بررسي آزمايشگاهي عملكرد هيدروليكي سرريزهاي قوسي

Exprimental Investigation of the Hydraulic Performance of Arced Weirs

سرريزها نقش تعيين كننده اي در ايمني سدها دارند و بخش زيادي از هزينه‌هاي مربوط به ساخت سد را به خود اختصاص مي‌دهند. براي اطمينان از ايمني سدها، كارشناسان مجبور به انتخاب سيلاب هاي با دوره بازگشت بالا، به عنوان سيلاب طراحي سرريزها است. سرريز قوسي، كماني از يك دايره است كه افزايش طول موثر تاج سرريز را در يك عرض مشخص ممكن مي‌سازد و بدين ترتيب مي‌توان به ازاي يك بار هيدروليكي يكسان، دبي بيشتري را از سرريز عبور داد. در اين مقاله، ضريب آبگذري و دبي عبوري سرريز قوسي قرار گرفته در داخل مخزن به صورت آزمايشگاهي مورد بررسي قرار گرفته است. ضريب آبگذري اين سرريزها به عنوان تابعي از H0/P و ? ارايه و عملكرد هيدروليكي اين سرريزها با سرريز خطي مقايسه شده است. نتايج نشان مي‌دهد كه به ازاي بار هيدروليكي يكسان، افزايش ? منجر به كاهش ضريب آبگذري مي‌شود. با اين حال افزايش طول موثر سرريز در نتيجه قوسي كردن آن، مي‌تواند بهبود راندمان سرريزهاي قوسي را تا حدود 45% در پي داشته باشد. با افزايش بار هيدروليكي، ارتفاع پشته جريان در پايين‌دست سرريزهاي قوسي، افزايش يافته كه اين امر كاهش راندمان سرريزهاي قوسي را در پي دارد. در پايان بر اساس نتايج و محدوديت‌هاي پژوهش حاضر، روشي براي طراحي سرريزهاي قوسي قرار گرفته در داخل مخزن ارايه شده است.

Weirshave important roles in dam safety and should spill floods with high return period. The designers enhance the width of the weirs to increase the discharge capacity. In some cases, this procedure has topography and economic limitations. Arced weirs can be considered as an alternative. In plan-view, arced weir is a part of a circle that increases the crest length for a given channel width that increases the flow capacity for similar heads. This structure is recommended also for modification and increasing the capacity of the existing spillways. In this paper, the hydraulic performance of arced weirs located in a reservoir has been studied experimentally. In the first step, dimensionless parameters affecting the performance of arced weirs is introduced using Buckingham ? theorem. Then effect of arc angle (?) and head water ratio (H0/P) on hydraulic performance of arced weirs was experimentally investigated and hydraulic performance of the tested arced weir geometries was compared with a linear configuration. For this purpose,arches with different radius of curvature from linear to semi-circular configurations (00 ? ? ? 1800) and various head water ratio (0.4 ? H0/P? 1.6)were studied. To simulate reservoir conditions, a reservoir simulator was designed and built.Laboratory observations show that the converging of flow over an arced weir causes a locally bulge in the downstream of the weir. This phenomenon was named as flow mound. Results show that arc angle (?) and head water ratio (H0/P) have strong effect on theflow mound and by increasing these parameters the mound height increases. The head-discharge relationship for arced weirs was derived using general form of the rectangular weir equation. Data from physical models were used to determine discharge and upstream head for the flat crested weirs installed in the reservoir. From discharge curves, it was found thatby increasing the weir angle, leads to increasing the crest length for a given channel width, flow capacity increases for a similar upstream head. Discharge coefficients as a function of H0/P for arced weirs are also presented and compared with linear configuration.The results show thatwith increasing H0/P,discharge coefficient is declined for each configuration. Also with increasing?,that leads to greater convergence of flow passing over the weirs, discharge coefficient decreases.Efficiency parameter is defined as the ratio of discharge of arced weir to that of liner weir with the same width. From efficiency curves indicates that the semi-circular weir can improve efficiency up to about 45%.However for all tested weirs, efficiency decreases with increasing H0/P and it reaches to 1. Finally, based on the results and limitations of this study, a method is presented for the design of arced weirs located in the reservoir. By using this method, the designer can find the geometric parameters of the arced weir for passing a certain flow rate in a given hydraulic head.