Structures In Open-channel Flows Research Articles

Meandering of instantaneous large-scale structures in open-channel flow over longitudinal ridges

The results of large-eddy simulations of open-channel flows over spanwise heterogeneous surface ridges at two representative spanwise spacings are presented. Flows at moderate Froude and Reynolds numbers over smooth channel beds with streamwise-orientated rectangular ridges are considered. The ridge spacing has a profound effect on the flow: at small spacing relatively small secondary cells occur, whilst at large ridge spacing secondary cells occupy the entire flow depth. The instantaneous flow features secondary flow instabilities and the meandering of alternating low- and high-momentum regions. The quasi-periodical nature of the meandering of the instantaneous large-scale motion is visualised and quantified for both ridge spacings. Although time-averaged clockwise and counter-clockwise secondary current cells are symmetrical about the ridge-axis, they exhibit quasi-periodical increase and decrease in size as well as lateral and vertical movement in space over the meandering period.

Experimental study on velocity distributions, secondary currents, and coherent structures in open channel flow with submerged riparian vegetation

Riparian vegetation, which is commonly found in natural rivers and open channels, has a strong influence on flow structures. This paper describes a laboratory experiment on velocity distributions, secondary currents, and coherent structures in narrow open-channel flow under the influence of submerged riparian vegetation. The channel is divided into a near-bank vegetated zone (VZ) and a main channel (MC) between which horizontal shear-induced coherent structures take place. A Prandtl mixing length model is applied to estimate the lateral distributions of horizontal Reynolds stress. Based on the experimental data, patterns of secondary currents induced by riparian vegetation in a narrow open channel are reported for the first time, which are found to be the cause of an S-shaped vertical profile of longitudinal velocity at the VZ-MC interface. This S-shaped velocity profile further results in two vertical mixing layers where vertical shear-induced coherent structures develop leading to correlations between longitudinal and vertical velocity fluctuations. Secondary transversal velocity also influences the characteristics of horizontal coherent structures close to the VZ-MC interface, that different inclination directions (clockwise or anti-clockwise) of these coherent structures are observed at different depths corresponding to the local transversal velocity.

Structure of open-channel flows through an array of square cylinders

ABSTRACT The impact of obstacles on the hydrodynamic flow structure is of major interest since such flows are representative of flooded urban areas. This study focuses on a pattern depicting a single-family home and garden area, as found in recent suburban development. The purpose herein is to analyze the evolution of the velocity distribution for a densely built area as well as for emerged to slightly submerged situations, which represent various flood situations ranging from commonplace to extreme. Free-surface flows through an in-line distribution of square cylinders were investigated in a laboratory flume, in the aim of assessing the effect of relative submergence on the flow structure within the cylinder array. The results show that the lines of square cylinders channelize the flow. Moreover, the submerged case generated similar flow characteristics below the top of the cylinders, which drastically changed once the water flowed above the square cylinder level.

A semi-coupled model for determining the distribution of two-dimensional coherent structures in open channel flow

A semi-coupled model for determining the distribution of two-dimensional coherent structures in open channel flow

Experimental and numerical analysis of weir structures in open channel flows

Weirs are important water structures used to hold water in dams and canals and to safely transfer water from upstream to downstream. Due to these features, these structures, which have been used for years, have many different designs. In this study, a rectangular labyrinth weir and an inclined-bed rectangular weir were studied experimentally and numerically. The weir geometries used in the experiment were produced in a 3D printer and transferred to the numerical simulation with the same dimensions. The discharge and discharge coefficients corresponding to five different upstream heights obtained from the experiment and numerical simulation were obtained and the data were compared.

Turbulence structure and longitudinal velocity distribution of open channel flows with reedy emergent vegetation

AbstractThe vertical distribution of longitudinal velocity and the turbulence structure of open channel flows with natural‐like reedy emergent vegetation were investigated in this study. The natural‐like reedy vegetation consisted of a basal stem and flexible blades with shape and reconfiguration behaviour comparable with those found in actual riparian areas. The 3D velocity field was measured using an acoustic Doppler velocimeter. The vertically non‐uniform distribution of the frontal area could dramatically affect the exchange of mass and momentum between emergent vegetation and their surroundings. The average time velocity profile varied inversely with the frontal area. The high velocity gradient appearing at the middle section of the zone with frontal area varied considerably. The structures of vortices and vertical transport were analysed using spectral analysis and conditional sampling to identify the prominent vortical structures and their contribution to momentum transport. Spectral analysis suggested that the stem‐scale vortices set by stem spacing where the vertical frontal area had no clear change were dominant, and shear‐layer vortices where vertical frontal area dramatically changed cannot be disregarded. Quadrant analysis showed that inward and outward interactions were the dominant contributors to Reynolds stress, excluding the zones approaching the channel boundary and the vertical frontal area that dramatically changed. A four‐layer model was developed to predict the vertical distribution of longitudinal velocity with a newly established profile of local drag coefficient. The predicting results agreed well with the experimental data.

Incorporating the Memory Effect of Turbulence Structures Into Suspended Sediment Transport Modeling

AbstractModeling of the random movement of fine sediment particles in open‐channel turbulent flow is mostly built upon the memoryless Brownian motion process. Such a process describes the chaotic behavior of small particles without considering temporal correlations in terms of the particle moving velocity and direction (i.e., memory). However, when particles are transported in time‐persistent turbulent flow, the movements of the suspended particles may exhibit persistency that depends on the various temporal durations of turbulent flow structures—such persistence results in direction and magnitude variations in the velocity of the fine moving particles. The diffusion property of the particles may then deviate from normal diffusion. The conventional memoryless random walk models may not provide a comprehensive description of the particle diffusion process for the duration of turbulence structures when the particles are subject to memory effects. In this study, a novel random walk model is proposed to present the temporal correlation of the suspended sediment particle velocity caused by turbulence structures in open channel flow. The probabilistic properties of the proposed model are discussed. In particular, enhanced physical insights are obtained regarding the particle diffusion behavior in turbulent flows. Numerical simulations are conducted to demonstrate that, similar to the conventional memoryless random walk models, the proposed model shows normal diffusion for long‐term observations, despite its local superdiffusion behavior. The effective diffusion coefficient of the proposed stochastic process on a long‐term time scale is formulated.

Velocity distribution and turbulence structure of open channel flow with floating-leaved vegetation

Floating-leaved vegetation alters the flow structure through the combined action of floating leaves, stems, and the channel bed. This study investigated the vertical distribution of streamwise velocity and the turbulence structure of an open channel flow with floating-leaved vegetation through theoretical analysis and laboratory experiments. Bionic lotus leaves fixed on circular wooden cylinders were used to imitate floating-leaved vegetation, and the 3D velocity field was measured by using an acoustic Doppler velocimeter. The measured data showed that with the presence of the channel bed and vegetation cover, strong velocity gradients appear at the bottom and top layers, whereas the velocity is almost uniformly distributed in the internal layer. On the basis of the modified mixing length hypothesis, the vertical distribution of streamwise velocity was numerically predicted, and the prediction results agreed well with the experimental data. The model parameters were then determined, and the effect of mixing length was analyzed. Spectral analysis showed that the peak frequency in the streamwise velocity spectra is within the range of 0.1 Hz to 0.2 Hz and that the vortices are mainly affected by stem-scale turbulence. Quadrant analysis showed that the ejections and sweeps are the major contributors to Reynolds stress in the bottom layer, inward and outward interactions are prominent in the top layer, and the contribution of sweeps and ejections are approximately equivalent to those of outward and inward interactions in the internal layer. Different to open channel flow featured with emergent rigid vegetation, the velocity and wake production decrease while the Reynolds stress and shear production increase towards the floating leaf. The effect of stem diameter, vegetation density, stem flexibility and boundary roughness on flow structure of open channel flow with floating-leaved vegetation need to be examined in future research.

Mean flow and turbulence structure of open channel flow with suspended vegetation

Mean flow and turbulence structure of open channel flow with suspended vegetation

Wavelet Coherency Structure in Open Channel Flow

Many studies based on single-point measurement data have demonstrated the impressive ability of wavelet coherency analysis to catch the coherent structures in the wall-bounded flows; however, the question of how the events found by the wavelet coherency analysis relate to the features of the coherent structures remains open. Time series of velocity fields in x–y plane of the steady open channel flow was obtained from a time-resolved particle imaging velocimetry system. The local wavelet spectrum found shows that one of the main energetic scales in open channel flows is 3h motions. The wavelet coherent coefficients of u and v series from the same measurement points successfully detected the occurrence and the scale of these 3h motions, and the phase angle indicates their inside velocity structure is organized by the Q2 and Q4 events. The wavelet coherency analysis between different measurement points further reveals the incline feature of the 3h scale motions. All the features of this 3h motion found by the wavelet coherency analysis coincide well with the flow field that is created by the passing of hairpin packets.

Wavelet spectral analysis of the free surface of turbulent flows By Giulio Dolcetti and Héctor García Nava

In an interesting recent study on turbulent structures in open-channel flow and their surface signatures, Dolcetti and Garcia Nava (2018) (hereafter DGN18) present laboratory measurements of statio...

Turbulence structure in open channel flow with partially covered artificial emergent vegetation

An innovative approach was recently proposed to increase water depth and improve navigation in rivers by increasing overall roughness through an adaptive placement of artificial vegetation. In this approach, a section of the river width is covered with artificial vegetation constructed from an array of dampers that can be added or removed while maintaining sufficient width for navigational purposes. The goal of the work here is to explore the mean flow and turbulent momentum transport properties of such flow in a controlled flume setting, where the flow rate is steady, the vegetation placement is uniform, and the Froude number is maintained at sub-critical conditions. The focus is on vegetation-induced roughness alterations to the bulk flow statistics as well as the structure of turbulence impacted by the presence of such artificially engineered vegetation. The canopy flow experimentally explored here differs from the well-studied planar uniform emergent canopy flow because the interface between the vegetation and non-vegetation areas occurs along a lateral direction instead of the vertical direction. The flume experiments were conducted by partially covering the channel width with emergent artificial vegetation that is flexible and can interact with the flow. The quantities studied are the equilibration distance of the flow statistics from the leading vegetation edge as such distance is essential to any engineering design. Moreover, the mean flow and turbulence regimes at several longitudinal sections were analyzed using spectral analysis and conditional sampling to identify the main vortical structures and their contribution to momentum transport. Spectral analysis demonstrated that the sizes of the dominant vortices can be classified into stem-scale (small-scale) and shear-scale (larger scale) vortices as conventional in canonical canopy flows. The shear layer formed in the planar direction and its associated coherent vortices are spawned near the edge of the vegetated area in a manner analogous to a plane mixing layer. Quadrant analysis showed two contributions to the overall shear stress and lateral momentum exchange between the vegetated and non-vegetated zone, namely, ejections and sweeps. The measured ejection-sweep contributions are short-lived but significant to the overall turbulent momentum transport consistent with other canopy flow studies. Approximately 80% of the turbulent stress contributions at the interface occur within 30% of the time. Because the flume experiments were conducted at sufficiently high Reynolds number and sufficiently low Froude number, the scalability of the experimental findings here to much larger settings encountered in engineering practice are briefly discussed.

The visualization of turbulent coherent structure in open channel flow

Due to its multiscale and multi-layer natures, the coherent structures of turbulent in the open channel flow is complex and difficult to be visualized for understanding its evolution. In this paper, five types of methods for the vortical structure in the fluids, namely the Q - criterion, the vorticity, the Omega method, the velocity-vorticity correlation structures (VVCS) as well as the most recent Rortex method, are adopted to visualize the turbulent flow in the open channel. With the free surface modelled as a free slip boundary, a direct numerical simulation (DNS) is carried out to study the multi-layered flow structure characteristics under the free surface. The visualization results by the Q - criterion, the vorticity, the Omega method and the Rortex are firstly analyzed. Then the turbulent flow layers near the free surface are identified with corresponding anisotropy indices. Afterwards, the VVCS within various turbulence layers are visualized accordingly. This research indicates that the VVCS can straightforwardly show the geometry information of the coherent structures of turbulent in different layers for the open channel flow.

Flume experiments on baffle-posts for retarding open channel flow

The Discussers congratulate the Authors for their findings from detailed experiments regarding the flow-retarding behaviour of baffle-post structures in open channel flow. The experimental data of ...

Quantitative Characterization of Streaky Structures in Open-Channel Flows

Streaky structures have been reported in open-channel flows by various investigations in the inner and outer layer. Although the ubiquity of streaks is well recognized, it remains unclear how the streaky structures in the inner and outer layers are related to each other. This paper employs a unified algorithm to identify streaky structures which can be applied in both inner and outer layers with exactly the same subjective threshold. The characteristic scales of streaky structures are quantified, including the width and the spanwise spacing. The results reveal that the spatial scales increase from the near-wall region to the outer layer. The continuous linear growth trend indicates a close relationship between the streaky structures in the inner and outer layers, which can be plausibly explained with the attached-eddy hypothesis.

Statistical analysis of turbulent super‐streamwise vortices based on observations of streaky structures near the free surface in the smooth open channel flow

AbstractLong streamwise‐elongated high‐ and low‐speed streaks are repeatedly observed near the free surface in open channel flows in natural rivers and lab experiments. Super‐streamwise vortex model has been proposed to explain this widespread phenomenon for quite some time. However, statistical evidence of the existence of the super‐streamwise vortices as one type of coherent structures is still insufficient. Correlation and proper orthogonal decomposition (POD) analysis based on PIV experimental data in the streamwise‐spanwise plane near the free surface in a smooth open channel flow are employed to investigate this topic. Correlation analysis revealed that the streaky structures appear frequently near the free surface and their occurrence probability at any spanwise position is equal. The spanwise velocity fluctuation usually flows from low‐speed streaks toward high‐speed streaks. The average spanwise width and spacing between neighboring low (or high) speed streaks are approximately h and 2h respectively. POD analysis reveals that there are streaks with different spanwise width in the instantaneous flow fields. Typical streamwise rotational movement can be sketched out directly based on the results from statistical analyses. Point‐by‐point analysis indicates that this pattern is consistent everywhere in the measurement window and is without any inhomogeneity in the spanwise direction, which reveals the essential difference between coherent structures and secondary flow cells. The pattern found by statistical analysis is consistent with the notion that the super‐streamwise vortices exist universally as one type of coherent structure in open channel flows.

Turbulence characteristics in free-surface flow over two-dimensional dunes

The turbulent structure of open-channel flows over two-dimensional dunes is investigated numerically using large-eddy simulation (LES), in order to improve our understanding of the interaction between the dune-generated turbulence and the free surface dynamics. The filtered Navier–Stokes equations in the LES model have been discretised using the finite volume method, with a dynamic sub-grid model being employed for the unresolved scales of turbulence. The partial cell treatment has been implemented in a Cartesian grid form to deal with the dune topography. Both the volume of fluid method and rigid lid approach have been employed in the numerical framework to investigate the effects of the free surface treatment on the flow characteristics. The numerical model predicted mean flow velocities, turbulence intensities and Reynolds stresses have been compared with experimental measurements published in the literature, with a detailed analysis being undertaken to assess the accuracy of the model results and the effects of the free surface treatment on the velocity and turbulence predictions. The instantaneous flow structure has been investigated, with emphasis being focused on the free surface dynamics and coherent structures.

Vertical mass and momentum transport in open-channel flows with submerged vegetations

The importance of flow and turbulence to the ecology of aquatic benthic organism has been widely reported. Aquatic vegetation is one of environmental variables that influence turbulence and the ecological condition of rivers. Aquatic canopies have the potential to greatly improve water quality through the removal of nutrients and trace metals. To describe the efficiency of this removal, we must be able to quantify the rate of vertical mixing. In fully submerged vegetation flows, coherent turbulent motions are generated near the vegetation edge and these large-scale eddies control the vertical exchange of mass and momentum. It is therefore important for river management to investigate hydrodynamic characteristics and coherent eddies in open-channel flows with vegetation canopies. Turbulence structure and transport mechanism of momentum in vegetated flows have been investigated intensively in the past decade.However, the effect of the submerged vegetation on the vertical mass transport and turbulent diffusion has not been fully investigated. Therefore, in the present study, continuous dye injection experiments were conducted to evaluate the mass transport structure in open-channel flow with rigid vegetation models by changing the vegetation density. A combination technique between PIV and planar laser-induced fluorescence (LIF) was developed by using two sets of CCD cameras, to measure the instantaneous velocity and concentration field simultaneously. The technique is capable of determining the turbulent scalar flux as well as the Reynolds stress, mean and fluctuating velocity and concentration fields. Consequently, the effects of coherent vortices on the vertical turbulent diffusivity were examined in detail.

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.

The role of different forms of natural riparian vegetation on turbulence and kinetic energy characteristics

Flow measurements were conducted by ADV Vectrino to investigate the role of different forms of single natural emergent vegetation elements on kinetic energy characteristics and the flow structure in open channel flow. Experiments were carried out for both forms of “erect” and “compressed” vegetation. The findings revealed that the form of “erect” has greater retaining influence on time-averaged mean velocity characteristics than “compressed” form. Furthermore, the turbulence and kinetic energy patterns behind the single vegetation were also examined. An empirical equation giving the relationship between the kinetic energy components of flow was derived based on the data. The ratio between turbulence kinetic energy and mean kinetic energy at a certain distance downstream of vegetation element was predicted by the empirical equation depending on the vegetation characteristics. These predictions were compared with a different experimental data set and produced satisfactory results. Also, the mean flow and turbulence characteristics behind an isolated natural plant and a volumetrically equivalent rigid cylinder were also compared.