Research papersEffects of different configurations of sloping crests and upstream and downstream ramps on the discharge coefficient for broad-crested weirs
Introduction
Any obstacle that impedes flow in a canal, causes water to rise, and increases the speed of water flow is called a weir. Weirs of different cross section shapes have been widely used as flow-measuring devices (Kabiri-samani and bagheri, 2014); they are usually categorized as being either sharp or broad-crested and as normal or side weirs (Zahiri et al., 2013). Broad-crested weirs are simple flow-control structures and are commonly used in open channels; weirs are also used to measure flow rate. According to ratio of the water head above the weir to the length of the weir crest (), weirs of finite crest length are categorized into 4 groups (Govinda Rao and Muralidhar (1963), Hager and Schwalt (1994), and Azimi and Rajaratnam (2013)). These groups are long-crested weirs (), broad-crested weirs (), short-crested weirs () and sharp–crested weirs. The geometry of these type weirs can influence the flow conditions and capacity.
Broad-crested weirs have attracted the attention of many investigators. Some authors found that the trapezoidal weirs are advantageous over standard broad crested weirs. Because of the upstream face slope, they have a higher discharge coefficient (Sargison and Percy, 2009) and the deposition of upstream sediments and debris is low. Furthermore, a sloping downstream face eliminates cavitation at high flow rates (Inozemtsev, 1969) and improves the sensitivity to the downstream submergence ratio (Farhoudi and Shokri, 2007). Goodarzi et al. (2012) investigated the characteristics of flow over a broad-crested weir with different upstream slopes. The results showed that reducing the upstream slope increases the discharge coefficient and reduces the flow separation zone.
Fritz and Hager (1998) conducted a series of experiments on trapezoidal-shaped weirs with different crest lengths and with upstream and downstream slopes of 1 V: 2H (vertical: horizontal). They revealed that the discharge coefficient for a broad-crested weir is nearly 10% less than its corresponding value over an embankment-shaped weir. Sargison and Percy (2009) investigated the flow of water over an embankment weir with changing upstream and downstream slopes. Results revealed that decreasing the upstream ramp slope increases the height of the water surface profile and the static pressure over the weir crest. It was also found that varying the downstream ramp slope has little effect on the discharge coefficients.
Hydraulic properties of flow through trapezoidal broad-crested weirs were investigated experimentally by Madadi et al. (2014). The results revealed that decreasing the slope of the upstream face reduced the size of the flow separation zone and increased the discharge coefficients. Azimi et al. (2013) performed laboratory investigations on different broad-crested weir geometries. The results showed that the discharge coefficient for broad-crested weirs with upstream and downstream ramps is greater than the discharge coefficient of weirs without ramps.
The discharge coefficients for combined broad-crested weir-box culvert structures were studied by Guven et al. (2013). The results indicated that combination of a broad-crested weir and a box-type culvert structure improved the discharge capacity compared to single structures (culvert and weir). Daneshfaraz et al. (2019) investigated the flow over a broad-crest weir with and without an opening in the body of the weir with different slopes using Flow-3D software. The results indicated that the opening in the body of the weir leads into an increase in discharge coefficient and a decrease in the upstream water surface level. Shaymaa et al. (2017a) investigated the adequacy of the turbulence models to simulate water surface profiles for rectangular and stepped broad-crested weirs. They found that the k-ε turbulence model gives highest accuracy among all of the tested turbulence models. The k-ε turbulence model is one of the two-equation turbulence models that accounts for turbulent motion through transport equations for turbulent kinetic energy (k) and dissipation of kinetic energy of turbulence (ε). Shaymaa et al. (2017b) compared the use of 2D and 3D numerical simulations for determining water surface elevation over broad-crested weirs; the results revealed that these two procedures provide similar accuracy.
Tanase et al. (2015) confirmed the capability of the k – ε (RNG or Re-Normalization Group) turbulence model to simulate free surface flow over a rectangular broad-crested weir. Maghsoodi et al. (2012) used the volume of fluid (VOF) method and k-ε turbulence model to simulate free surface flow over rectangular and broad-crested submerged weirs with variable crest widths and upstream/downstream-facing slopes. The results of that study indicated that good agreement exists with the experimental data, a finding that has also been confirmed by Joongcheol and Nam (2015).
Reviewing the literature indicates that up to now, the effect of crest slope on hydraulic performance of broad-crested weirs has not been widely investigated. This subject will be answered in the present study. Additionally, hydraulic properties of flow over weirs with sloped crests (positive and negative slopes), and without sloped crests (horizontal) are investigated. Different configurations of positive and negative sloped weir crests are considered with upstream and downstream ramps.
This study was performed using ANSYS FLUENT software that uses the finite volume method (FVM) for solving the governing equations. Numerical simulations were validated by comparing experimental results and then hydraulic characteristics of this type of weir such as the water surface profile, discharge coefficients, and stage-discharge equation investigated. After validation, the performances of these weirs compared with each other.
Section snippets
Stage-Discharge equation
The stage-discharge equation is based on the upstream water depth over the crest for a broad-crested weir; it can be expressed by the following equation (Eq. (1)):in which q is flow discharge per unit width (m2/s), is the discharge coefficient, g is gravitational acceleration (m/s2), and h is the water depth over the crest (m).
Dimensional analysis
Schematic diagrams of broad-crested weirs with different configurations of upstream and/or downstream ramps and crest slopes are shown in Fig. 1. For these
Broad-Crested weirs with upstream or downstream ramps
Fig. 7 illustrates the results from the numerical simulations. The water surface profile, the velocity contour, and the velocity vectors surrounding the BCW-UR-HC and the BCW-DR-HC for h/Lr = 0.375 and h/p = 0.750 with and are shown in Figs. 7(a) and (b). As can be seen in Fig. 7 (a) for the BCW-UR-HC, flow downstream of the edge of the crest forms a jet that reaches the bottom and the canal; this promotes a two-phase flow of an air–water mixture. This flow
Conclusions
In this study, hydraulic characteristics for flow over six types of broad-crested weirs with upstream or/and downstream ramps and with/without positive and negative crest slopes were investigated. Several turbulence models were employed for handling turbulence and the VOF method was used to track the liquid–gas interface. The results can be summarized as follows:
- 1)
Numerical simulations were validated by existing experimental results; the k – ε (RNG) turbulence model was most accurate amongst the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References (29)
- et al.
Performance assessment of OpenFOAM and FLOW-3D in the numerical modeling of a low Reynolds number hydraulic jump
Environ. Model. Softw.
(2016) - et al.
Experimental investigation on discharge coefficient for a combined broad-crested weir-box culvert structure
J. Hydrol.
(2013) - et al.
Volume of fluid (VOF) method for the dynamics of free boundaries
J. Comput. Phys.
(1981) - et al.
Investigation of flow characteristics above trapezoidal broad-crested weirs
Flow Meas. Instrum.
(2014) - et al.
A novel approach for estimation of discharge coefficient in broad-crested weirs based on Harris Hawks Optimization algorithm
Flow Meas. Instrum.
(2021) - et al.
Discharge coefficient for compound sharp crested side weirs in subcritical flow conditions
J. Hydrol.
(2013) - et al.
Experimental investigation of the approach angle effect on the discharge efficiency for broad-crested weirs
Sci. Technol.
(2016) - Azimi A. H, and Rajaratnam N, (2013a). Discussion of new theoretical solution of the stage-discharge relationship for...
- et al.
Discharge characteristics of weirs of finite crest length with upstream and downstream ramps
J. Irrig. Drain Eng.
(2013) - Azimi A. H, Rajaratnam N and David Z. Zhu, (2014). Submerged Flows over rectangular weirs of finite crest length, J....
3-D Numerical simulation of water flow over a broad-crested weir with openings
ISH Journal of Hydraulic Engineering
Effects of viscosity and surface tension on slot weirs flow
J. Hydraul. Res.
Hydraulics of embankment weirs
J Hydraulic Eng
Cited by (7)
Discharge estimation using brink depth over a trapezoidal-shaped weir
2023, Flow Measurement and InstrumentationFlow-Through Short-Crested Trapezoidal Weirs: Effect of Downstream Slope
2023, Journal of Irrigation and Drainage EngineeringNumerical and Experimental Study of Trajectory for Free-Falling Jets
2023, Iranian Journal of Science and Technology - Transactions of Civil EngineeringExperimental study on the effect of arm slope on turbulent structure of weir-flume combination
2023, Shuikexue Jinzhan/Advances in Water Science