Structures In Open-channel Flows Research Articles

Turbulence structure of open channel flows over permeable and impermeable beds: A comparative study

The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels.

Size and effect on the mean flow of large-scale horizontal coherent structures in open-channel flows: an experimental studyThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

The size of the largest horizontal coherent structures (HCSs) of turbulence in open-channel flows is investigated experimentally on the basis of three series of flow velocity measurements. These are further used to explore the dynamics and morphological consequences of HCSs. The flow velocity measurements were carried out in a 21 m long and 1 m wide channel, with a bed formed by sand with average grain size of 2 mm. The bed surface was flat. The turbulent and subcritical flow under investigation was uniform, with a flow depth of 4 cm. The bed slope of 0.0015 was such that, for the present flow depth, the bed shear stress acting on the bed was substantially below the threshold for initiation of motion, thus ensuring that the bed remained flat throughout the measurements. To the knowledge of the writers, this work is a first attempt to systematically investigate HCSs in open-channel flows. It should be viewed as an extension to the case of horizontal structures of work previously carried out by a number of authors on large-scale organized turbulence motion in open-channel flows, so far focusing exclusively on vertical coherent structures (VCSs). The horizontal burst length was found to be between five and seven times the flow width. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. Both of these findings lend support to the longstanding belief expressed by many prominent researchers that the formation of large-scale river forms is directly related to the large-scale turbulence. In particular, the present measurements for the first time provide some direct evidence in support of hypotheses previously raised by Yalin and da Silva regarding the formation of alternate bars and meanders through the action of HCSs on the mean flow and the mobile bed and banks.

FLOW RESISTANCE AND TURBULENT STRUCTURE IN OPEN CHANNEL FLOW WITH SUBMERGED VEGETATION

Vegetation colonies are often observed to grow in stripe pattern or patches within many stream channel. In this paper, the influence that interaction of longitudinal vegetation zones gave to the flow resistance in an open-channel was examined experimentally. Laboratory experiments were conducted using a two-component electromagnetic current-meter to clarify the momentum transport between main flow and flexible artificial vegetation. Results showed that the flow resistance significantly depended on the lateral distance between longitudinal vegetation zones and the reason could be explained by the difference of convective momentum transport through the interaction between longitudinal vegetation zones.

"MULTIRESOLUTIONAL-PROPER ORTHOGONAL" HYBRID ANALYSIS ON TURBULENT STRUCTURES IN OPEN-CHANNEL COMPLEX GEOMETRY FLOWS

We propose a hybrid expansion that consists of a multiresolution approximation (MRA) and a proper orthogonal decomposition (POD) to analyze turbulent flow structures in open-channel flows with complex geometries. The open-channel cavity flows are examined here and their velocity vectors are measured by using particle image velocimetry (PIV). In the first step of the hybrid expansion, the velocity time-series are classified into three distinct components using the MRA, i.e. pseudo-mean velocity, organized turbulence in a mixing shear layer, and incoherent turbulence. In the next step, these velocity components are decomposed by applying the POD. The principal components of the pseudo-mean velocity and the organized turbulence disclose the predominant flow characteristics, such as spatial structures of recirculating flows in the cavity and organized turbulent motions along the mixing shear layer, hysteretic behavior in modal time-series, and so forth. The results strongly support that the present hybrid analysis is effective for detecting spatiotemporal hierarchical structures in turbulent flows with complex geometries.

A103 植生キャノピー開水路流れの乱流構造の可視化計測

It is important in river environment and hydraulic engineering to reveal the hydrodynamic properties in vegetated open-channel flows, in which velocity profiles are largely changed in the vertical and spanwise directions. In particular, vegetated canopies generate coherent turbulent motions (such as sweeps and ejections) which promote erosions and depositions of sediment on beds.Therefore, it is needed to investigate these turbulence mechanism and coherent vortices in order to maintain the vegetations in actual rivers. In this study, we conducted turbulence measurements by using PIV technique, and revealed the turbulence structure in open-channel flows with vegetated canopies.

SPATIAL CORRELATION STRUCTURES OF WATER SURFACE FLUCTUATION AND TURBULENCE IN OPEN-CHANNEL FLOWS

In this paper, we investigate relations between water surface fluctuations and turbulence structures in openchannel flows. Examined here are instantaneous velocity vectors and corresponding water surface profiles measured by a simultaneous image measurement technique. At first, the proper orthogonal decomposition (POD) of the water surface fluctuations successfully extracts their predominant spatial characteristics that are represented as sinusoidal wave shapes. Then, by using the modal amplitudes of the POD eigenvectors, we calculate spatial correlation coefficients between the amplitudes and the velocity fluctuations. The result shows that there exists a thin surface layer near the water surface in which velocity fluctuations seem to be parallel to the surface and that there are strong relationships between the water surface fluctuations and the large scale turbulent structures that have the same longitudinal scale.

Discussion of “Mean Flow and Turbulence Structure of Open-Channel Flow Through Nonemergent Vegetation” by Fabián Lopez and Marcelo H. Garcia

Discussion of “Mean Flow and Turbulence Structure of Open-Channel Flow Through Nonemergent Vegetation” by Fabián Lopez and Marcelo H. Garcia

STEREO PIV MEASUREMENT OF NEAR-WALL COHERENT STRUCTURES IN OPEN-CHANNEL FLOW

Coherent structures near the wall (streamwise vortices, low-speed streaks and coherent Reynolds stress producing events) in an open channel flow at Reτ=300 are investigated using stereo particle image velocimetry (SPIV) in a cross-stream plane. A time series of 3 velocity components in the plane is converted into an instantaneous full 3 dimensional velocity field by Taylor's hypothesis. It is observed that a pair of counter-rotating quasi-streamwise vortices cause coherent Reynolds stress contributions beside the vortices by their swirling motions.A kinked lowspeed streak is also observed below the vortex pair. These structures are consistent with the conceptual model proposed by Jeong et al.(J. Fluid Mech. vol.332 p185 (1997)), and is the first quantitative experimental verification of the complete combination of configuration among these structures to our knowledge.

三角形断面桟型粗度を有する開水路流れの乱流構造

三角形断面桟型粗度を有する開水路流れの乱流構造

EFFECTS OF ADVERSE PRESSURE GRADIENTS ON TURBULENT STRUCTURES IN OPEN CHANNEL FLOWS

Experimental studies were conducted on turbulent structures in open channel flows with adverse pressure gradients. A laser Doppler anemometer (LDA) was used to obtain velocity data, and we succeeded in acquiring velocity profiles in the viscous-sublayer. Accordingly, the friction velocity was precisely calculated with the ‘viscous-sublayer’ law. Although the friction velocity is important to analyze turbulence characteristics, most studies have not employed this method due to restrictinns of measurement techniqucs and anemometers. Using this accurate friction velocity, this study examined the opinions that other researches have presented about open channel flows with adverse pressure gradients. It was found that von Karman constant is universal as pressure-gradient parameter changes. On the other hand, the integral constant in the log-law becomes smaller than that of uniform flow. The results also showed that the log-wake law correctly represents velocity profiles in the equilibrium region.

Large flow structures in a turbulent open channel flow

The outer region flow structure in an open channel flow is studied by means of flow visualization and velocity measurements. The structure can be thought of as formed by large streamwise vortices scaling with the flow depth, creating upwelling and downwelling motions at the free surface. Associated with the downwelling motion induced by these vortices are regions with higher longitudinal velocity on the free surface. Similarly, “boils” and eddies with a vertical axis are detected in the zones corresponding to the upwelling. Three temporal components are used to analyze velocity measurements: a temporal mean value, a component associated with slow fluctuations, and a third component associated with fast fluctuations. It is shown that the contribution of fast fluctuations to the turbulent intensities is important near the wall, at z+ < 1000. Slow fluctuations are the primary contributors in the remainder of the boundary layer's outer region. It is proposed that the large streamwise vortices, a manifestation of the largest turbulence scales, are a major contributor to slow fluctuations.

EXPERIMENTAL STUDY ON THE STRUCTURE OF OPEN CHANNEL FLOW WITH IMPERMEABLE SPUR DIKES

It is important for river engineers to know the hydraulic roles of spur dikes. A series of laboratory tests for horizontal structure of open channel flow with spur dikes was performed by varying theinterval of spur dikes. The results indicate that the structure of flow is different according to the in The reattachment length is found to be a function of the interval of spur dikes. The momentum transport rate between the main flow region and spur dike region in the most upstream spur dike region takes a peak when the interval is 4. 0 times of the length of a spur dike. The flow near the tip of the subsequent spur dike plays an important role in the momentum transport rate in the first spur dike region.

Numerical Simulation on Turbulent Structures in Open-Channel Flows over Dunes

The κ-e turbulence model has been developed at present to predict reasonably turbulent structure over dunes in open-channel flows. In closed duct flows and boundary layers, the turbulent structure is reproducted reasonably by relevant models. However, Nezu and Nakagawa (1987) found that the standard κ-e turbulence model can not predict reasonably turbulent structure in open-channel flows, because the turbulence near the surface is more depressed than in the closed duct flows. Therefore, in the present study, the standard κ-e model with generalized coordinate in which the effect of surface is taken into account is used to predict turbulent structure in open-channel flows over dunes. Mean velocity, turbulence characteristics and the effect of surface in open-channel flows over dunes were discussed.

Effect of Bed-Load on Turbulent Structure in Open Channel Flows

Mean flow properties and turbulent characteristics of open channel flow with bed-load transport were experimentally studied in the 550×25.8×20cm flume with erodible gravel bed. The sand with the diameters of 0.59mm and 1.19mm were used and the rate of bed-load transport reach to 13g/cm·sec. The velocity were measured by ADV, and the characteristics of the turbulent intensity and the spectra of turbulent velocity were analyzed. The experimental results show that: 1) the mean velocity profile of open channel flow with bed-load can be expressed by a log-law; 2) the turbulent intensity near bed decreases with the increase of bed-load transport rate; 3) the spectrum of turbulent velocity also changes as the increase in bed-load transport.

Turbulent structure of open channel flow on smooth bed with suspended sediment

Itakura and Kishi's theory about open channel flows with suspended sediment predicts decreasing of the friction factors for rough beds. While, for smooth beds, Imamoto et. al. observed increasing of them. However, from the fact that the important property of the flow, the increase of velocity gradient observed also for the smooth bed, decreasing of the integrated constant A of log low velocity profile was deduced. In this paper we proposed a model to explain the phenomena due to a modification of the coherent structure near the all by the suspended sediment

The kinematic structure of open-channel flows

1. Flow turbulence. Turbulent eddy formations exert a substantial influence on the kinematic structure of openchannel flows. 2. Analysis of the field-based and experimental data that have been discussed here, which are the result of studies performed by numerous researchers as well as the present authors, persuasively demonstrate the presence of a formation of transverse eddies both in the central part of the flow as well as near the lateral walls. The formation is responsible for the distribution of the longitudinal flow rates along the length of a uniform turbulent flow. 3. It may be assumed that an eddy will arise in the zone located between the free surface and the lateral wall which is directed opposite to the eddy of the free surface (Figs. 1, 2, and 3). In the central part of the channel this will lead to twisting of the isotachs in the direction of the free surface, while in the area of the lateral walls, twisting in the direction of the slopes. This patterns occurs both in simple as well as in compound channels (Figs. 4 and 7). 4. There is a need for exhaustive investigations of the characteristics of the turbulent regime, and the conditions under which transverse turbulent eddy formations arise and affect the development of the longitudinal flow rates and their distribution across the width of the flow. Such investigations would be helpful in studying the kinematic structure of flow when attempting to estimate the conveyance capacity of currents, the stability of channel beds, and load transport. 4. Currently known dependences for describing the distribution of the averaged local flow rates do not adequately reflect the true picture in the most critical portion of a current — i.e., in the sloping portion. It is this portion which determines the conditions of stability of deformed channels of rivers and canals.

Turbulence in Open‐Channel Flows

A review of open channel turbulence, focusing especially on certain features stemming from the presence of the free surface and the bed of a river. Part one presents the statistical theory of turbulence; Part two addresses the coherent structures in open-channel flows and boundary layers.

An automatic velocity measurements system operated by P. C. Velocity measurements for open channel flows

A P. C.-controlled automatic measuring system is developed for quantifying three-dimensional structure of open channel flows. Manual-controlled measuring systems require a great deal of time and labor as well as cause error in measurements. The proposed system allows us to obtain the velocity field of the considering flow with little manpower and high accuracy. In addition, an error inherent to spacing of two sensors, which usually occurs in measurements of velocity components by X-type Hot-film anemometer, is rectified on the basis of logarithmic velocity profile.

Numerical Analysis of Three-Dimenional Turbulent Structure in Open Channel Flow.

Numerical simulation has been carried out for the fully developed turbulent How of an open channel by using a Reynolds stress model. In calculation, since the turbulent structure including free surface is different from the duct flow, it is necessary to modify the model constants and boundary condition. Calculated results are compared with the experimental data available. The most important feature is that the maximum velocity appears not at the free surface, but rather just below it, in the open channel How. This phenomenon is called the 'velocity-dip', and it is peculiar to open-channel flows. The present method can predict this phenomenon well. The secondary flow pattern of the open channel is quite different from that of a closed channel. The essential reason for this difference is the presence of a free surface, instead of a plane symmetry. This secondary flow pattern is predicted in good agreement with the experimental data by applying the modified Reynolds stress model.

Effect of Relative Spacing of Strip Roughness on Turbulent Structure in Open Channel Flows

Effect of Relative Spacing of Strip Roughness on Turbulent Structure in Open Channel Flows