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.
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.
R. Badizadegan; K. Esmaili; M. Faghfour Maghrebi; M. Saneie
Abstract
Abstract
One of the most frequently encountered cases of rapid varied flow is the hydraulic jump. It occurs when a supercritical open channel flow changes into sub critical flow. In the present research, the experimental study of the hydraulic jump on seven sinusoidal corrugated beds with different ...
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Abstract
One of the most frequently encountered cases of rapid varied flow is the hydraulic jump. It occurs when a supercritical open channel flow changes into sub critical flow. In the present research, the experimental study of the hydraulic jump on seven sinusoidal corrugated beds with different wave steepness. The wave steepness of sinusoidal corrugated beds is in the range of 0.1667 to 0.75 and Froude number was in the range of 4.5 to 12.26. The results showed that the tail water depth of a jump on a corrugated bed is about 20% smaller than that on smooth bed in Froude number 12 and the length of jump on corrugated beds is about 35% less than that for smooth bed.
Keywords: Hydraulic jump, Corrugated bed, Wave steepness
