5% - eTheses Repository - University of Birmingham

Recent work by Nezu & Nakagawa (1991) has helped to clarify the
mechanisms behind boil formations in steady and unsteady flow.
They defined a so-called "boil of the first kind" as a direct
surface manifestation of primary shear vortices which have grown
to fill the depth of flow and are constrained by the free surface
as indicated in figure 13. Horseshoe vortices (figure 11),
caused by three dimensional instabilities discussed above,
channel fluid from below the vortex upwards to the free surface.
This fluid may well contain suspended solids entrained into the
horseshoe vortex during its impingement on the stoss slope of the
downstream sandwave, and this would account for high sediment
concentrations seen in boils of relatively low intensity
(Jackson, 1976) . Other weak boil structures identifed by Nezu
and Nakagawa are not considered further here.
3.2 Vortex-particle interactions
Numerous studies have been conducted on the dynamic interactions
of discrete elements and vorticity, usually with the conclusion
that this aspect is vitally important to a proper description of
sediment transport. Grass (1974) studied sand motion in boundary
layers using high-speed movie film from which he produced
pictures of sediment laden horse-shoe vortices (figure 14). He
presented convincing evidence of particle entrainment and
transport within migrating vortex loops and also estimated the
particle ejection velocities presented in figure 15. These
velocities were found to be lower than the overall mean stream
velocity which is consistent with the notion of particle ejection
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