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The main purpose of broad crested weir used in open channels is to raise and control upstream (U/S) water level. In this study, a new performance was added to this weir, by making a step at downstream (D/S) of weir. The energy dissipation, the height of the weir/the upstream water height ratio and Froude number relationships (E% – P/h – Fr) for three range of flume slop S = 0.0, 0.002 and 0.004 were simulated. The experiments were performed in a laboratory horizontal channel of 4.6 m length, 0.3 m width and 0.3 m depth for a wide range of discharge. The D/S step height of the weir was 7.5 cm. FLUENT software was used as numerical model which represent a type of Computational Fluid Dynamics (CFD) model in order to simulate flow over weirs. The Volume of Fluid (VOF) method with the Standard k – ε turbulence model was used to estimate the free surface profile and the structured mesh with high concentration near the wall regions. The experimental results of the water surface profile gave a high agreement with the results of the numerical models. The maximum value 28.78 of E% was obtained in single step broad crested weir in the experimental result and 27.35 in numerical result at S = 0.004. Finally, the range of the relative error of the energy dissipation between experimental and numerical results was achieved and the maximum was 6.76 in all runs.

A broad crested weir is usually considered for most hydraulic structures for flow measurement and to control the water surface level in open channels. The water flow type is usually critical conditions. The weirs are of different types such as broad crested, sharp crested and ogee crest weir. The streamline flows over broad crested weir are parallel to the crest, critical depth occur along the crest and the pressure distribution is hydrostatic [

To dissipate energy, step at D/S ends of weirs is constructed or lining by rubbles and riprap is implemented to prevent erosion and scouring in D/S ends [

The flow characteristics over broad crested weirs and single step broad crested weir with rounded upstream (U/S) corner have attracted the attention of several investigators to obtain the flow characteristics and the energy dissipation in numerical and physical methods. [

Several numerical studies were carried out to find the flow characteristics around weirs by Computational fluid dynamics (CFD). The study of [

Iraq has been suffering from severe water shortage problems. One of the main reasons for this shortage is the construction of dams in Turkey and Syria [

Scaled physical models of weirs have been constructed in hydraulic laboratories to study these behaviors and to calculate the Cd, but they are expensive, time consuming and there are many difficulties associated with scaling effects. Today, with the advance in computer technology, numerical modeling of weirs is becoming increasingly important in the engineering work. For this point, physical modeling can often be replaced by these models.

In this paper, laboratory measurements and 2D numerical modeling were conducted to simulate the flow pattern over a broad crested weir and single step broad crested weir located in a rectangular channel. The results obtained from experimental and numerical work were compared. D/S height of traditional broad crested weir was reduced by a single step; this reduction gave the weir a new performance by making it as an energy dissipater. So, the main objectives of this paper are to study the free surface profile over weir and dissipation energy D/S of the weir for different bed slope of flume ranges (0, 0.002, 0.004) slope channels in laboratory. The experimental result was compared with numerical modeling.

The experimental tests were carried out in the Hydraulic Engineering Laboratory, Collage of Engineering, Al-Mustansiriayah University, Baghdad. The wall and bed of the flume were made of smooth strong glass supported by stainless steel supporting bars at equal distances. The cross section of the flume was 30 cm wide, 30 cm depth and 480 cm long (

Two weir models were manufactured from steel and well-polished to smooth surfaces and then tested in the laboratory. The first weir was broad crested and the second was single step broad crested weirs which have step at the D/S crest and its height was equal to 7.5 cm. These models can be classified (

Model No. | P (cm) | P1 (cm) | L (cm) | L2 (cm) | R (cm) |
---|---|---|---|---|---|

1 | 15 | 15 | 36 | - | 2 |

2 | 15 | 7.5 | 40 | 16 | 2 |

reduce the adverse effects of separations zone, so the two weirs had a radius (R) of rounded corner 2 cm at U/S corner. To ensure the stability and uniformity of water surface levels, models were placed on the bed of the flume at a distance 0.9 m from the flume inlet. In addition, the width of the weirs 30 cm was the same as that of the flume width.

The developments in computer science and numerical techniques have advanced the use of CFD as a controlling tool for analysis of flow over the weirs. In this paper, numerical method was completed by using FLUENT program. FLUENT is one of the great CFD commercial software and it has the capacity to solve 2D and 3D problems of open channel flow, and to predict flow profile over weirs. (VOF) method was used to determine the water surface profile in each cell, which is used in many hydraulic problems because it represent the sharp interface between the air and water phases such as this work [

The governing equations of FLUENT software for unsteady incompressible 2D flows over weirs are continuity. Navier-Stokes equations (Equation (1) and Equation (2)) are based on principles of physics mass conservation and Newton’s Second Law [

where: ρ = fluid density,

Turbulent flows are characterized by fluctuating velocity. These fluctuations mix transported quantities such as momentum, energy, kinetic energy and species concentration, and cause the transported quantities to fluctuate as well. Since these fluctuations can be of small scale and high frequency, they are also computationally expensive to simulate directly in practical engineering calculations. Instead, the exact governing equations can be time-averaged, ensemble-averaged, or otherwise manipulated to remove the small scales, resulting in a modified set of equations that are computationally easier to solve and describe [

FLUENT have different turbulent models which includes: k − ε model [standard k − ε model, renormalization group (RNG) k − ε model and realizable k − ε model] and k − ω model [standard k − ω and shear stress transport (sst) k − ω] [

where the eddy viscosity μt, is computed by using Equation (5).

where the eddy viscosity μt, is computed by using Equation (5).

where,

Before applying the FLUENT software, the dimensions of weirs must be designed similar to that of the experimental weirs. The first step in numerical method is producing the geometry and creating mesh of the model. For this purpose, GAMBIT software was used. It is noteworthy to mention that GAMBIT can create 2D and 3D meshes. The mesh size of the modeling is an important part of the numerical simulation due to its effect on the accuracy of the result and the simulation time. It was found that the best optimum cell size was 2 mm for the edge and 4.5 mm for the face (

Boundary conditions in the FLUENT software are one of the most important order of the numerical model of flow and it is should be similar to that of the physical model. In this research, the boundary conditions of the numerical modeling of weirs are shown in

The experimental and numerical results of water surface profiles over broad crested weir and single step broad crested weir along the centerline of the flume with different

discharge and various sloping of flume were plotted. The water level increased by increased discharge and this lead to the increase of the U/S and D/S water levels. Also, it can be seen from Figures 5-10, the numerical result had a great agreement with experimental results. Where, Y is the water level of flow and X is the horizontal distance of the flume.

Water surface profile of single step broad crested weir with (P/P1 =2) are shown in

By applying the energy equations in point (1) and in point (2) in

The experimental and numerical models can classified into two weirs based on (P/P1) as broad crested and single step weir, these weirs can be classified into three groups based on the variation of channel slope S = 0.0, 0.002 and 0.004. Each group include twelve running (six for experimental results and six for numerical results) based on the variation of the U/S water head (h).

(P/P1 = 2) for different slopes in experimental and numerical methods. The effect of (P/P1) began to appear with little effect at (S = 0.0), while in the case of S = 0.002 and S = 0.004 the effect obviously appeared on (E%) by increasing it because the effect of single step which decreased the D/S head.

_{2}) increased when S = 0.0. But, in S = 0.002 and S = 0.004 the increase of (E%) was more relative to the former slope. This could be attributed to the reason that as the water level above D/S crest

The variation of the

It can be noticed that the increase of (E%) was accompanied with the increase of ( ) in all slopes (S = 0.0, 0.002 and 0.004) because the same reason above and the effect of single step which decreased the D/S head respectively. The maximum value of (E%) was 26.9 experimentally and 27.35 in numerically model.

All running details are shown in

Model No. | P/P1 | S | Run N0. | Range of h (cm) | Range of Fr_{2} | Range E% | Range of Relative Error for E% | ||||
---|---|---|---|---|---|---|---|---|---|---|---|

Exp. | Num. | Exp. | Num. | Exp. | Num. | Exp. | Num. | ||||

1 | 1 | 0 | 1 - 6 | 7 - 12 | 6 - 8.6 | 5.9 - 8.8 | 1.26 - 1.48 | 1.25 - 1.479 | 1.53 - 1.84 | 1.47 - 1.77 | 1.94 - 6.76 |

0.002 | 13 - 18 | 19 - 24 | 5.9 - 8.5 | 5.8 - 8.7 | 1.29 - 1.6 | 1.306 - 1.61 | 2.04 - 2.9 | 1.9 - 2.74 | 2.1 - 6.2 | ||

0.004 | 25 - 30 | 31 - 36 | 5.7 - 8.3 | 5.6 - 8.4 | 1.38 - 1.74 | 1.37 - 1.72 | 2.95 - 4.2 | 2.8 - 4.41 | 2.14 - 5.6 | ||

2 | 2 | 0 | 37 - 42 | 43 - 48 | 5.8 - 8.8 | 5.9 - 8.7 | 2.3 - 3.5 | 2.25 - 3.47 | 7 - 20.9 | 7.3 - 21.5 | 1.8 - 5.4 |

0.002 | 49 - 54 | 55 - 60 | 5.6 - 8.5 | 5.7 - 8.4 | 2.5 - 4.1 | 2.47 - 4 | 9.9 - 24.8 | 10.4 - 24.9 | 2.3 - 5.6 | ||

0.004 | 0.004 | 67 - 72 | 5.4 - 8.2 | 5.3 - 8.1 | 2.65 - 4.6 | 2.61 - 4.45 | 12.3 - 26.9 | 12 - 27.35 | 1.76 - 5.1 |

and the range of relative error for energy dissipation between experimental and numerical models for each group.

In this research, the energy dissipation (E%) and the flow over broad crested and single step broad crested weirs were studied experimentally and simulated by using 2D code (FLUENT software). From this research the following main conclusions were achieved:

1. The VOF method which was used in FLUENT to predict the water surface profile and the result has high agreement with experimental method in all runs.

2. The results showed that the water level along the flow direction was gradually decreasing until it reaches stability condition in D/S side of the flume and the increase in the bed slope made water level smoother and it was reduced in D/S side of the flume.

3. For the two models of weirs (broad crested and single step broad crested weirs), the (E%) value decreased with increased the (h/p) and slop value, the (E%) value increased with increasing the value and with increasing the slope.

4. This research proved the effectiveness of the second model of weir in energy dissipation. While, the maximum value of (E%) in first model was 4.1 in the experimental method and 4.41 in the numerical method at (S = 0.004). Also, the maximum value of (E%) in second model was 26.9 in the experimental method and 27.35 in the numerical method because of the influence of both single step and slope at (S = 0.004) on (E%).

5. Moreover, the ranges of the relative error for first model (P/P1 = 1) in cases S = 0.0, S = 0.002 and S = 0.004 were (1.94 to 6.76), (2.1 to 6.2) and (2.14 to 5.6) respectively. The ranges of the relative error of second model (P/P1 = 2) in cases where S = 0.0, S = 0.002 and S = 0.004 were (1.8 to 5.4), (2.3 to 5.6) and (1.76 to 5.1) respectively.

Al-Hashimi, S.A.M., Madhloom, H.M., Nahi, T.N. and Al-Ansari, N. (2016) Channel Slope Effect on Energy Dissipation of Flow over Broad Crested Weirs. Engineering, 8, 837-851. http://dx.doi.org/10.4236/eng.2016.812076