بررسي آزمايشگاهي عملكرد هيدروليكي سرريز اوجي و كانال پايين¬دست در شرايط قوس محوري

Laboratory Study of Hydraulic Performance of Ogee Spillway and Downstream Canal with Axial Arc

سرریز¬ها یكی از سازه¬هایمتداول برای تخلیه و اندازه‌‌گیری جریان می¬باشند. به همین جهت این نوع سازه¬های هیدرولیكی بسته به هدف از استفاده، دارای اشكال مختلفی هستند. در برخی از موارد به دلیلمحدودیت¬‌های اجرایی، سرریز¬های باانحنا در پلان ساخته می¬شوند. در چنین شرایطی مطالعه توزیع جریان در طول سرریز و دیگر پارامتر¬های مربوط به آن، حائز اهمیت خواهد بود. در این پژوهش یك مدل فیزیكی از سرریز سد كه از نوع اوجی آزاد با پلان قوسی است، مورد آزمایش قرار گرفت. هم¬چنین به¬منظور بررسی اثر انحنای سرریز بر عملكرد جریان، مدل دوم سرریز در شكل نرمال و با شرایط هندسی و هیدرولیكی مشابه، مورد مقایسه قرار گرفت. نتایج مربوط به مدل اول نشان داد كه با افزایش نسبت هد جریان به هد طراحی ضریب دبی سرریز تا مقدار 72/1 افزایش یافته و پس از آن استغراق سرریز رخ داده و ضریب دبی تا مقدار 23/1 كاهش می‌یابد. هم‌¬چنین با افزایش سرعت در هر دبی و كاهش فشار در طول سرریز، امكان خلأ¬زایی و خوردگی سازه افزایش و ضریب خوردگی كه معرف آن شاخص خلأ¬زایی می¬باشد، كاهش یافت. بررسی نتایج مربوط به كارایی سرریز نشان داد برای مدل سرریز در شكل نرمال، استغراق سریع¬تر و به‌ ازای مقادیر دبی یكسان با سرریز قوسی، مقدار ضریب دبی برای سرریز درشكل نرمال، كمتر خواهد بود. در این پژوهش میزان تأثیر افزایش ضریب دبی در سرریز با قوس محوری در مقایسه با سرریز با تاج مستقیم و در شرایط هیدرولیكی و هندسی مشابه، برابر 21 درصد محاسبه شد.

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.