CE 336 Lab 2 Flow measurement using Weirs (original) (raw)
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Section 3. HYDRAULIC DESIGN A. Weirs and Orifices A.1. WEIRS
NOTE: Some of the graphs contained in this section are copied from the Los Angeles Hydraulics Manual and we wish to give them credit for their efforts. Also, applicable graphs are for 8 " curb heights which may not meet Rio Rancho standards. A weir is a barrier in an open channel, over which water flows. A weir with a sharp upstream corner or edge such that the water springs clear of the crest is a "sharp crested weir". All other weirs are classified as "weirs not sharp crested". Weirs are to be evaluated using the following equation: Q = CLH 3/2 where: Q = Discharge in cfs C = Discharge coefficient from Handbook of Hydraulics, King and Brater, 5th Edition (or comparable) L = Effective length of crest in feet H = Depth of flow above elevation of crest in feet (approach velocity shall be disregarded in most applications) Applications Weirs are generally used as measuring and hydraulic control devices. Emergency spillways in which critical depth occurs and overflow-type roadway crossings of channels are the most common applications of weirs. Channel drop structures and certain storm drain inlets may also be analyzed as weirs. Special care must be exercised when selecting weir coefficients in the following cases: a. Submerged weirs b. Broad crested weirs c. Weirs with obstructions (i.e., guardrails, piers, etc.)
Journal of Irrigation and Drainage Engineering, 2013
formula because they did not obtain a normal distribution in a series of tests that they conducted to evaluate the coefficient of variation of discharges manufactured by the emitter. This important claim requires an in-depth justification-which is not provided in the article-because it constitutes a significant change from previous assumptions in the literature. The discussers think that a detailed description is required of the conditions under which the tests were conducted, which should particularly specify whether the tests were performed following any of the existing standards [e.g., ISO 9261 (ISO 2004)], and should provide the accuracy of the measurements. The authors should also provide details of the kurtosis values and the skewness of each type of emitter and discuss the results. Moreover, there is no horizontal or vertical scale in with which to assess the magnitude of the deviations. If one of the conclusions of the article is that the variation in flow as a result of manufacturing does not follow a normal distribution, it is necessary to apply some specific tests (e.g., the Kolmogorov-Smirnov test). In Eq. (21), the authors present a relationship between f a and the kurtosis coefficient, but there is no reference to the goodness of fit obtained. Therefore, there is uncertainty concerning how good is the fit may be. The authors cite two factors that affect the uniformity of flow: hydraulics and manufacturing. However, they do not make any reference to clogging, which can be an important factor in some irrigation units. Furthermore, in the paragraph about field verification of emission uniformity, data relating to field evaluations are compared [Eq. (3)] with the results of Eqs. (1) and (14). However, this comparison is incorrect because Eq. (3) considers hydraulic factors, manufacture, and clogging. Eq. (1), on the other hand, only considers the first two factors and Eq. (14) only considers the first. It does not seem very logical to compare different things.
Pengujian Pengaruh Ketinggian Weir pada Koefisien Discharge dari Weirmeter Sharp-Crested V-Notch 90o
Bina Teknika
Weirmeter is a discharge gauge on an open-channel flow that is usually applied to dams and rivers. In each weirmeter there is a discharge coefficient (Cd), which is the coefficient multiplied by the theoretical discharge to obtain the actual discharge because the pressure difference and flow velocity are neglected. The Cd is directly proportional to the discharge. The actual debit measurement results do not approximate the result (the multiplication of the theoretical debit with Cd). This difference may be due to unequal Cd values for each weir dimension, one of which is the weir height (P). Therefore it is necessary to test the weirmeter with P varies in order to know the pattern and the value of Cd. The tested weirmeter is sharp-crested v-notch 90 o with a length of 200 cm and a width of 35.5 cm. P used is 20.3 cm; 21.3 cm; and 22.3 cm. From the test, the Cd value is inversely proportional to P although some of these test results are far from the theory because it requires a 90% confidence interval on the Student distribution (t distribution).
Broad-Crested Weir as a Device for Measurement of Discharge
Weirs are overflow structures, which alter the discharge so that flow rate can be calculated, flood can be prevented, or even make a body of water more navigable. Measurements of discharge are known based on the surface profile of the water. For estimation of discharge over the weir, weir coefficient is required. In this report only the broad crested weir is focused on. Design and the analysis of this weir are shown in the report. It was found that the broad-crested weir is best for measuring discharge in small medium channels. Flow over a broad-crested weir is highly dependent on the weir's geometry, it's a useful hydraulic tool which enables engineers to control water height, velocity, and most importantly they can be used to calculate discharge. Hydraulic structures such as weirs, flumes, and pipes, may cause the flow upstream of the structure to rise above the normal flow depth this is a common property.
The objectives of this experiment are to determine the discharge coefficient of both notches and to demonstrate and determine the flow characteristics of rectangular notch and (V) notch. Basically a weir is a structure built across open channel to measure the volumetric flow rate of water flow and discharge coefficient as stated in the objective above. The stilling baffle is used to ensure minimum turbulence. It will act as a reservoir to collect water volume and slowly disperse in the water from the opening at the bottom of the stilling baffle. The water that flow in the water channel should not be disturb so that the flow is stable. The water was let to flow until it is stabled. Vernier Gauge was set to a datum reading using the top of the hook. The gauge was positioned about half way between the notch plate and stilling baffle. Then the H was obtained and recorded.
2014
Gates and weirs have been used extensively for flow control and discharge measurement in open channel flow. Works concerning the use of sluice gates as a discharge measurement structure may be found in, Rajaratnam (1977), French(1986),and Swamee (1992). Many researchers had developed a generalized discharge equation for sluice gates, Abdel-Azim et al. (2002), Bos (1989), and Munson et al.(1994). Weirs and gates may be combined together in one device yielding a simultaneous flow over the weir and below the gate. The reason lead to the use of a bottom opening combined with weirs is to provide a self-cleaning facility for the weir. This will reduce the typical problem of the accumulation of sediments at the upstream side of the weir. The flow through combined devices may be classified as free flow when both the flow over weir and below the gate is free, while it is termed submerged when the flow below the gate is submerged and the flow over the weir may or may not be submerged. Problems concerning sedimentation and depositions in the upstream side of the weir are minimized by combining these weirs with bottom gates as outlined by Alhamid(1997). Fadil (1997) had developed a meter for the combined flow through a contracted sluice gate and a weir. Combined-submerged flow through weirs and below gates were analyzed and discussed by Negm(2000).The characteristics of the combined flow over weirs and below gates of equal contraction are discussed by Abdel-Azim et al.(2002), different geometrical combinations were used to investigate the discharge characteristics of such combined weirs and gate structures. They found that the flow parameter (H/D) (the ratio of the upstream water depth to the height of the opening) and the geometrical parameter (y/D) (the ratio of the vertical distance between the lower edge of the weir and the upper edge of the gate opening to the height of the gate opening) have major effects on the discharge coefficient. Al-Suhili and Shwana(2013) had obtained a neural networks model for the discharge coefficient of a Cipoletti weir with rectangular bottom opening.
Discharge Formula for Sharp-Crested Rectangular Weirs
Sharp-crested rectangular weirs are commonly used as discharge measuring facilities in open channels and laboratories. Weir plate height and width are two key elements in shaping the head-discharge relationship. The experiments demonstrate that for a certain range of discharge over the weir, height of the plate is no longer affecting the discharge. Average velocity over the weir plotted against the weir head has a unique behavior. This behavior can be used in such a way that can define a weir velocity to estimate the discharge rather than using the discharge coefficient. Hence a more accurate outcome and a more compact and short form of expressions are expected to be obtained. The present study focuses on the experimental investigation of various possible formulations of the weir velocity for available weir heights and widths.