Entrainment of sediment particles by very-large-scale motions

Stereoscopic particle image velocimetry (PIV) configured in two orthogonal planes was utilised to capture the flow structure at the instant of entrainment of spherical bed particles in open-channel flow. Experiments were conducted with lightweight target particles amongst a bed of coplanar fixed spheres with diameters of 16 mm. The protrusions of the target particles were set to give an average entrainment rate of 1/60 s−1. These protrusions were established from extensive initial experiments which utilised an automated mechanism to place spheres on the bed of the flume and record the time elapsed until they were entrained by the flow. The results showed that at lower flow depth to particle diameter ratios, bed particles are more stable and require larger protrusions to entrain at the same rate as at a larger depth. This effect is consistent with observations of reduced velocity variance and reduced drag force variance for lower flow submergences. The PIV measurements indicated that particle entrainment is associated with very large-scale motions which extend up to
50 flow depths in the streamwise direction. Contributions of smaller scale velocity and pressure spatial fluctuations are suppressed by a spatial averaging effect related to the particle size, and a temporal averaging effect related to the time taken to fully entrain a particle from its resting pocket. These observations are relevant to sediment transport modelling. However, further data are required to clarify the role of particle lift forces and particle shape in the entrainment process.