Experiment 9: Flow Over Weirs 1. Introduction A weir is a barrier across the width of a river or stream that alters the characteristics of the flow and usually results in a change in the height of the water level. Several types of weirs are designed for application in natural channels and laboratory flumes. Weirs can be broad-crested, short-crested, or sharp-crested. Sharp-crested weirs, commonly referred to as notches, are manufactured from sharp-edged thin plates. A coefficient of discharge needs to be determined experimentally for each weir to account for errors in estimating the flow rate that is due to these assumptions.
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The difference in flow rate of water that flows into both of the channel was observed. Other than that, the experiment was capable to study the discharge coefficient of fluid flow that determined by calculation of this experiment. The experiment was started as follows the procedures, with the depth of water with different height was tested by recording the time taken to collect 3L of water, which later will be used to calculate the flow rate of the flow.
The data obtained were further tabulated by calculating the discharge coefficient, using the equation provided. Then, graphs were constructed to analyze the characteristics of the flow.
From the constructed graphs, rectangular notch graph shows the discharge coefficient decrease slowly before a constant value is reached. Meanwhile, the triangular notch graph shows the discharge decrease smoothly, but the values are higher than the rectangular notch.
To conclude, triangular notch has a higher discharge coefficient than rectangular notch. The experiment was successfully accomplished as all the objectives were gained. Liquid flow in open channels, however, is not confined, and thus the flow rate is controlled by partially blocking the channel. This is done by either allowing the liquid to flow over the obstruction or under it. An obstruction that allows the liquid to flow over it is called a weir, and an obstruction with an adjustable opening at the bottom that allows the liquid to flow underneath it is called an underflow gate.
Such devices can be used to control the flow rate through the channel as well as to measure it. A weir is a flow control device in which the water flows over the obstruction. In this experiment, the rectangular weirs and triangular weirs are been used. Rectangular weirs and triangular or v-notch weirs are often used in water supply, wastewater and sewage systems. They consist of a sharp edged plate with a rectangular, triangular or v-notch profile for the water flow. Broad-crested weirs can be observed in dam spillways where the broad edge is beneath the water surface across the entire stream.
Flow measurement installations with broad- crested weirs will meet accuracy requirements only if they are calibrated.
Besides, the other objectives is to determine the discharge coefficients of the fluid flow. While on V-notch, learn how to plot graph of against H. Next, to estimate an average value of for the range of the test. Last but not least, to compare values between rectangular notch and V-notch. The weir structure will increase the water level, which will be measured.
Water level-discharge relationships are available for standard-shaped openings or notches. Two types of weirs are widely used: 1. Rectangular shape opening 2. Stilling baffle is used to ensure minimum turbulence. The stilling baffle will act as a reservoir to collect water volume and slowly disperse the water from the opening at the bottom of the stilling baffle. Types of Weirs Rectangular Weir The rectangular weir is able to measure higher flows than the v-notch weir and over a wider operating range.
V-notch weir is typically used to measure low flows within a narrow operating range. Typical Cd values for V-notch are in the range of 0. Stilling Baffle 5. Hydraulic Bench FM 2. Vernier 6. Flow Control Valve 3. Hook 7. Water Channel 4. Weir Plate V or rectangular 8. General Shut Down Procedures 1 The hydraulic bench flow control valve were shut and water supply is closed. Experiment 1: Flow Characteristics over Weirs 1 The weir apparatus on the hydraulic bench were levelled and the rectangular notch weir is installed.
The water level is ensured that it is even with the crest of the weir. The water flow were adjusted by using the hydraulic bench flow control valve to obtain heads H. This is because the value for the flow rates, Q and the head above bottom of notch, H obtained for both rectangular notch and V-notch are different in each experiment. However, the Cd values of rectangular notch is smaller compared to the Cd values of V-notch. Calculation below shows the average value of Cd for rectangular notch for the range of the test:- Cd 0.
Besides, as the head above bottom notch increases , the Cd values gain also decreases. This shows that, the Cd values is dependent on the value of flow rate, Q and the value of the head above the notch, H. The Cd values can be obtained from the tangent of the graph on each point plotted. The reason of this error could be because of the reading from the graph is not as accurate as the calculation using formula which lead to about 0.
As the flow rate increases, the discharge coefficient becomes more accurate to the theoretical value. When the flow rate is to low it clings to the notch and flows down it.
This changes the coefficient of discharge because now the water is not only being affected by gravity it is having to resist viscosity and the friction of the surface of the notch.
The limitations of the theory is it has to be level so the only force on the water is gravity, there has to be a constant flow, and constant pressure. The theory behind this experiment makes an assumption that there is a minimum height of water above the notch and any heights below this start to deviate from theory at an increasing rate. The relationship between the head of the weir and the discharge of the water over the weir is directly proportional.
Weirs are commonly used to alter the flow of rivers to prevent flooding, measure discharge, and help render rivers navigable. There are several of recommendations to estimate discharge when using a weirs because it is very important to ensure that all flow enters by travelling over the weir and not around the weir or under the weir.
It must be noted that the weir should be extended into the ground to minimize groundwater to pass under the weir. As long as the flow conditions downstream of the weir do not affect the flow over the weir, a free outfall is maintained. First of all, before doing an experiment it is better to learn and understand first on how to conduct the experiment.
Moreover, the result of rectangular-notch and v-notch obtained must be taken in 4 decimal points to get an accurate values. Errors can never be ignored when it comes to laboratory work. The aim is to reduce the error as much as possible to obtain accuracy in work. Ways to reduce the error are by repeating the experiment for three times or more and then taking the average readings, by being extra cautious during the experiment, by asking more than one person to record the readings and carry out the experiment.
It is important to keep the voice to a minimum while in a laboratory and always listen to the instructor. If any guidelines are needed, then refer to the supervisor.
Flow Over Notches and Weirs
It is usually a device for measuring discharge. A weir is a notch on a larger scale - usually found in rivers. It may be sharp crested but also may have a substantial width in the direction of flow - it is used as both a flow measuring device and a device to raise water levels. Depending on the weir design, flow may contract as it exits over the top of the weir, and, as with orifices, the point of maximum contraction is called the vena contracta. We will also assume that the velocity through any elemental strip depends only on the depth below the free surface. These are acceptable assumptions for tanks with notches or reservoirs with weirs, but for flows where the velocity approaching the weir is substantial the kinetic energy must be taken into account e. A General Weir Equation To determine an expression for the theoretical flow through a notch we will consider a horizontal strip of width b and depth h below the free surface, as shown in the figure below.
Experiment #9: Flow Over Weirs