Compound Open-channel Flows Research Articles

Development of Two-layered model for compound open-channel flow

Present study deals with the development of two-dimensional two-layered model for straight compound open-channel flow using Smagorinsky-based subgrid-scale turbulence model, based on large eddy simulation approach. The two-layer depth averaged governing equations are solved using finite volume method based on MacCormack predictor-corrector explicit scheme. Model results for symmetric and asymmetric straight compound channel flows are verified with the available numerical and experimental data. Variations of flow variables in symmetric and asymmetric straight compound channels in terms of shear stress distributions and spanwise and vertical velocities are analyzed at the horizontal interface between the lower layer and the upper layer. Spanwise and vertical velocities at the horizontal interface revealed the existence of horizontal vortices near the junction of the main channel and the floodplain where momentum exchange takes place. Further, the model result, showing the spanwise shear stresses distribution at the interface of symmetric and asymmetric compound channel, revealed the existence of peak shear stress near the channel junction. Sensitivity of the two important model parameters is also undertaken and their roles in longitudinal velocity distribution along the channel width are discussed.

Turbulent non-uniform flows in straight compound open-channels

The reported experimental study assesses the effects of flow non-uniformity on the momentum flux in straight compound channels. Two flumes were used, featuring vertical and sloping banks. Starting with uniform flow condition, various imbalances in the upstream discharge distribution were introduced. This resulted in a time-averaged lateral flow and advective transport of momentum, which interacted with the shear-layer turbulence generated by the compound geometry. To investigate this interaction, the three contributions to transverse momentum flux (depth-averaged flow, shear-layer turbulence and dispersive term of spanwise velocity) are assessed. The first two contributions were strengthened by the sloping banks, while the third becomes important for the case of the vertical bank. With a lateral flow towards the main channel, the first contribution rises at the expense of the second. With a lateral flow towards the floodplain, the first two contributions have the same order of magnitude, and the Boussinesq approach is invalidated.

Application of Shiono and Knight Method to Compound Open Channel Flow with One-Line Emergent Vegetation

A quasi two-dimensional model was developed to calculate depth-averaged velocity and bed shear stress in a straight compound channel with one-line emergent vegetated at the floodplain edge. Reduction in volume of water due to vegetation is modelled as porosity. Drag force due to vegetation is modelled as an additional momentum source term in the Navier-Stokes equation. Estimation methods for model parameters—friction factor, roughness height due to drag force, porosity, 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 and boundary shear stress. The results show that the quasi 2D model well reproduces a reasonable simulation of the flow field under relative water depths of 0.24 and 0.52.

低水路側壁が傾斜した複断面開水路流れにおける二次流れと組織構造の特徴

In this study, characteristics of secondary flow and coherent structure in one-sided compound open channel flows with inclined main channel side wall are investigated using PTV (Particle Trucking Velocimetry) and fluorescent dye injection method. The velocity measurement results show that intensity of secondary flow in the case of deep depth tends to be weak compared with the flow with vertical side wall and the rotation of circular secondary flow over the inclined side wall in the case of shallow depth is in the direction opposite to that one of the flow in the case of deep depth and vertical side wall. The results of flow visualization indicate that streamwise vortex is formed over the junction edge, inclined side wall and the corner of the main channel. Furthermore, the analysis of DPTV (Dye-streak pattern Particle Trucking Velocimetry) which is combined with PTV and fluorescent dye injection method shows that the streamwise vortices play important role in generating instantaneous secondary flow.

Comparison between linear genetic programming and M5 tree models to predict flow discharge in compound channels

There are many studies on the hydraulic analysis of steady uniform flows in compound open channels. Based on these studies, various methods have been developed with different assumptions. In general, these methods either have long computations or need numerical solution of differential equations. Furthermore, their accuracy for all compound channels with different geometric and hydraulic conditions may not be guaranteed. In this paper, to overcome theses limitations, two new and efficient algorithms known as linear genetic programming (LGP) and M5 tree decision model have been used. In these algorithms, only three parameters (e.g., depth ratio, coherence, and ratio of computed total flow discharge to bankfull discharge) have been used to simplify its applications by hydraulic engineers. By compiling 394 stage-discharge data from laboratories and fields of 30 compound channels, the derived equations have been applied to estimate the flow conveyance capacity. Comparison of measured and computed flow discharges from LGP and M5 revealed that although both proposed algorithms have considerable accuracy, LGP model with R2 = 0.98 and RMSE = 0.32 has very good performance.

Large-eddy simulation of the turbulent structure in compound open-channel flows

A large-eddy simulation study has been undertaken to investigate the turbulent structure of open-channel flow in an asymmetric compound channel. The dynamic sub-grid scale model has been employed in the model, with the partial cell treatment being implemented using a Cartesian grid structure to deal with the floodplain. The numerical model was used to predict the: primary velocity and secondary currents, boundary shear stress, turbulence intensities, turbulent kinetic energy, and Reynolds stresses. These parameters were compared with experimental measurements published in the literature, with relatively close agreement being obtained between both sets of results. Furthermore, instantaneous flow fields and large-scale vortical structures were predicted and are presented herein. These vortical structures were found to be responsible for the significant lateral exchange of mass and momentum in compound channels.

Turbulence structure and concentration exchange property in compound open-channel flows with emergent trees on the floodplain edge

There are aquatic plants, trees, and shrubs at the floodplain edge in natural rivers. In particular, spanwise profile of streamwise velocity component is influenced by the drag force of the trees, and consequently, the streamwise velocity decreases locally behind trees and near the junction between the main channel and the floodplain. Furthermore, concentration exchanges such as nutrients between the main channel and the floodplain differ from those observed in the non-tree compound open-channel flows. It is thus very important to investigate these hydrodynamic properties and turbulence structure, considering the emergent tree effects in river engineering and eco-hydraulics. So, in the present study, turbulence measurements by 3D acoustic Doppler velocimetry were conducted in a wide laboratory flume, in which several cylinders are placed as tree models. Three-dimensional large eddy simulation was also carried out to simulate concentration transport properties in these complex flows and it is found that there is a maximum exchange rate of concentration between the main channel and floodplains.

A neural network approach for prediction of discharge in straight compound open channel flow

Most natural rivers and streams consist of two stage channels known as main channel and flood plains. Accurate prediction of discharge in compound open channels is extremely important from river engineering point of view. It helps the practitioners to provide essential information regarding flood mitigation, construction of hydraulic structures and prediction of sediment load so as to plan for effective preventive measures. Discharge determination models such as the single channel method (SCM), the divided channel method (DCM), the coherence method (COHM) and the exchange discharge method (EDM) are widely used; however, they are insufficient to predict discharge accurately. Therefore, an attempt has been made in this work to predict the total discharge in compound channels with an artificial neural network (ANN) and compare with the above models. The mean absolute percentage error with artificial neural networks is found to be consistently low as compared to other models.

PIV and PTV measurements in hydro-sciences with focus on turbulent open-channel flows

PIV is one of the most popular measurement techniques in hydraulic engineering as well as in fluid sciences. It has been applied to study various turbulent phenomena in laboratory experiments related to natural rivers, e.g., bursting phenomena near the bed, mixing layers observed at confluences, wake turbulence around dikes and piers, and so on. In these studies, PIV plays important roles in revealing the space-time structure of velocity fluctuations and coherent vortices. This review article focuses particularly on the applications of PIV to turbulent open-channel flows, which have been conducted for the past decade in Hydraulics Laboratory of Kyoto University. In Section 2, we introduce our experimental setup and PIV/PTV algorithm. In Section 3, we apply the PIV measurements to reveal turbulence characteristics and coherent structures in open-channel flows as well as in vegetated canopy flows. For complex flow situations, various applications of PIV to compound open-channel flows and wind-induced water waves are considered to reveal coherent vortices. In Section 4, we discuss some advanced PIV measurements in open-channel flows. The free-surface-elevation fluctuations and velocity components were measured simultaneously with two sets of cameras to examine phase-averaged parameters of turbulence. A multi-layer scanning PIV was developed to reveal 3D turbulence structure in compound open-channel flows. Our discriminator PIV/PTV was applied successfully to sediment-laden open-channel flows and revealed the fluid/particle interaction and the relationship between coherent structures and sediment concentration. Finally, we conducted simultaneous measurements of velocity and dye concentration with a combination of PIV and LIF in vegetated open-channel flow, which enables us to examine turbulent scalar flux of a passive contaminant.

Numerical simulation of turbulent flows in trapezoidal meandering compound open channels

AbstractThree‐dimensional (3D) numerical study is presented to investigate the turbulent flow in meandering compound open channels with trapezoidal cross‐sections. The flow simulation is carried out by solving the 3D Reynolds‐averaged continuity and Navier–Stokes equations with Reynolds stress equation model (RSM) for steady‐state flow. Finite volume method (FVM) is applied to numerically solve the governing equations of fluid flow. The velocity magnitude, tangential velocity, transverse velocity and Reynolds stresses are calculated for various flow conditions. Good agreement between the simulated and available laboratory measurements was obtained, indicating that the RSM can accurately predict the complicated flow phenomenon. Comparison of the calculated secondary currents of four cases (one being inbank flow and other three being overbank flow) with different water depths reveals that (i) the inbank flow exhibits different flow behaviors from that of the overbank flow does and (ii) the water depth has significant effects on the magnitude and direction of secondary currents. Copyright © 2010 John Wiley & Sons, Ltd.

COMPUTATIONS ON MEANDERING COMPOUND OPEN CHANNEL FLOWS USING 3D URANS MODEL

A meandering compound open channel flows show a complicated three-dimensional flow structures because of the combined effects of the secondary currents of the first kind, and the horizontal vortices at the junction between the main channel and the floodplain. The secondary current of the first kind is generated by the unbalance of the centrifugal force and the lateral pressure gradient. The horizontal vortices are generated due to the shear layer instability. Clarification of the flow is important for both flood control and river environment. URANS computations have more computational efficiency compared with LES or DNS. The applicability of 3D CFD model based on a URANS non-linear k-epsilon model was examined through the comparison with the experimental results by Sanjo et al, in which detailed steady and unsteady flow structures have been illustrated clearly through PIV measurement. The present URANS computations could reproduce the secondary flow patterns excellently. The horizontal vortex shedding observed in the experiment could also be captured though shape of the row of vortices was not replicated well.

Turbulence structure of compound open-channel flows with one-line emergent vegetation

Vegetation such as trees and shrubs are often observed at the floodplain edge in natural rivers. Spanwise profiles of streamwise velocity component are influenced significantly by the drag force of trees. That is to say, the streamwise velocity decreases locally behind trees and near the junction between the main-channel and the floodplain. This property is quite different from that observed in typical compound open-channel flows, in which the velocity profile has a single inflection point and forms a mixing layer related to large-scale horizontal vortices. In contrast, in compound open-channel flow with a one-line emergent vegetation, a “V”-shaped velocity profile appears with twin inflection points. It is thus very important to investigate these hydrodynamic properties and turbulence structure considering the emergent vegetation effects in river engineering and eco hydraulics. So, in the present study, turbulence measurements by 3-D acoustic Doppler anemometer (ADV) were conducted in 150cm wide laboratory flume, in which 2cm diameter and 25cm height cylinders are placed as vegetation models with 10cm span along the junction edge.

Large eddy simulation of compound open-channel flows with emergent vegetation near the floodplain edge

Trees and shrubs observed in floodplain edge in natural rivers have significant impacts on hydrodynamic characteristics. In particular, spanwise profile of streamwise velocity component is influenced by the drag force of the trees, and consequently, the streamwise velocity decreases locally behind trees and near the junction between the main-channel and the floodplain. Furthermore, concentration exchanges such as nutrients between the main-channel and the floodplain differ from those observed in the non-tree compound open-channel flows. It is of essential importance to reveal how emergent trees influences these mass transfers. However, it is very difficult to evaluate time and space variations of the concentration by laboratory measurements. So, in the present study, a 3-D LES is used to overcome this difficulty, and we reveal the relation between the tree-alignment density and the exchange-rate of the concentration.

Hydraulic calculation of steady uniform flows in trapezoidal compound open channels

Hydraulic calculation of steady uniform flows in trapezoidal compound open channels is studied. Based on the force balance of water in each sub-section, the average velocities of the main channel, side slope, and floodplain are derived. The lateral momentum exchanges between the sub-sections are expressed by using the apparent shear stress. To verify the model, seven groups of UK Flood Channel Facility (UK-FCF) measured data with a relative water depth between the floodplain and the main channel varying from 0.057 to 0.4 are used for comparison. The result shows that the calculated velocity is larger than the measured data when the relative water depth is small, while it is less than or close to the measured value in the case of a larger relative water depth. The influence of the apparent shear stress on the calculation of velocity on the floodplain is not obvious, while it is much greater on the main channel. The three-stage model is compared with Liu’s two-stage model, showing that the former can give a better prediction for a three-stage trapezoidal compound channel. Finally, the apparent shear stress is calculated and compared with the measured data. The result shows that the chosen values of the momentum transfer coefficients are appropriate.

복단면 개수로흐름에서 홍수터 식생의 흐름저항을 반영한 1차원 모형

본 연구에서는 식생이 포설된 홍수터를 포함하는 복단면 개수로 흐름의 수위를 예측하기 위하여 유효 전단응력기법에 근거한 일차원 모형을 제시하였다. 제안된 모형은 주수로와 홍수터 접합부에서 발생하는 운동량 교환효과를 와점성계수 개념을 이용하여 반영할 수 있다. 주수로와 홍수터의 경계에서 발생하는 경계와점성계수는 3차원 레이놀즈 응력 모형을 이용하여 결정하였다. 경계와점성계수의 영향을 파악하기 위하여 민감도분석을 수행하였다. 경계와점성계수의 변화에 대하여 배수곡선의 변화는 크지 않았으나, 홍수터에서 부담하는 유량은 경계와점성계수에 비례하여 증가하였다. 마지막으로 식생된 홍수터의 식생밀도 및 침수비에 따른 경계와점성계수의 변화를 살펴보았다. 계산된 경계와점성계수는 식생밀도와 침수비에 비례하며, 대략<TEX>$(2-5){\times}\;10^{-4}$</TEX> 정도의 값을 갖는 것으로 나타났다. In this study, the 1D apparent shear stress model for vegetated compound open-channel flows was suggested. To consider the effect of momentum exchange between main channel and floodplain, the eddy viscosity concept was used in the present model. The interfacial eddy viscosity in the interface of main channel and floodplain was determined from the 3D Reynolds stress model. The evaluated interfacial eddy viscosity appears to be good agreement with those proposed previously. To investigate the effect of interfacial eddy viscosity, sensitive analysis was carried out. the computed backwater profiles are nearly identical with respect to the value of the interfacial eddy viscosity. However, the discharge conveyed by the floodplain changes is proportional to the interfacial eddy viscosity. Finally, the changes of the interfacial eddy viscosity due to the vegetation density and vegetation height were examined. The computed results of interfacial eddy viscosity are in proportion to the vegetation density and vegetation height, and the interfacial eddy viscosity has a range of <TEX>$(2-5)\;{\times}\;10^{-4}$</TEX>.

Apparent friction coefficient in straight compound channels

In compound open channel flow, the strong interaction between the main channel and the shallow floodplains affects considerably the discharge capacity. Since this phenomenon was identified, many authors have estimated experimentally the flow interaction in terms of an apparent shear stress acting at the vertical interface between the main channel and the floodplains. Empirical formulae have been developed to quantify this apparent shear stress, yet without general applicability. Herein, a dimensionally sound expression, depending on the square of the velocity gradient between the main channel and the floodplains, and on the so called “apparent friction coefficient”, is proposed. Its variation with the geometrical and roughness ratios is analysed herein. A generalized formulation to predict the apparent shear stress is presented and validated for a wide range of laboratory data. These include small-scale flumes and the large-scale flood channel facility, with both smooth and rough floodplains.

Scalar flux modeling of solute transport in open channel flows: Numerical tests and effects of secondary currents

Numerical experiments involving various algebraic scalar flux models for solute transport in open channel flows are presented. Five algebraic scalar flux models including these of Daly and Harlow, Abe and Suga, Suga and Abe, Sommer and So, and Wikstrom et al. are tested. For the flow computation, a Reynolds stress model is used. The models are applied to laboratory experiments of solute transport in rectangular and compound open channel flows. The performance of each model is evaluated both qualitatively and quantitatively. It is found that Daly and Harlow's model, although simple, predicts the solute transport most accurately. Further, with reference to the simulation results, the roles of the Reynolds fluxes and secondary currents in the solute transport equation are investigated. It is found that the Reynolds fluxes and secondary currents reduce and move the peak concentration, respectively.

Turbulence structure and coherent motion in meandering compound open-channel flows

Meandering compound open-channel flow is of high interest in river engineering and flood disaster. In particular, cross-sectional secondary currents develop significantly associated with an interaction between meandering inbank main-channel flow and straight overbank floodplain flow. These currents promote mass and momentum exchanges between them, as well as sediment transport and bed erosion. Further, organized motions may be generated in the same manner as typical horizontal vortices in straight compound channels. However, there is limited detailed information on the generation process of organized motion and secondary currents. This study focuses on the 3D structure of horizontal coherent motion and its advection property in the channel-transition zone from the straight to meandering compound open-channel flows by using a multi-layer scanning PIV, and compares them with those of continuous meandering channel flows.A strong relation between the horizontal vortices and secondary currents was obtained, resulting in a phenomenological flow model.

高水敷水漂の大きい複断面開水路流れの組織構造に関する研究

高水敷水漂の大きい複断面開水路流れの組織構造に関する研究

Investigation of Twin Vortices near the Interface in Turbulent Compound Open-Channel Flows Using DNS Data

The direct numerical simulation of turbulent flows in a compound open channel is described. Mean flows and turbulence structures are provided, and are compared with numerical and measured data available in the literature. The simulated results show that twin vortices are generated near the interface of the main channel and the floodplain and that their maximum magnitude is about 5% of the bulk streamwise velocity. Near the interface, the simulated wall shear stress reaches a maximum, contrary to experimental data. A quadrant analysis shows that both sweeps and ejections become the main contributor to the production of Reynolds shear stresses near the interface. Through the conditional quadrant analysis, it is demonstrated how the directional tendency of dominant coherent structures determines the production of Reynolds shear stress and the pattern of twin vortices near the interface. In addition, the time-dependent characteristics of three-dimensional vortical structures in a compound open-channel flow were investigated using direct numerical simulation (DNS) data.