Rectangular Broad-crested Weirs Research Articles

Discharge Coefficient of Rectangular Broad-Crested Weirs in Narrow Channels with High Relative Length of the Threshold

Discharge Coefficient of Rectangular Broad-Crested Weirs in Narrow Channels with High Relative Length of the Threshold

FLOW OVER BROAD-CRESTED WEIR WITH INFLOW BY APPROACH SHAFT – NUMERICAL MODEL


 
 
 In the case of flow over rectangular broad-crested weir, where the inflow is realized by approach shaft, occurs influence of water surface level by approach flow velocity. The paper describes numerical model of flow including weir, approach and outlet shaft. Simulations of flow were created by 2D and 3D model with using three methods of turbulent modelling. In this paper a water surface level for each model setup is evaluated and then it is compared with measured values.
 
 

FEM analysis of water surface profile using VOF Method

The prediction of water surface elevation in open channels is important for evaluation and determination of the heights of side wall of structures in open channel systems. At present empirical equations are basically used for the purpose. In the early half of the 19th century experimental approach was mainly used but it had the drawbacks of laborious data collection and instrument operation limitation. The 3D flow behavior or some complex turbulent structure which is the nature of any open channel flow, cannot be effectively captured through experiments, so in these circumstances, computational approach can be adopted to overcome these obstacles. In comparison to experimental studies computational approach is repeatable, can simulate at full scale, can generate the flow taking all the data points into consideration. The motive of the present work is to use the simple geometry of the rectangular broad crested weir to test the commercial CFD software ANSYS-CFX in order to test its feasibility for implementation in more complex open channel flows. The experimental results of Hager & Schwalt’s are validated and then the analysis has been done for weirs with different slopes.

CFD simulated model and experimental tests for critical depth and flowrate estimation over a broad-crested weir under the longitudinal slope effect

This work consists of an experimental approach using rectangular broad-crested weirs having varied edge geometries to investigate the longitudinal slope effect on the relationship between the critical depth and the depth at centre of the weir. Based on this relationship, a statistical regression analysis method was used to derive a new and precise formula to estimate the flowrate over square and rounded broad-crested weirs. In all cases, the depth at the centre of the weir was considered as a control section. In general, it was found that the longitudinal slope So has an insignificant effect on the relationship between the critical depth and the depth at the centre of the weir and the suggested new flowrate estimation formula. Also, computational fluid dynamic model using FLUENT ANSYS version 16 was also designed and operated to simulate the problem based on the VOF method and the standard k~e method. The CFD model provided a very good simulation of the problem for the new flowrate formula and the existing pressure head at the centre of the weir. Comparing the results of the new formula with that of the actual experimental tests and the results of the simulation model indicted a very good agreement.

Discharge formula for flows over open-check dams

The present study proposed a discharge prediction model for open-check dams. The open-check dam is composed of a trapezoidal spillway and a rectangular opening located at the bottom of the spillway. The flow conditions, determined using the ratio of water head and dam crest length, for flow over open-check dams were divided into broad-crested and sharp-crested weir flow conditions. For the broad-crested weir flow condition, the discharge equation for rectangular broad-crested weir was proposed to estimate the discharge. For the sharp-crested weir flow condition, a linear combination of discharge equations for rectangular and trapezoidal sharp-crested weirs was proposed to describe the discharge. A series of laboratory experiments were performed in order to calibrate the proposed discharge model. The results showed the discharge coefficient and cross-sectional area ratio (Ar) were highly correlated under two flow conditions. The averaged error between measured and predicted discharge was less than 2%. The previous models displayed averaged errors between 4% and 25% under the present experimental conditions, whereas that of the present model was considerably low. The present model demonstrated favorable accurate and convenient estimation of discharge for flows over open-check dams.

Numerical Modeling of Flow Over a Rectangular Broad-Crested Weir with a Sloped Upstream Face

The objective of this study was to evaluate the effect of the upstream angle on flow over a trapezoidal broad-crested weir based on numerical simulations using the open-source toolbox OpenFOAM. Eight trapezoidal broad-crested weir configurations with different upstream face angles (θ = 10°, 15°, 22.5°, 30°, 45°, 60°, 75°, 90°) were investigated under free-flow conditions. The volume-of-fluid (VOF) method and two turbulence models (the standard k-ε model and the SST k-w model) were employed in the numerical simulations. The numerical results were compared with the experimental results obtained from published papers. The root mean square error (RMSE) and the mean absolute percent error (MAPE) were used to evaluate the accuracy of the numerical results. The statistical results show that RMSE and MAPE values of the standard k-ε model are 0.35–0.67% and 0.50–1.48%, respectively; the RMSE and MAPE values of the SST k-w model are 0.25–0.66% and 0.55–1.41%, respectively. Additionally, the effects of the upstream face angle on the flow features, including the discharge coefficient and the flow separation zone, were also discussed in the present study.

GENİŞ BAŞLIKLI SAVAK ETRAFINDAKİ AKIMIN İNCELENMESİ

Weirs are among the oldest hydraulic structures that have been used by hydraulic engineers to determine the discharge of an open-channel flow. They are placed perpendicular to the flow axis and built for purposes of measuring the flow of water, controlling the water flow and regulating the water level. In this study, flow over rectangular-broad crested weir was investigated experimentally in an open channel. In the experiments, water surface and velocity distributions upstream and downstream of the weir faces were measured at three different flow rates. The flow field was modelled numerically by using the same experimental conditions. Basic equations that are used for numerical model are solved by FLOW-3D package program based on finite volume method. Different mesh compositions were also tested in this study. Standard k-e turbulence closure model is used in the numerical modeling and the flow profile is computed using VOF (Volume of Fluid) method. The free surface and velocity distributions of flow which are obtained numerically are compared with the values measured experimentally. The results obtained using the standard k-e model is found to be in good agreement with measured data. Keywords: Broad crested weir, open channel, computational fluid dynamics, water surface profiles, FLOW-3D

Development of rectangular broad-crested weirs for flow characteristics and discharge measurement

One of the important problems which designers and engineers are faced with is proper measurement of water flow rates in open channels and rivers. To solve this problem, the water discharge to be precisely measured in water conveyance systems such as off takes and distribution ditches. The common procedure to meet this requirement is achieved by using water-measuring structures such as flumes, orifices, weirs and current meters. Among the mentioned structure, engineers prefers the weirs because of their simple structure and economical features. The broad-crested rectangular weirs are very common which are widely used in water ways and canals. In this study, the effect of steeping in the upstream side of rectangular broad-crested weirs over discharge coefficient and flow characteristics is investigated. Five weirs with 15, 30, 45, 60 and 90 weir angles were developed and flow discharge coefficient, negative velocity over the weir crest edge and water surface profiles along the weir crest evaluated in laboratory hydraulic flume. The obtained results showed by decreasing the slope of the upstream side, discharge efficiency and the discharge capacity of the weir are increased subsequently. Consideration of experimental data it is obvious that by using the proposed weir with slope of 15 degree in comparison with standard weir, the efficiency is increased up to 19.17%. It is noteworthy that by reducing the slope, return current on the crest is not observed.

Control of undular weir flow by changing of weir geometry

Undular flow above the broad-crested weirs is an open channel flow phenomenon which occurs under certain hydraulic conditions. The flow surface on the weir transforms to sinusoidal waves and the number of waves may vary from one to over 10. Hence weir cannot be utilized to measure the flow discharge due to the formation of surface undulations above its crest. Propagated waves can be disturbed remarkably to the channel downstream. In this study, effects of upstream face slope of rectangular broad-crested weirs on the undular weir flow were investigated. The results obtained indicated when the weir upstream slope reduced from standard angle 90° to 40°, the relative wave height decreased up to 78% and the relative wave length increased up to 55%. The experimental results showed that wave was not generated above the weir with 21° upstream face. Furthermore, effects of the radius of curvature of weir upstream corner on the undular wave characteristics were studied. This study established that curvature and slope has similar effects on the wave elimination.

Zone of flow separation at the upstream edge of a rectangular broad-crested weir / Oblast odtržení proudu na návodní hraně pravoúhlého přelivu se širokou korunou

The paper deals with the determination of the basic characteristics of flow at the crest of a rectangular broad-crested weir and in detail with the characteristics of flow separation at the upstream edge of the weir crest. The determination of the characteristics is made on the basis of measurement of the water surface level, the pressure head and the velocity field using the Particle Image Velocimetry method. The characteristics are expressed dimensionless in relation to the energy overflow head and the critical depth.

Flow Characteristics of Rectangular Broad-Crested Weirs with Sloped Upstream Face

The hydraulic characteristics of flow over rectangular broad-crested weirs with varying upstream slopes were experimentally studied. A series of laboratory experiments was performed to investigate the effects of changing upstream slopes from 90o to 75o, 60o, 45o, 30o, 22.5o, 15o, and 10o on the flow surface pattern, discharge coefficient values, approach velocity profile and flow separation zone. In addition, a new mathematical relationship for water surface profile and a new correction factor to estimate discharge coefficient over weirs with various upstream slopes were introduced. The results showed decreasing upstream slopes from 90o to 10o leading to increasing discharge coefficient values and dissipation of the separation zone.

Discharge relations for rectangular broad-crested weirs

Bileşik geniş tepeli savak, düşük akım ölçme ve ardından küçük bir iç dikdörtgen bölüm var, onlar daha yüksek akış derinliklerde geniş bir dikdörtgen bölümüne genişletmek. Sulama kanallarında su ölçüm cihazı olarak kullanılabilir. Bu yazıda akış kontrolü ve ölçümü için hidrolik yapıların tasarımında faydalı olacaktır verilerini sunmaktadır. laboratuvar deneyleri bir dizi sipariş katsayısı deşarj değerleri üzerine dikdörtgen bileşik kesitli geniş tepeli savaklar alt savak kret ve basamak yüksekliği ve genişliğinin etkisini araştırmak için, yaklaşım hızı katsayısı ve modüler limit yapıldı . Bu amaçla, deşarj geniş bir yelpazede için dikdörtgen bileşik kesitli 15 farklı geniş tepeli savak modelleri test edildi. yaklaşım kanalı ve kuyruk su kanalında eşik referansı başları her deney ölçüldü. deşarj katsayısı ve yaklaşım hız modeli parametreleri üzerine katsayısı bağımlılık araştırıldı

Calculation of free-surface profile over a rectangular broad-crested weir

Measurements of the free-surface profile over a laboratory-scale, rectangular broad-crested weir were compared with numerical calculations using commercial software. Although the geometry of the weir is simple, the fluid mechanics is complex with regions of separated flow that affect its hydrometric performance. For the given flow rate, the prediction of the upstream water depth was excellent and the rapidly-varied flow profile over the crest was reproduced quite well. In the supercritical flow downstream, a stationary wave profile was observed and reproduced in form by the calculations.

Flow Past Wall‐Mounted Rectangular and Round‐Nosed Rectangular Bodies

Experimental results related to the form drag coefficient of two-dimensional wall-mounted rectangular and round-nosed rectangular bodies are obtained and expressed as a function of the Reynolds number and the radius of the upstream top corner. The form drag coefficient is obtained by graphical integration of the pressure distribution on the front and rear faces of the body. The wall bodies tested bear a close resemblance to the form of rectangular broad-crested weirs which are widely used to measure open channel flows.

Characteristics of Hydrofoil Weirs

The symmetric Joukowsky profile is investigated for its performance as a weir profile both under free flow and submerged flow conditions. The discharge coefficient is found to be a function of head and eccentricity factor, which defines the weir geometry. The discharge coefficient increases with profile thickness and crest curvature. The boundary layer effects on this particular weir coefficient are very small. The discharge coefficient is found to be about 15 to 20% higher (on average) than the conventional rectangular broad-crested weir. Under submerged flow conditions, the thinner hydrofoil weirs are found to possess a higher submergence limit (80% and more), which varies with head. The discharge reduction factor for a given submergence is found to be smaller than that of a Crump Weir. The overall performance of hydrofoil weir is found to be superior and is therefore recommended for flow measurement in open channels.