Mechanism of Secondary Currents and its Interactions with Sediment Transport

This paper describes the conditions for initiation and maintenance of secondary
currents in open channel flows. By analyzing the Reynolds equation in the wall-normal and
wall-tangent directions, this study reveals that, like other types of vortices, the secondary
currents are originated in the near-boundary region, and the magnitude (or strength) of
secondary flow is proportional to the lateral gradient of near-wall velocity. The near-wall
secondary flow always moves from the region with lower velocity (or lower boundary
shear stress) to the location with higher velocity (or higher boundary shear stress).
Subsequently, the near-boundary secondary flow creeps into the main flow and drives
circulation within a region enclosed by lines of zero total shear stress, leading to anisotropy
of turbulence in the main flow region. This paper also discusses typical secondary currents
in open channel flows and presents the relationship between sediment transport and
secondary currents. This study explores the importance of wall-normal velocity on turbulence structures in open channel flows, and explains its impacts on sediment transport. A new definition of shear stress on an interface has been proposed, and the N-S equation can be also derived using the new definition, in which the time-averaged wall-normal velocity is included. The analysis shows that the wall-normal velocity can be induced by many factors, e.g., secondary currents, non-uniformity, sediment settlement etc. Previous research ignores its influence on flow structures and sediment transport. This discovery was attempted to explain observed oddness in water resources engineering, such as the deviations of measured Reynolds shear stress, turbulent intensities, eddy viscosity and sediment concentration from the theoretical predictions. It is found that many odd phenomena observed from natural rivers and laboratories can be well explained after the influence of wall-normal velocity is included.