بررسي آزمايشگاهي هيدرولي كسرريز نعل اسبي و تاثير طول سرريز در ضريب دبي

Experimental Investigation of Horseshoe-Shaped Spillway Hydraulics and Effect of Spillway Length on Discharge Coefficient

يكي از انواع سرريزهاي غيرخطي در جانمايي پلان سرريز نعل اسبي است كه تعيين پارامترهاي هيدروليكي اين نوع سرريزها در طراحي آنها ضروري ميباشد. گاهي به دليل محدوديت ساختگاه سد و جهت كاهش حجم حفاري، تونلو محل آبگيري نيروگاه برق آبي در مجاورت بدنه سد و سرريز ساخته ميشود. در چنين مواردي ميتوان به كمكسرريزهاي نعل اسبي، سازه مركبي از آن دو (آبگير و سرريز) را ايجاد نمود و صرفه اقتصادي مناسبي در ساختتاسيسات جانبي سد به وجود آورد. در اين تحقيق با ساخت مدل فيزيكي سرريز نعل اسبي در ابعاد آزمايشگاهي به بررسي مشخصات هيدروليكي سرريزهاي نعل اسبي از جمله ضريب دبي، سرعت و پروفيل جريان در قسمتهاي مختلف آن پرداخته شده است. نتايج ﺗﺤﻘﯿﻖ ﺣﺎﺿــﺮ ﻧﺸــﺎن داد ﮐﻪ در ﺳــﺮرﯾﺰﻫﺎي ﻧﻌﻞ اﺳــﺒﯽ، ﺑﺎ اﻓﺰاﯾﺶ ﻧﺴــﺒﺖ ﺑﺎر آﺑﯽ ﺑﻪ ﻃﻮل ﺳــﺮرﯾﺰ، ﻣﻘﺪار ﺿــﺮﯾﺐ دﺑﯽ در ﻣﺤﺪوده0/6 اﻟﯽ 0/7ﮐﺎﻫﺶ ﭘﯿﺪا ﻣﯽﮐﻨﺪ. اﯾﻦ ﮐﺎﻫﺶ ﺿــﺮﯾﺐ دﺑﯽ ﺑﺮاي ﺳــﺮرﯾﺰ ﺑﺎ ﻃﻮل ﮐﻮﭼﮑﺘﺮ از112 ﺳــﺎﻧﺘﯽﻣﺘﺮ و ﺑﺰرﮔﺘﺮ از آن روﻧﺪ ﻣﺘﻔﺎوﺗﯽ دارد. ﻣﻘﺎﯾﺴــﻪ ﻧﺘﺎﯾﺞ ﺑﻪدﺳــﺖ آﻣﺪه از آزﻣﺎﯾﺶﻫﺎ ﺑﺮاي ﺳــﺮرﯾﺰ ﻣﺴـﺘﻄﯿﻠﯽ ﻫﻢ ﻋﺮض ﺳﺮرﯾﺰ ﻧﻌﻞ اﺳﺒﯽ ﻣﺆﯾﺪ آن اﺳﺖ ﮐﻪ ﺳﺮرﯾﺰ ﻧﻌﻞ اﺳﺒﯽ ﻗﺎدر اﺳﺖ ﺑﺎر آﺑﯽ روي ﺳﺮرﯾﺰ را ﺑﻪ ﻣﻘﺪار 54 درﺻـﺪ ﮐﺎﻫﺶ دﻫﺪ. ﺑﺮرﺳـﯽ ﭘﺮوﻓﯿﻞ ﺳﻄﺢ آب ﻧﺸﺎن ﻣﯽ دﻫﺪ ﮐﻪ در اﺛﺮ ﺑﺮﺧﻮرد ﺟﺮﯾﺎنﻫﺎي ﺳﺮرﯾﺰ ﺷﻮﻧﺪه ﺑﻪداﺧﻞ ﺳﺮرﯾﺰ ﻧﻌﻞ اﺳﺒﯽ، ﭘﺮش ﻫﯿﺪروﻟﯿﮑﯽ از ﻧﻮع ﭘﺮش دم ﺧﺮوﺳﯽ ﺑﻪوﺟﻮد ﻣﯽآﯾﺪ.

One of the non-linear weirs is horseshoe-shaped weir that determination of its hydraulic parameters for designing of that is necessary. Sometime due to limitation in dam site and also reduction in soil excavation volume, tunnel and intake location of hydro-electric power this structure is needed to be constructed near the dam and its intake. In these cases, a combined structure of both the intake and spillway can be constructed by using horseshoe-shaped weir to achieve a proper economic condition in dam construction. In this study, by making physical models of the horseshoeshaped spillway in laboratory dimension, some hydraulic parameters like discharge coefficient, velocity and water surface profiles in different parts of the horseshoe-shaped spillway were investigated. Results showed that in horseshoe-shaped spillways, with increasing the ratio of head to spillway length, discharge coefficient was decreased about 0.6 to 0.7. This reduction in discharge coefficient had different trends for spillways with length of greater or less than 112cm. Comparison of the results between the rectangular weir and the horseshoe-shaped spillway by consideration an equal width for them showed that horseshoe-shaped spillway could reduce head above the weir about 54%. Investigation of the water surface profiles showed that overflowing of waters into the horseshoe spillway, created the rooster tail type hydraulic jump.