Takaaki Okamoto

Turbulence structure and “Monami” phenomena in flexible vegetated open-channel flows

Aquatic plants in natural rivers are essentially important components of ecosystems. In vegetated canopy open-channel flow, velocity profiles are changed largely in the vertical direction resulting in the Kelvin-Helmholtz instability. Consequently, large-scale organized vortices are induced, which dominate the momentum transfer and scalar concentration among canopies. The downstream advection of coherent vortices causes the coherent waving of aquatic vegetation, referred to as Monami. However, the flow-visualization techniques such as PIV have not been sufficiently applied to these complicated flows in previous studies, such that no detailed information on the interaction between the flow field and the plant motion exists. Simultaneous measurements of turbulence and vegetation motion were conducted herein for open-channel flows with flexible vegetation by using a combination of PIV and PTV techniques. The former part of this study focuses on the effects of Monami phenomena on mean-flow and turbulence structures. The latter part examines an interaction between the coherent structure and the vegetation motion.

Spatial evolution of coherent motions in finite-length vegetation patch flow

A number of experimental studies on submerged canopy flows have focused on fully-developed flow and turbulent characteristics. In many natural rivers, however, aquatic vegetation occurs in patches of finite length. In such vegetated flows, the shear layer is not formed at the upstream edge of the vegetation patch and coherent motions develop downstream. Therefore, more work is neededz to reveal the development process for large-scale coherent structures within vegetation patches. For this work, we considered the effect of a limited length vegetation patch. Turbulence measurements were intensively conducted in open-channel flows with submerged vegetation using Particle Image Velocimetry (PIV). To examine the transition from boundary-layer flow upstream of the vegetation patch to a mixing-layer-type flow within the patch, velocity profiles were measured at 33 positions in a longitudinal direction. A phenomenological model for the development process in the vegetation flow was developed. The model decomposed the entire flow region into four zones. The four zones are the following: (i) the smooth bed zone, (ii) the diverging flow zone, (iii) the developing zone and (iv) the fully-developed zone. The PIV data also confirmed the efficiency of the mixing-layer analogy and provided insight into the spatial evolution of coherent motions.

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

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.

Flow–vegetation interactions: length-scale of the “monami” phenomenon

ABSTRACT This paper explores the interaction between flow field and plant motion in open-channel flows with submerged flexible vegetation. Experiments were performed in a 10 m-long and 40 cm-wide tilting flume. Flows with submerged vegetation were studied under a range of conditions by varying lengths of flexible vegetation models (5.0, 7.0, 9.0 and 10.5 cm). Flow velocity components and the tip motion of waving plants were measured simultaneously using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques. The applied techniques are capable of investigating the interaction between flow field and plant motion. We examined quantitatively how plant motions were coupled to strong oscillations in flow velocity associated with the “monami” phenomenon as well as its vertical extent. A strong correlation between flow velocity and plant motion was observed in the mixing-layer zone (near the vegetation edge) only. Analysis of cross-correlation and coherence functions enabled estimation of the phase lag between the turbulent flow field and plant motion. The experiments revealed that the number of flexible vegetation elements waving at the same time depends on the length-scales of the turbulence structure in the mixing-layer zone.