stage–discharge relationships in open channels: practices and ...
On a flow meter for discharge measurement in irrigation channels
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Flow Measurement and Instrumentation 17 (2006) 255–257 www.elsevier.com/locate/flowmeasinst On a flow meter for discharge measurement in irrigation channels Arun Goel Civil Engineering Department, National Institute of Technology (Deemed University), Kurukshetra-136119, Haryana, India Abstract Flow characteristics of a simplified constricted width flow meter are investigated in this paper. It is a very simple device, which is not only easy to construct and operate but also maintenance free. The rate of flow can be estimated for both free and submerged flow conditions. The device can find applications in measurement of discharge in irrigation channels provided in small catchments. c 2006 Elsevier Ltd. All rights reserved. Keywords: Flow meter; Irrigation channel; Rectangular section; Flow measuring structures 1. Introduction Measurement of discharge of water in open channels is critical from the viewpoint of water conservation. Assessment of accurate discharge measurement is a must for the efficient management of water resources so as to provide a unique relationship between upstream head and the rate of flow. Weirs, flumes and gates are used to determine discharge in open channels for a long time as mentioned by Boiten [1]. The flumes suggested by Parshall [2] are to be constructed exactly to the recommended dimensions, requiring greater skill and workmanship as mentioned by Ackers [3] and Bos [4]. Each flume must be leveled accurately in both longitudinal and transverse directions and, moreover, they are not geometrically similar. For each discharge range, different dimensions of the flume are recommended. For submerged flow conditions, a correction to the discharge is applied for which graphs are available for different widths of throats and discharges. Furthermore, these are more expensive to fabricate due to large transition lengths. In the Cutthroat flume as proposed by Skogerboe et al. [5], the stage discharge relation needs direct calibration due to the absence of a throat section, which causes curvilinear flow and a non-hydrostatic pressure distribution. Due to the complex hydraulic behavior, these flumes should either correspond exactly with a flume that has been calibrated earlier or should be calibrated before being put to use. E-mail address: drarun [email protected]. Keeping in view some of these difficulties, the proposed constricted width flow meter having sudden expansion at the downstream end and with a simplified geometry has been considered for studying the flow characteristics under free and submerged flow conditions. The present research is in line with the previous work reported by Hager [6–8], Samani et al. [9,10] and the ISO standard [11] for development of the proposed flow meter. 2. Constricted width flow meter In the proposed flow meter, the geometry of the device has been simplified. It consists of a converging inlet section of uniform width b (where b = B/2, B = the width of the channel) as shown in Fig. 1, in which the transition to critical depth takes place. When the flow is subcritical, the reduction of width of the channel causes a reduction in depth of flow at the constricted section. By constriction of width to half, a critical flow condition may be obtained by converting a major part of the upstream head into kinetic energy at the control section. There is a definite relation between the upstream depth of flow and discharge. Such a section with critical flow above the weir crest is required to make the upstream head (h 1 ) independent of downstream conditions; Boiten [1]. This condition can be observed by formation of the hydraulic jump immediately downstream of the control section. In the proposed device, a sudden diversion is provided after the throat portion and head losses are ignored. In 0955-5986/$ - see front matter c 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.flowmeasinst.2006.05.001
Transcript of On a flow meter for discharge measurement in irrigation channels
Flow Measurement and Instrumentation 17 (2006) 255–257www.elsevier.com/locate/flowmeasinst
On a flow meter for discharge measurement in irrigation channels
Arun Goel
Civil Engineering Department, National Institute of Technology (Deemed University), Kurukshetra-136119, Haryana, India
Abstract
Flow characteristics of a simplified constricted width flow meter are investigated in this paper. It is a very simple device, which is not only easyto construct and operate but also maintenance free. The rate of flow can be estimated for both free and submerged flow conditions. The device canfind applications in measurement of discharge in irrigation channels provided in small catchments.c© 2006 Elsevier Ltd. All rights reserved.
Keywords: Flow meter; Irrigation channel; Rectangular section; Flow measuring structures
1. Introduction
Measurement of discharge of water in open channels iscritical from the viewpoint of water conservation. Assessmentof accurate discharge measurement is a must for the efficientmanagement of water resources so as to provide a uniquerelationship between upstream head and the rate of flow. Weirs,flumes and gates are used to determine discharge in openchannels for a long time as mentioned by Boiten [1]. Theflumes suggested by Parshall [2] are to be constructed exactlyto the recommended dimensions, requiring greater skill andworkmanship as mentioned by Ackers [3] and Bos [4]. Eachflume must be leveled accurately in both longitudinal andtransverse directions and, moreover, they are not geometricallysimilar. For each discharge range, different dimensions ofthe flume are recommended. For submerged flow conditions,a correction to the discharge is applied for which graphsare available for different widths of throats and discharges.Furthermore, these are more expensive to fabricate due to largetransition lengths.
In the Cutthroat flume as proposed by Skogerboe et al. [5],the stage discharge relation needs direct calibration due to theabsence of a throat section, which causes curvilinear flow anda non-hydrostatic pressure distribution. Due to the complexhydraulic behavior, these flumes should either correspondexactly with a flume that has been calibrated earlier or shouldbe calibrated before being put to use.
E-mail address: drarun [email protected].
0955-5986/$ - see front matter c© 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.flowmeasinst.2006.05.001
Keeping in view some of these difficulties, the proposedconstricted width flow meter having sudden expansion at thedownstream end and with a simplified geometry has beenconsidered for studying the flow characteristics under free andsubmerged flow conditions. The present research is in line withthe previous work reported by Hager [6–8], Samani et al. [9,10]and the ISO standard [11] for development of the proposed flowmeter.
2. Constricted width flow meter
In the proposed flow meter, the geometry of the device hasbeen simplified. It consists of a converging inlet section ofuniform width b (where b = B/2, B = the width of thechannel) as shown in Fig. 1, in which the transition to criticaldepth takes place. When the flow is subcritical, the reduction ofwidth of the channel causes a reduction in depth of flow at theconstricted section. By constriction of width to half, a criticalflow condition may be obtained by converting a major part ofthe upstream head into kinetic energy at the control section.There is a definite relation between the upstream depth of flowand discharge. Such a section with critical flow above the weircrest is required to make the upstream head (h1) independentof downstream conditions; Boiten [1]. This condition can beobserved by formation of the hydraulic jump immediatelydownstream of the control section.
In the proposed device, a sudden diversion is providedafter the throat portion and head losses are ignored. In
256 A. Goel / Flow Measurement and Instrumentation 17 (2006) 255–257
(a) Plan.
(b) Elevation.
Fig. 1. Layout of the structure.
non-modular flow conditions, both upstream and downstreamheads are measured for the purpose of discharge measurement.
2.1. Determination of discharge
The stage–discharge relation reads [1,11]
Q = (2/3)3/2(g)1/2C.Cdr.b.h1.501 (1)
where
Q = discharge (m3/s)
g = acceleration due to gravity (m/s2)
C = characteristic discharge coefficient (−)
Cdr = submerged flow coefficient (−)
b = throat width (m)
h1 = upstream measured head (m).
3. Description of the flume
The experiments have been conducted in a 12 m long, 40 cmwide and 60 cm deep steel flume having glasses on the sides forvisual observation and an 8 m long, 58.4 cm wide and 80 cmdeep concrete flume in the Hydraulic Laboratory of N.I.T.Kurukshetra. The model is made of concrete with a plasteredsurface. Water, after passing through the device installed in theflume, flows over a sharp crested weir for the measurement offlow. Baffle plates and wooden planks are provided in the flumeto dampen the waves generated due to the flow of water. Atailgate has been provided at the end of the flume to controlthe downstream depth of flow so as to create submerged flowconditions. In free flow conditions, the upstream depth of flow(h1) for free flow is measured. The tailgate is lowered toincrease the downstream depth of flow (h2). For each flowcondition, heads h1 and h2 are measured up to an accuracy of0.1 mm by an electrically operated point gauge.
4. Experimental procedure
The centrifugal pump is started and the inlet valve is opened.After some time, when a flow steady state condition is reached,observations of the head are taken upstream and downstreamof the device and over the sharp crested weir. As there is lot offluctuation of the water level, side openings of the Prandtl pitottubes are used for measurement of upstream and downstreamheads. The Prandtl pitot tube is kept facing the flow andthe static head is measured by connecting its side openingto the measuring glass cylinder kept outside the flume. Forfree flow, the tailgate is kept completely open for observingthe upstream head and then it is slightly lowered to measureboth upstream and downstream heads. The tailgate is loweredin stages and observations are taken for each condition. Aslowering of gate is continued, a situation is reached where for agiven downstream depth, the upstream head starts increasing,which is the modular limit. Similar experiments have beenconducted for other discharge values for free and submergedflow conditions. The device has been tested for a dischargerange from 0.002 m3/s to 0.039 m3/s as available in thelaboratory. Fig. 1 gives the layout of the structure.
5. Results and discussion
5.1. Modular limit
The modular limit (Sl) is a limiting condition between freeflow and submerged flow where the upstream depth (h1) juststarts increasing on further raising the downstream water levelfor a given discharge. Due to the constriction of flow at thethroat section, the upstream potential energy gets convertedinto kinetic energy under critical flow conditions. For modularflow, the upstream head is independent of the downstreamdepth of water flow. However, for non-modular flow, due to thearea reduction at the throat, the flow upstream of the controlsection remains subcritical, and subsequently the upstream headis influenced by the downstream flow conditions. Majumdarand Joshi [12] have developed some equations for calculatingthe modular limit based on inlet and outlet conditions and theconstriction ratio of the flow meter. These have been verifiedby them experimentally too. The head loss is assumed to be thesum of losses at the inlet and an outlet transition, which is dueto the formation of a hydraulic jump on the downstream sideof the device. The equation proposed by them is valid up to themodular limit. But as soon as the modular limit is reached, thetotal head loss becomes a sum of inlet and outlet losses andthere is no head loss due to the hydraulic jump.
For free flow, data collected in a 40 cm wide flume and58.4 cm wide field channel have been plotted, as shown inFig. 2, where C = f (h1/L), L is the throat length. Fig. 3 showsthe variation of the submerged flow coefficient Cdr = f (S) withthe submergence ratio S = 100h2/h1.
5.2. Limits of application
The following limitations are recommended to avoid surfacetension and viscosity effects on the device: h1 ≥ 0.05 m andb ≥ 0.20 m, and for submerged flow: 0.3 < h1/L < 0.8.
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Fig. 2. Discharge coefficient for free flow C = f (h1/L).
6. Conclusions
The following conclusions have been drawn from the presentstudy.
1. The proposed flow measuring device has simple geometryand that is why it is easy to fabricate.
2. The device is shorter in length and does not require bedslope, and hence is economical to fabricate as compared toother critical depth flumes.
3. The proposed flow meter has a high modular limit (Sl =
80%), which makes it practically more useful.4. The discharge can be measured for free and as well as
submerged flow conditions.
Acknowledgements
The author is sincerely grateful to the reviewers for givinginvaluable comments and suggestions for improvement to thepresent form of the manuscript.
References
[1] Boiten W. Flow measurement structures. Flow Measurement andInstrumentation 2002;(13):203–7.
[2] Parshall RL. The improved venturi flumes. Transactions, ASCE 1926;(89):841–80.
Fig. 3. Discharge coefficient for submerged flow Cdr = f (S = 100h2/h1).
[3] Ackers P. Weirs and flumes for flow measurement. New York: John Wileyand Sons; 1978.
[4] Bos MG. Discharge measurement structures. New Delhi: Oxford & IBHPub. Co; 1975.
[5] Skogerboe GV, Bennett RS, Walker WR. Generalized discharge relationsfor cut throat flumes. Journal of Irrigation and Drainage Engineering,ASCE 1972;98(4):569–83.
[6] Hager WH. Modified venturi channel. Journal of Irrigation and DrainageEngineering, ASCE 1985;111(1):19–35.
[7] Hager WH. Modified trapezoidal venturi channel. Journal of Irrigationand Drainage Engineering, ASCE 1986;112(3):225–41.
[8] Hager WH. Venuri flume of minimum space requirements. Journal ofIrrigation and Drainage Engineering, ASCE 1988;114(2):226–43.
[9] Samani Z, Magallanez H. Measuring water in trapezoidal canals. Journalof Irrigation and Drainage Engineering, ASCE 1993;119(1):181–6.
[10] Samani Z, Magallanez H. Simple flume for flow measurement in openchannels. Journal of Irrigation and Drainage Engineering, ASCE 2000;126(2):127–9.
[11] International Standards Organization (ISO). International Standard ISO9827-1994 Streamlined triangular profile weirs, section 9, Dischargecalculation. New Delhi: Bureau of Indian Standards; 1994.
[12] Majumdar SK, Joshi LM. Studies on critical submergence for flow meters.Irrigation and Power Journal 1981;38(2).
