Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions

Abstract

The fluctuating drag forces acting on spherical roughness elements comprising the bed of an open-channel flow have been recorded along with synchronous measurements of the surrounding velocity field using stereoscopic particle image velocimetry. The protrusion of the target particle, equipped with a force sensor, was systematically varied between zero and one-half diameter relative to the hexagonally packed adjacent spheres. Premultiplied spectra of drag force fluctuations were found to have bimodal shapes with a low-frequency peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency region of the drag force spectra cannot be explained b..