Three-dimensional turbulence structure of rectangular side-cavity zone in open-channel streams

Abstract

Open-ended side-cavity zones are often observed in natural rivers. They appear in embayment and aligned spur-dike fields. Many pollutant clouds and suspended sediments are conveyed and trapped in the cavities, and it is thus very important in environmental hydraulics and river management to accurately predict mass and momentum exchanges through mainstream/side-cavity interface. It is well known that small-scale shedding vortices are generated due to shear instability induced by velocity differences between high-speed mainstream and low-speed embayment flows. A large momentum in the main-channel causes large-scale horizontal gyre in the cavity zone. The coherent turbulence structure in the mainstream/embayment boundary and the horizontal large-scale gyre structure play a key role in the mass/momentum exchanges. In particular, previous studies have pointed out that a rectangular-shaped cavity zone with an aspect ratio of 3.0 produces two kinds of gyres and shows a more effective exchange property in comparison with a square-shaped cavity. However, much uncertainty remains regarding the detailed hydrodynamics accompanied by three-dimensional turbulence motions. Using large eddy simulation, which is also compared with particle image velocimetry measurements, we predict three-dimensional current properties and turbulence structure. Based on these results, a significant relation between instantaneous vertical flows and the spanwise momentum transfer is shown. Furthermore, a phenomenological flow model is proposed for the side-cavity zone in the open-channel field.