny ears”. This new scour pattern referred as BES can be observed at lower velocities that the horseshoe scour (HSS), above a critical velocity U c 2 c 1 , with a timescale formation much higher that the one of HSS.

Highlights

Erosion is encountered in numerous natural or industrial situations
Water flow is driven by means of a paddle wheel located in one linear part of the channel and is considered after a honeycomb in the other linear test section where a granular bed composed of glass beads of density ρg = 2500 kg m−3 and mean diameter d = (270 ± 30) μm is put in a drawer of length 600 mm and 40 mm depth at the bottom of the channel
The present Shields number is an inertial Shields number chosen as Red > 1, which is different from the local Shields number that could be taken by knowing the exact fluid stress at the bed surface [7, 8]

Summary

Introduction

Erosion is encountered in numerous natural or industrial situations. Understanding erosion process is essential for instance to interpret geomorphological patterns on Earth or other planets and to predict the evolution of estuaries and river beds [1]. Quantifying the erosion processes generated at the vicinity of structures referred as scouring is relevant to a wide range of engineering applications, including the design and risk assessment of hydraulic structures such as bridge piers, off-shore platforms and wind turbines. Numerical approaches are far from describing carefully these two complexity levels and the complex grain/fluid interaction even if key advances have been made with the recent development of coupled methods with Large Eddy Simulation (LES) for the fluid and Discrete Element Method (DEM) for the grain phase [5]

Methods

Results

Conclusion