Koji Shiono

Turbulent open-channel flows with variable depth across the channel

The flow of water in straight open channels with prismatic complex cross-sections is considered. Lateral distributions of depth-mean velocity and boundary shear stress are derived theoretically for channels of any shape, provided that the boundary geometry can be discretized into linear elements. The analytical model includes the effects of bed-generated turbulence, lateral shear turbulence and secondary flows. Experimental data from the Science and Engineering Research Council (SERC) Flood Channel Facility are used to illustrate the relative importance of these three effects on internal shear stresses. New experimental evidence concerning the spatial distribution of Reynolds stresses τ yx and τ zx is presented for the particular case of compound or two-stage channels. In such channels the vertical distributions of τ zx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth-averaged values of τ yx are shown to be significantly different from the depth mean apparent shear stresses. The importance of secondary flows in the lateral shear layer region is therefore established. The influence of both Reynolds stresses and secondary flows on eddy viscosity values is quantified. A numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity. The results of this study are then used in the analytical model to reproduce lateral distributions of depth-mean velocity and boundary shear stress in a two stage channel. The work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular.

Turbulence measurements in a shear layer region of a compound channel

The paper describes some turbulence measurements carried out in the SERC Flood Channel Facility at Hydraulics Research Ltd., Wallingford, U.K. The facility represents a large scale model of a river system with floodplains, and is designed to produce fully developed boundary layer flows with transverse shear. This article presents some of the open channel flow data, including measurements of the primary velocity, Ū/u ∗, the distribution of turbulent intensities, u1/u∗ , v1/u∗ , and w1/u∗ , the kinetic energy, k/u2∗ , and the Reynolds stresses τzx and τyx in the region of strong lateral shear induced by transverse variation in depth. Attention is focussed on the non linear nature of the Reynolds stresses in the shear layer, flow structures and the lateral variations in eddy viscosity and local friction factor.

Complex flow mechanisms in compound meandering channels with overbank flow

Turbulence and secondary flow measurements were undertaken using a two-component laser-Doppler anemometer in meander channels with straight flood plain banks. The most interesting feature of the compound meandering channel flow was found to be the behaviour of the secondary flow. The difference in direction of rotation of the flow before and after inundation at a bend section was confirmed by the detailed velocity measurements. In addition, by performing the measurement over a half wavelength of meander, the originating and developing processes of the secondary flow were also clarified. In contrast to the centrifugal force for inbank flow, the interaction between the main channel flow and the flood plain flow in the cross-over region was found to play an important role in developing a shear produced secondary flow in the overbank cases. New experimental evidence concerning the spatial distribution of Reynolds stress −ρ uw , −ρ uv and −ρ vw are presented for sinuous compound meander channels. In such channels, large interfacial shear stresses were induced at around the bankfull level, especially in the cross-over region, and were found to be larger than the bed shear stress in magnitude. Particular importance is placed on −ρ vw , which is usually small compared with other stress components, as the cause of the secondary flow in the lower layer. The influence of secondary flow on eddy viscosity was found also to be significant. These turbulence data are particularly useful in understanding the flow mechanisms that occur in meandering channels and in developing proper turbulence models for such flows.

Quasi two-dimensional model for straight overbank flows through emergent vegetation on floodplains

The paper presents a quasi two-dimensional (2D) model calculating depth-averaged velocity and bed shear stress in a straight compound channel with a vegetated floodplain. The model numerically solves the depth-averaged Navier–Stokes equation for the streamwise motion of flow (quasi 2D). Reduction in volume of water due to vegetation is modelled as porosity. Drag force due to vegetation is modelled as an additional momentum sink term in the Navier–Stokes equation. Estimation methods for model parameters—drag coefficient, shading factor, porosity, friction factor, eddy viscosity and advection term—are discussed. The predictive capability of the model is assessed against experimental data with regard to distributions of depth-averaged velocity, bed shear stress, transverse shear stress and stage-discharge. The results show that the quasi 2D model reproduces a reasonable simulation of the flow field.

Numerical modelling of solute transport in compound channel flows

A 3-D numerical model has been developed to investigate transport processes of solute in a compound open channel. The Navier-Stokes equations have been numerically solved in conjunction with the linear and nonlinear k-∊ models to predict flow fields and turbulent parameters. The predicted velocity field and eddy viscosity are then used for solving the solute transport equation to predict solute transport rates. The hydrodynamic and solute transport models have been applied to the experimental studies undertaken by Wood and Liang (1989) and the results obtained from both the linear and non-linear k-∊ models are compared with their results. This comparison clearly shows the influence of the secondary currents upon the mixing processes.

Turbulent characteristics in a baffled contact tank

Turbulence measurements were undertaken using a two-component laser Doppler anemometer (LDA) in a serpentine contact tank (CT) commonly used for water chlorination. A detailed examination of turbulent parameters was carried out and showed the significant effects of the baffle lee and the inlet and outlet of the tank on the CTs hydraulic efficiency. Turbulence levels were quantified along the tank and were found to be considerably high in the tank inlet, first and second compartments and to be decaying rapidly in the latter compartments. The decay rate of turbulence along the tank is similar to that of the grid generated turbulence. The turbulence generation mechanisms in the first compartment were identified using comparative studies of the turbulence characteristics in the backward-facing step flow obtained by Eaton and Johnston (1981) and in the plane wall jet obtained by Rajaratnam (1976). The main contribution of the high turbulence level in the compartment was found to be due to shear generated turbulence in the upper layer. Tracer measurements were also carried out and an optimum dispersion coefficient at the outlet of the tank was found to be 16.2 cm-/s. From the Marske and Boyles equation (1973) in this tank, for b = 37, the dispersion coefficient is 2.9 cm2/s, which is one order smaller than that obtained from this study.

Energy losses due to secondary flow and turbulence in meandering channels with overbank flows

An investigation of energy losses due to boundary friction, secondary flow, turbulence, expansions and contractions in meandering compound channels with overbank flow is described.The compound meandering channel was divided into three sub-areas, namely the main channel below the bankfull level, the meander belt width above the bankfull level and a region outside the meander belt above the bankfull level, and turbulence data obtained by a Laser Doppler anemometer system.The energy loss due to the shear stress on the horizontal plane at the bankfull level was estimated using the measured Reynolds stresses and sectional averaged velocity, and the energy loss due to secondary flow below the bankfull level was then estimated.Both energy losses were found to make a significant contribution to the total energy loss in the lower layer for shallow overbank flow.The energy losses due to the contraction and expansion in the meander belt were evaluated and found to be a significant component of the total energy loss ...

Predictions of solute transport in a compound channel using turbulence models

Prediction of solute distributions in an asymmetric compound channel is carried out using two turbulence models. Results of the numerical models are compared with turbulence data recently obtained using laser Doppler anemometer (LDA) and laser induced fluorescence (LIF) in a small laboratory flume. The predicted distributions of solute and Reynolds flux using a k-s model and an algebraic stress model for various injection points near the water surface are used to identify different mixing mechanisms. A skewed distribution of solute on the floodplain observed in the experimental data is well predicted by the algebraic stress model but not by the k-ε model. The cause of the skewed distribution is examined through the variations of secondary flow and eddy diffusivity. The predicted eddy viscosity and diffusivity and the turbulent Schmidt number are discussed with the experimental data. As a result, solute concentration distribution can be well predicted by adjusting the turbulent Schmidt number even if the eddy viscosity was not correctly calculated by any particular model. An effect of secondary flow on peak concentration in the shear layer along the channel is also demonstrated.

The effect of floodplain roughness on flow structures, bedforms and sediment transport rates in meandering channels with overbank flows: Part I

Studies were carried out to understand the effect of floodplain roughness on flow structures, sediment transport rates and bedforms in a mobile meandering channel with overbank flows. Three floodplain roughnesses were examined in this study. Flow structures and bedforms were measured using a three component laser Doppler anemometer (LDA) system and digital photogrammetry, respectively. Comparisons of flow structures and bedforms between different floodplain rougheness are made. Considerable changes in the flow structure and bed form were observed. In particular, multiple secondary flow cells along the meandering channel occur at deeper water depths as the floodplain roughness increases. These cells also cause a series of wavy bedforms along the meandering channel. This paper is separated into two parts. Part I concentrates, in detail, on the flow structure with bedforms in the meandering channel for overbank flow as to change of floodplain roughness. The sister paper, Part II, concentrates on bed form formation during flood, sediment transport rates and flow resistance in the main channel and floodplain.

Flow characteristics in meandering channels with non-mobile

Experiments were conducted in meandering channels with non-mobile and mobile beds to measure flow rates, velocities, turbulent kinetic energies, bedforms and sediment transport rates for overbank flows. The behaviour of bedform in meandering channels with overbank flows was observed using digital photogrammetry, with velocity measurements taken with a Laser Doppler Anemometer. The bedform structure and velocity distributions along the meandering channel were obtained for bank full flow and three overbank flow depths. Important interactions between the flow structure and bedform were observed along the meandering channel. The sediment transport rates collected during the experiment showed three phases; an increase in the sediment transport rate up to the bankfull level, a small decrease as the flow goes overbank up to a relative depth ratio of 0.3 and then an increase again for higher flow depths. The regions of higher turbulent kinetic energy were identified. The total energy losses due to friction, secondary flow and interfacial turbulence in the lower layer flow of the main channel were compared in both the non-mobile and mobile bed cases.