T. Eshrati; R. Fazloula; M. Sanei; A. Emadi
Abstract
Introduction: Weirs are one of the common structures for discharge and flow measurement. Therefore,these types of hydraulic structures depending on the purposeofuse havedifferent shapes. In some cases, due to practical constraints, spillways with curvature in plan are designed. In such situations study ...
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Introduction: Weirs are one of the common structures for discharge and flow measurement. Therefore,these types of hydraulic structures depending on the purposeofuse havedifferent shapes. In some cases, due to practical constraints, spillways with curvature in plan are designed. In such situations study of flow distribution over the spillway and other related parameters, will be important. In this study, a physical model of dam spillway, which is type of ogee-crested weir with curvature in plan, were tested. Also in order to investigate the effects of curvature on the performance of the flow, the second model of spillway in normal shape, with similar geometric and hydraulic conditions, were compared.
Materials and Methods: First physical model of prototype is built at the scale of 1:75 and the second model was constructed in straight form (without curvature in plan) with similar geometric conditions to the first model. Spillways have been designed according to USBR standard for design head at value of 4 cm in model and vertical upstream face. Experiments were performed in Soil Conservation and Watershed Management Research Institute at reservoir with dimensions 1.2 m length, 0.70 m width and 0.5 m height walls of Plexiglas. To measure the flow discharge, a sharp triangular weir with apex angle of 90˚ in the output of channle was used. Measurements in first model were conducted with five discharges and five values of h/Hd (0.53, 0.74, 0.90, 1.08 and 1.44) and for six sectors on spillway body. To evaluate the effect of curvature, flow performance and discharge coefficient changes were compared for five early discharges (ratio of critical depth to design head at value of 0.28, 0.44, 0.58, 0.72 and 0.81) and six other discharge (mentioned ratio at value of 0.36, 0.51, 0.66, 0.76 and 0.83) in both models.
Results and Discussion: The results related to the first model showed that by increasing the ratio of head to design head (h/Hd), rate of spillway discharge coefficient increases to the value of 1.72 and decreases to 1.23, when the weir was submerged. It also observed that with increase in flow rate of each discharge and reducing the pressure along the spillway, possibility of vacuum-creation and corrosion of structure increased and the corrosion rate witch introduced by Cavitation Index decreased. The minimum value for Cavitation Index that has been calculated was 1.45 that is greater than the critical value of it. The results of the pressure distribution and changes of Cavitation Index in first model showed, the minimum height of the pressure for each discharge occurred at the end of ogee profile and the minimum value of the Cavitation Index occurred at the last section of spillway in downstream for the value of h/Hd=0.53.As well as for all test cases in this study, the maximum velocity and minimum of Cavitation Index were calculated at the same section of spillway where hydraulic jump was observed. On the other hand, it was observed that with increasing flow rate, the critical section moves upward on the spillway body. The results related to the spillway efficiency generally indicated that by increasing the ratio of critical depth to design head (yc/Hd) discharge coefficient increases. In fact, by increasing the ratio of h/Hd and increase the discharge rate up to design discharge, the amount of evacuation and efficiency of both models goes up. For larger discharges, the flow is blocked by the spillway forehead and model efficiency will decrease due to submersion and flow rejection. Results obtained from comparison of two models indicated for the spillway in normal shape submergence of the weir occurred faster and discharge coefficient of each test achieved in lesser value per test, So that the discharge coefficient increasing in curved spillway continued until the value of 0.81 yc/Hd (at 10.3 lit/s of discharge) and in normal shape until the value of 0.72 yc/Hd (at 9.2 lit/s of discharge). Therefore it seems that the upward central arch factor will increase the discharge coefficient and efficiency of spillway.
Conclusion: In the present research the hydraulic performance of ogee spillway with curved plan to investigate the pressure distribution and the vacuum-creation and in particular to compare the results related to flow performance and the effect of spillway curvature on its performance were studied using two experimental models. The impact of increasing the discharge coefficient for the weir with upward central arch compared to weir with straight crest, in terms of similar geometric and hydraulic conditions, was calculated to the value of 21 percent, in this study.
S. Riahi; A.R. Vatankhah
Abstract
Introduction: Side weir structures are extensively used in hydraulic engineering, irrigation and environmental engineering, and it usually consists of a main weir and a lateral channel. Side weirs are also used as an emergency structure. This structure is installed on one side or both sides of the main ...
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Introduction: Side weir structures are extensively used in hydraulic engineering, irrigation and environmental engineering, and it usually consists of a main weir and a lateral channel. Side weirs are also used as an emergency structure. This structure is installed on one side or both sides of the main channel to divert the flow from the main channel to the side channel. Lateral outflow takes place when the water surface in the main channel rises above the weir sill. Flow over a side weir is a typical case of spatially varied flow with decreasing discharge. There have been extensive studies on side weir overflows. Most of the previous theoretical analysis and experimental research works are related to the flow over rectangular side weirs in rectangular main channels. In the current study, the flow conditions over a trapezoidal side weir located in a rectangular main channel in subcritical flow regime is considered.
Materials and Methods: The experiments were performed in a rectangular open channel having provisions for a side weir at one side of the channel. The main channel was horizontal with 12 m length, 0.25 m width, and 0.5 m height, and it was installed on a frame; lateral channel that has a length of 6 m, width of 0.25 m, and height of 1 m. It was set up parallel to the main channel; walls and its bed were made up of Plexiglas plates. The side weir was positioned at a distance of 6 m from the channel’s entrance. A total of 121 experiments on trapezoidal side weirs were carried out.
Results and Discussion: For trapezoidal side weir, effective non-dimensionnal parameters were identified using dimensional analysis and Buckingham's Pi-Theorem. Finally, the following non-dimensional parameters were considered as the most effective ones on the discharge coefficient of the trapezoidal side weir flow.
in which Fr1= upstream Froude number, P= hight of the trapezoidal side weir, y1= upstream water depth, z=side slope of the trapezoidal side weir and T=top flow width of the trapezoidal side weir. Water surface profiles were measured along the weir crest, the main channel centerline, and far from the weir section. Different elevations in water surface profile depend on the upstream Froude number in the main channel; depth differences in low Froude numbers are at minimum values, and in high Froude numbers are at maximum amounts. The water surface level along the crest drops at the entrance of the side weir to the first half of the side weir; and it has been attributed to the side weir entrance effect at the upstream. Afterwards, the water level rises towards the downstream of the weir. According to the experimental results, measurements of the water in the centerline of the main channel are reliable and water surface drop is negligible. According to the parameters affecting the discharge coefficient for each value of z, discharge coefficient equations were developed with acceptable accuracy such that the effects of this parameter were shown separately. Finally, the general equation was proposed. The general functional form for discharge coefficient is presented as follows where the effect of the side slope parameter, z, is also considered.
The mean and maximum percentage errors of the discharge coefficient computed using the proposed equation are as 2.6% and 11.5% , respectively.
Conclusion: In this study, the characteristics of trapezoidal side weir overflows in subcritical flow regime were discussed. For this purpose, experimental data related to the water surface profile of the side weir and discharge coefficient were collected and analyzed. The results showed that the most efficient section for measuring water surface profile is located at the center line of the main channel. It was found that for trapezoidal side weir, the discharge coefficient depends on the Froude number, the ratio of crest height to initial depth, the overflow length to initial depth, and the side slope of the weir. In this study, conventional trapezoidal weir theory has been used in order to evaluate the discharge coefficient and provide side weir discharge equation. For this purpose, three reference depths were considered for conventional weir, and for each depth an equation was developed for the discharge coefficient. Comparison between predicted values and experimental data showed that average flow depth results in accurate outcomes for assessing the discharge coefficient. The average value of error for discharge coefficient estimation by the proposed equation is 2.6%. Thus this equation is proposed for use in practice by water engineers.
Keywords: Control structure, Conventional weir, Discharge coefficient, Spatially varied flow, Trapezoidal side weir, Water surface profile
F. Alizadeh Sanami; M. Masoudian; M. Saneie
Abstract
Introduction: Because of the fairly simple equations for accurate flow measurement and controlling the water level, weir and gate method is more useful than the separate weir, gate and partial flume methods. Since the flowing water in the channel always contains sediment particles and floating debris, ...
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Introduction: Because of the fairly simple equations for accurate flow measurement and controlling the water level, weir and gate method is more useful than the separate weir, gate and partial flume methods. Since the flowing water in the channel always contains sediment particles and floating debris, they are deposited at the gate inlets and behind the weirs which reduces the size of the channel in the structure range and which reduces some problems such as neighboring land flooding due to overflow of water from the channel banks, threatening the structure stability and reducing the measurement accuracy. Using a combination of weir- gate model, in comparison with other conventional devices, will make it possible to get the actual conditions closer two main hypotheses derived from the relations and accurately measure the discharge coefficient. In this model, the deposited materials are easily passed through the gates and the suspended debris are easily passed over the weirs. One of the combined weir- gate structures is semi cylindrical weir- gate structure. Regarding about the form of the combined weir- gate structures, it has some advantages , including simple design, sediments and floating material flow, high flow discharge coefficient compared with other replaceable structures and its being economic. Semi Cylindrical gate turning around center Axis, for reason of rotation the center becomes Conversion to wire, wire gate with opening with different height.
Materials and Methods: The experiments were conducted in a rectangular flume with the length of 8 m, width of 0.282 m and height of 0.3 m in Soil Conservation and Watershed Management Research Institute. In this research, PVC pipes were used as semi cylindrical gate structures. The experiments were conducted for three diameters 70, 120 and 160 mm with height of the opening between zeros until radius, angles zero, 30, 45, 60 and 90 degree and differently discharging. Experiments were performed at a discharge limit of 2-27 l/s. In order to decrease turbulence of the flow, the gate was installed at the end 4 m of the flume. The ratio of cylindrical structure diameter to channel width (D/B) was in the range of 0.25 to 0.57 and the Froude number was in the range of 0.08 to 0.55.
Results and Discussion: Coefficient discharge of semi cylindrical structure and then dimensionless parameters of [H/P], [a/H], [ /H] and [Fr] against the discharge coefficient in the studied gate opening between zero until radius were investigated. According to result with decrease dimensionless parameter of a/H, discharge coefficient increased, So that the maximum Coefficient discharge rate of angle 90 degrees and minimum angle 0 degrees. Also in a constant of a/H for the curvature of the downstream and upstream, with increasing the diameter of semi cylinder, discharge coefficient remains and has no change, shows that changes in diameter of semi cylinder have no significant impact on discharge coefficient. With increasing H/P for both curves upstream and the downstream, discharge coefficient increased. Also in a constant of H/P for the curvature of the downstream and upstream, with increasing the diameter of semi cylinder, discharge coefficient remains. With constant of angles with increasing Freud for all angles, both the curvature of the downstream and upstream, /H decreased. Also in a constant of Freud and angles, with increasing the diameter of semi cylinder, /H remains. According to the result, discharge coefficient of semi cylindrical gates varies, in Experimental limit, from 0.45 to 1.45 which is more than that of sluice gates reported by the USBR. One of the reasons, this is ascribed to this is the difference between the amount of entrance, head loss in this structure, because when the flow approaches cylindrical gate, due to curvedness of the wall upstream, a gradual gathering of flow lines gives the aerodynamic method to entrance section, thus decreasing resistance against the flow and entrance head loss and increasing discharge coefficient. However, in sluice gates the vertical wall in entrance section is conducive to fast gathering of flow line, thereby increasing resistance against flow, increasing entrance head loss, and decreasing discharge coefficient relative to a semi cylindrical method.
Conclusion: The results showed that increasing Froude coefficient and decreasing the a/H (ratio of gate opening to upstream water depth) dimensionless parameter decrease, respectively increase the discharge coefficient and decrease head loss in all aspects of structural alignment. Within addition by increasing the H/P dimensionless parameter (ratio of upstream water depth to structure diameter) discharge coefficient increased. Results showed that the maximum and minimum values of discharge coefficient are related respectively to 90 degree and 0 degree angle.
S. Pouresmaeil; M.F. Maghrebi
Abstract
Bottom intakes are hydraulic structures widely adopted for diverting water in steep rivers. The problems of vibration, corrosion, deformity, and clogging of the bottom racks by sediment, freezing and foliage inspire a new system of bottom intake in which a filled trench of porous media replaces the bottom ...
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Bottom intakes are hydraulic structures widely adopted for diverting water in steep rivers. The problems of vibration, corrosion, deformity, and clogging of the bottom racks by sediment, freezing and foliage inspire a new system of bottom intake in which a filled trench of porous media replaces the bottom racks. In the present paper we have analyzed the data obtained from a systematic series of experiments carried out in a laboratory two-story flume. Measurements of the diverted discharge were performed for different rates of flow, grain size distributions, surface slopes and dimensions of intake as well as important factors. Also the relationships presented in previous studies, are compared with the results of new experiments and the characteristics of the physical behavior of these equations are investigated. Due to lack of the validity in previous models, a new equation for estimating discharge coefficient and flow diversion in porous bottom intake is presented, by using dimensional analysis and multivariate regression with experimental observations. The robustness of the proposed relationship is confirmed by the principles of hydraulic behavior of this intake and the comparison between the calculated discharges through the proposed relationship and those measured in an extensive series of experiments, characterized by ranges of the relevant flow parameters much larger than those investigated in the present contribution.
Y. Aminpour; M. Yasi; J. Farhoudi; H. Khalili Shayan
Abstract
The development of an enhanced approach for the use of radial gates as flow measurement structures is important in irrigation networks. In this study, new theoretical relationships were developed to estimate the discharge coefficient (Cd) for a single radial gate with three different sills, at free and ...
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The development of an enhanced approach for the use of radial gates as flow measurement structures is important in irrigation networks. In this study, new theoretical relationships were developed to estimate the discharge coefficient (Cd) for a single radial gate with three different sills, at free and submerged flow conditions. These equations were calibrated and verified by using about 2600 laboratory data from the world-wide literature. Results indicated that the flow rate under the radial gates can be estimated by an error in the order of ±5%. The reliability of the proposed relationships and in particular the scale effects, were tested using 530 field data of radial gates operating on different canal networks. The predictions of the flow rates from the proposed method are shown to be superior compare with the other predictive methods. In the presence of multi radial gates in a given cross section, the total discharge is estimated by an error up to ±30% when using single radial gate relationships. This discrepancy is considered to be mainly due to the influence of different gate openingsand the difference between gate and canal widths. A self-developed correction factor, k, was introduced to account for the dimensionless effective parameters such as the ratio of gate-to-canal width, the geometry of the gates, and the ratios of upstream and downstream depths to the average gates openings. The results are promising the predictive errors of the total flow rates are reduced by ±5% and ±10% for 74% and 94% of the flow data, respectively.
A. Masoodi; P. Parsamehr; F. Salmasi; S. Pureskandar
Abstract
Compound broad-crested weir, have a small inner rectangular section for measuring low flows and then, they broaden to a wide rectangular section at higher flow depths. This paper presents data that will be of use in the design of hydraulic structures for flow control and measurement. A series of laboratory ...
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Compound broad-crested weir, have a small inner rectangular section for measuring low flows and then, they broaden to a wide rectangular section at higher flow depths. This paper presents data that will be of use in the design of hydraulic structures for flow control and measurement. A series of laboratory experiments was performed in order to investigate the effects of length of the lower weir crest and step height of broad-crested weirs of rectangular compound cross section on the values of the discharge coefficient. For this purpose, 15 different broad-crested weir models with rectangular compound cross sections for a wide range of discharges tested. Multiple regression equations based on dimensional analysis theory were developed for computing discharge coefficient. The results of compound broad-crested weirs were compared with Genetic programming (GP) and Artificial neural network (ANN) and it was found that the ANN formulation of the problem of solving for the discharge coefficient is less successful than that by GP. The implementation of GP offers another formulation for discharge coefficient.
