Experimental study on influence of boundary on location of maximum velocity in open channel flows

The velocity dip phenomenon may occur in a part of or in the whole flow field of open channel flows due to the secondary flow effect. Based on rectangular flume experiments and the laser Doppler velocimetry, the influence of the distance to the sidewall and the aspect ratio on the velocity dip is investigated. Through application of statistical methods to the experimental results, it is proposed that the flow field may be divided into two regions, the relatively strong sidewall region and the relatively weak sidewall region. In the former region, the distance to the sidewall greatly affects the location of maximum velocity, and, in the latter region, both the distance to the sidewall and the aspect ratio influence the location of the maximum velocity.

Vertical rotation effect on turbulence characteristics in an open channel flow

This paper solves the three-dimensional Navier-Stokes equation by a fractional-step method with the Reynolds number Reτ=194 and the rotation number Nτ=0-0.12. When Nτ is less than 0.06, the turbulence statistics relevant to the spanwise velocity fluctuation are enhanced, but other statistics are suppressed. When Nτ is larger than 0.06, all the turbulence statistics decrease significantly. Reynolds stress budgets elucidate that turbulence kinetic energy in the vertical direction is transferred into the streamwise and spanwise directions. The flow structures exhibit that the bursting processes near the bottom wall are ejected toward the free surface. Evident change of near-surface streak structures of the velocity fluctuations are revealed.

Utility Impact below Bridge or Culvert Soffit on Open Channel Flow

The background to this research was a flooding incident that occurred in Bridgend, Co. Donegal, Ireland in August 2017. While several properties were flooded, a flooding case study of a single dwelling house adjacent to the Bridgend River at Riverdale, Bunamayne, Co. Donegal, Ireland is used herein. For this study the flooded site shall be referred to as the “Hegarty property”. A structure in the form of a stone arched culvert is located directly adjacent to the two-storey detached dwelling house on the Hegarty Property. While the culvert is referred to locally as a bridge, within this research the word culvert will be used in connection with the structure. The culvert has a concrete surrounded utility (watermain) crossing at a gradient below the culvert soffit on the upstream face of the structure. The utility obstructed flow through the culvert and contributed to the flooding event. Given the implication of climate change and the increased probability of more extreme flooding events, it was decided to explore the case study to ascertain the factors that contribute to flooding events when utilities are positioned at culvert or bridge structures. This work was completed to assist undergraduate students, researchers, and local authorities in a relatively unknown area of flood causation.

Effect of Mixed Vegetation of Different Heights on Open Channel Flows

Vegetation of different heights commonly grows in natural rivers, canals and wetlands and affects the biodiversity and morphological process. The role of vegetation has drawn great attention in river ecosystems and environmental management. Due to the complexity of the vegetated flow, most previous research focuses on the effect of uniformed one-layered vegetation on the flow structure and morphological process. However, less attention was paid to the impact of the mixing vegetation of different heights, which is more realistic and often occurs in natural riverine environments. This paper aims to investigate the effect of mixing three-layered vegetation on flow characteristics, particularly the velocity distrbution, via a novel experiment. Experiments were performed in a titling water flume fully covered with vegetation of three heights (10, 15 and 20 cm) arranged in a staggered pattern, which is partially submerged. Velocities at different positions along a half cross-section were measured using a mini propeller velocimeter. Observed results showed that the velocity has a distinct profile directly behind vegetation and behind the vegetation gap. The overall profile has two distinct reflections about ? below or near the top of short vegetation (h): the velocity remains almost constant in the bottom layer ( h) the velocities directly behind the middle after short vegetation increase much faster than those directly behind the short after tall vegetation. The finding in this study would help river riparian and ecosystem management. .

Numerical Simulation of the Flow of a Molten Blast Furnace Slag in an Open Channel

Despite its industrial importance, the flow of molten blast furnace slag in open channels has not been sufficiently studied. In this work, the unsteady non-uniform flow of a molten blast furnace slag in a rectangular open channel is numerically studied by solving the Saint-Venant equations by means of an explicit backwards finite difference scheme. An Arrhenius-type dependence of the viscosity of the slag on temperature is assumed. To calculate that viscosity, four temperatures are considered, namely 1450˚C, 1500˚C, 1550˚C and 1600˚C. To study the dynamic response of the system, a half-sinusoidal pulse with duration of 5 s is imposed at the channel entrance. According to the numerical simulations, for all the temperatures considered, the slag flow in the channel for an angle of 5 degrees is supercritical in nature. However, for an angle of 1 degree, the flow is transcritical, that is, it presents a transition from subcritical to supercritical.

Investigation of open channel flow with unsubmerged rigid vegetation by the lattice Boltzmann method

The aquatic vegetation can significantly affect the flow structure,the sediment transport,the bed scour and the water quality in rivers,lakes,reservoirs and open channels.In this study,the lattice Boltzmann method(LBM)is applied in the two-dimensional numerical simulation of the flow structure in a flume with rigid vegetation.A multi-relaxation time model is applied to improve the stability of the numerical scheme for flows with a high Reynolds number.The vegetation induced drag force is added in the lattice Boltzmann equation model in order to improve the simulation accuracy and an algorithm of the multi-relaxation time is developed.Numerical simulations are performed for a wide range of flow and vegetation conditions and are validated by comparing with the laboratory experiments.Analysis of the simulated and experimentally measured flow Helds shows that the numerical simulation can satisfactorily reproduce the laboratory experiments,indicating that the proposed lattice Boltzmann model enjoys a high accuracy for simulating the flow-vegetation interaction in open channels.

Predictions of bulk velocity for open channel flow through submerged vegetation

Vegetation is of great significance in river ecosystems in terms of hydrodynamics,water environment and ecology.The question of how to predict the bulk velocity in channel flow through submerged vegetation is currently a hot topic in hydraulics research.The present study addresses this question.The various formulae used for bulk velocity estimation in previous work were reviewed and compared.The main novelty of this paper is that a new expression of friction factor is proposed as a function of two dimensionless factors,and the number of tuning parameters is less than that in previous work.A comparison of measured and calculated data was conducted for flow through submerged rigid and flexible vegetation.The comparison showed that the proposed new model can make more accurate predictions than previous models.It is envisaged that the proposed formulation can be usefully employed in eco-hydraulics predictions.

United friction resistance in open channel flows

It is now over half a century since Keulegan conducted his open channel flow experiments. Over the past decades, many empirical formulae were proposed based on his results, however, there is still not a combined expression to describe the effects of friction over all hydraulic regions in open channel flows. Therefore, in this letter, based on the analysis of the implicit model and the logarithmic matching method, the results of Keulegan （for authentic experiment data are no longer available, here we adopt the analytical solutions given by Dou） are rescaled into one monotone curve by combining the Reynolds number and the relative roughness of the river bed. A united expression that could cover the entire turbulence regions and be validated with Dou＇s analytical solutions is suggested to estimate the friction factor throughout the turbulent region in open channel flows, with higher accuracy than that of the previous formulas.

TURBULENT OPEN CHANNEL FLOW SUBJECTED TO THE CONTROL OF A SPANWISE TRAVELING WAVE

Turbulent open channel flows subjected to the control of a spanwise traveling wave have been investigated by means of Direct Numerical Simulation （DNS）. The objective of this study is to reveal the response of the near-wall and surface-influenced turbulence to the spanwise traveling wave control. Three typical frequencies of the spanwise traveling wave, i.e., high-, middle- and low-frequency, corresponding to the exciting periods at 25, 50 and 100, are considered to study the turbulence dynamics in the wall and surface regions. To elucidate the behaviors of turbulence statistics, some typical quantities, including the mean velocity, velocity fluctuations and the structures of turbulence fluctuations, are exhibited and analyzed.

LARGE EDDY SIMULATION OF PULSATING TURBULENT OPEN CHANNEL FLOW

Pulsating turbulent open channel flow has been investigated by the use ofLarge Eddy Simulation (LES) tech-nique coupled with dynamic Sub-Grid-Scale (SGS) model for turbulentSGS stress to closure the governing equations. Three-dimensional filtered Navier-Stokes equationsare numerically solved by a fractional — step method. The objective of this study is to deal withthe behavior of the pulsating turbulent open channel flow and to examine the reliability of the LESapproach for predicting the pulsating turbulent flow. In this study, the Reynolds number (Re_τ) ischosen as 180 based on the friction velocity and the channel depth. The frequency of the drivingpressure gradient for the pulsating turbulent flow ranges low, medium and high value. Statisticalturbulence quantities as well as the flow structures are analyzed.

A numerical model for air concentration distribution in self-aerated open channel flows

The self-aeration in open channel flows, called white waters, is a phenomenon seen in spillways and steep chutes. The air distribution in the flow is always an important and fundamental issue. The present study develops a numerical model to predict the air concentration distribution in self-aerated open channel flows, by taking the air-water flow as consisting of a low flow region and an upper flow region. On the interface between the two regions, the air concentration is 0.5. In the low flow region where air concentration is lower than 0.5, air bubbles diffuse in the water flow by turbulent transport fluctuations, and in the upper region where air concentration is higher than 0.5, water droplets and free surface roughness diffuse in the air. The air concentration distributions obtained from the diffusion model are in good agreement with measured data both in the uniform equilibrium region and in the self-aerated developing region. It is demonstrated that the numerical model provides a reasonable description of the self-aeration region in open channel flows.

EVALUATION OF THE SHADING METHOD FOR REDUCING IMAGE BLOOMING IN THE PIV MEASUREMENT OF OPEN CHANNEL FLOWS

The shading method is a simple but effective way of reducing image blooming in the measurement of open channel flows with the Particle Image Velocimetry （PIV）.The current paper proposes a simplified analytical model for light attenuation using this method.The model is verified against experimental data,and the influence of several parameters is illustrated numerically.The possible adverse effect due to the light attenuation is shown to be limited when the parameters in the shading method are in an adequate range,as shown by processing standard images of Case B in PIV Challenge 03.A simple criterion for setting the shade in experiment is given for controlling the errors caused by the shading technique within an acceptable range.

THE TURBULENCE CHARACTERISITICS OF OPEN CHANNEL FLOW

The turbulence characteristics of open channel flow has been measured by means of the two-dimensional laser doppler anemometer and the turbulence mechanism has been discussed. According to the measured data, the expressions of various statistical parameters are obtained, which provide the reliable and verifiable data for the investigation of the turbulence structure and the suspension mechanism.

MECHANISM OF WALL PRESSURE FLUCTUATIONS BENEATH THE OPEN CHANNEL FLOW

Based on the measured results that wall pressure fluctuations are mainly de- cided by coherent structures of turbulence, the relationship between root-mean- square wall pressure and wall shear stress in turbulent shear flow and that between the intensities of pressure and fluctuating velocity in homogeneous and isotropic turbulence are established in this paper. These relationships are consistent with former works, and have good agreement with experimental data. The paper also dis- cusses the concept of 'apparent pressure' on the wall in mean flow.

Analysis of the spanwise vortex of open channel flows based on the Omega-Liutex vortex identification method

In this paper, a particle imaging velocimetry (PIV) system of high-temporal-spatial resolution is used to investigate the spanwise vortex distribution of fully developed turbulent flows in an open channel and its relationship with the turbulence. The distributions of the time-averaged velocity, the turbulence intensity and the Reynolds stress are obtained in the longitudinal profile. The third-generation vortex identification method (based on the Omega-Liutex vector) is applied to accurately identify and analyze the vortex in the spanwise direction. The results suggest that the vortex density increases with the Reynolds number at a given aspect ratio (B / H) of the flow. The distribution trend of the spanwise vortex density in the vertical direction remains unchanged for different discharges. Specifically, the vortex density increases along the vertical direction and reaches the peak at y / H = 0.15, then decreases and reaches the bottoms at the flow surface.

3-D hybrid LES-RANS model for simulation of open-channel T-diversion flows

The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage. To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows, a hybrid LES-RANS model, which combines the large eddy simulation （LES） model with the Reynolds-averaged Navier-Stokes （RANS） model, is proposed in the present study. The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel, as well as the downstream region of a branch channel. The LES model was used to simulate the channel diversion region, where turbulent flow characteristics are complicated. Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence. A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations. This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions. Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T-diversion flows.

Lateral Shear Layer and Its Velocity Distribution of Flow in Rectangular Open Channels

The lateral velocity distribution of flow in the shear layer of open channel is required to many problems in river and eco-environment engineering, e.g. distribution of pollutant dispersion, sediment transport and bank erosion, and aquatic habitat. It is not well understood about how the velocity varies laterally in the wall boundary layer. This paper gives an analytical solution of lateral velocity distribution in a rectangular open channel based on the depth-averaged momentum equation proposed by Shiono & Knight. The obtained lateral velocity distributions in the wall shear layer are related to the two hydraulic parameters of lateral eddy viscosity (λ) and depth-averaged secondary flow (Γ) for given roughened channels. Preliminary relationships between the above two parameters and the aspect ratio of channel, B/H, are obtained from two sets of experimental data. The lateral width (δ) of the shear layer was investigated and found to relate to the λ and the bed friction factor (f), as described by Equation (26). This study indicates that the lateral shear layer near the wall can be very wide (δ/H = 14.6) for the extreme case (λ = 0.6 and f = 0.01).

Uniform Flow of Molten Metals in Rectangular Open Channels

The flow of liquids in open channels has been studied since ancient Rome. However, the vast majority of published reports on flow in open channels are focused on the transport of drinking water and sewage disposal. The literature on the transport of molten metals in open channels is quite scarce. In this work, the uniform flow of pig iron and molten aluminum in rectangular open channels is studied. Specific energy curves are constructed and critical heights are analytically determined. The transition from subcritical to supercritical flow is analyzed as a function of the angle of inclination of the channel and the roughness of its walls. Manning’s equation is applied to the pig iron flow using data reported in the literature for molten aluminum. The need to correct the roughness coefficient for pig iron is observed in order to obtain results consistent with those previously reported.

HYDRAULIC CHARACTERISTICS OF OPEN CHANNEL FLOWS OVER ROUGH BEDS

Due to the very fine space resolution and high noise-reduction capability of the improved laboratory-made laser Doppler anemometer, the temporal mean velocity and velocity fluctuation in uniform, rough bed open channel turbulent flows were measured in full detail nd a large amount of information about the wall region was obtained. The influence of boundary roughness on flow characteristics was examined by changing the relative smoothness scale, H/k_s, of the flow. It is proposed that there are three sorts of bed roughness, namely, the large scale roughness, the medium one and the small one. The small scale roughness is of more universal significance to engineering practice, and much attention is devoted to the characteristics of the flow over boundaries of such a sort of roughness.

The properties of dilute debris flow and hyper-concentrated flow in different flow regimes in open channels

Particle Image Velocimetry(PIV) technique was used to test the analogues of hyperconcentrated flow and dilute debris flow in an open flume. Flow fields, velocity profiles and turbulent parameters were obtained under different conditions. Results show that the flow regime depends on coarse grain concentration. Slurry with high fine grain concentration but lacking of coarse grains behaves as a laminar flow. Dilute debris flows containing coarse grains are generally turbulent flows. Streamlines are parallel and velocity values are large in laminar flows. However, in turbulent flows the velocity diminishes in line with the intense mixing of liquid and eddies occurring. The velocity profiles of laminar flow accord with the parabolic distribution law. When the flow is in a transitional regime, velocity profiles deviate slightly from the parabolic law. Turbulent flow has an approximately uniform distribution of velocity and turbulent kinetic energy. The ratio of turbulent kinetic energy to the kinetic energy of time-averaged flow is the internal cause determining the flow regime: laminar flow(k/K<0.1); transitional flow(0.1< k/K<1); and turbulent flow(k/K>1). Turbulent kinetic energy firstly increases with increasing coarse grain concentration and then decreases owing to the suppression of turbulence by the high concentration of coarse grains. This variation is also influenced by coarse grain size and channel slope. The results contribute to the modeling of debris flow and hyperconcentrated flow.