تاثیر کاهش 20 درصدی طول هم‌پوشانی صفحات مستغرق بالادستی پایه پل بر توپوگرافی بستر در قوس تند 180 درجه

روش‌های مختلفی برای کاهش آبشستگی موضعی اطراف پایه پل ارائه شده است .استفاده از صفحات مستغرق (submerged vanes) از جمله روش‌هایی است که مورد توجه محققین قرار می‌گیرد. صفحات مستغرق سازه‌های مستطیلی نازکی هستند که به لحاظ داشتن زاویه با جهت جریان ناحیه پرفشار و کم‌فشار در دو طرف آن‌ها ایجاد می‌شود. این سازه‌ها باعث ایجاد گرداب ثانویه و تغییر الگوی جریان در کف رودخانه و در نتیجه تغییر انتقال رسوب و آبشستگی می‌شوند. عملکرد صفحات مستغرق برای کاهش آبشستگی پایه پل و حرکت رسوب از اطراف صفحات به علت ایجاد جریان رو به پایین در جلوی آن‌ها تحت تاثیر نحوه قرارگیری صفحات می‌باشد. این صفحات در مقطع عرضی کانال طراحی و با زاویه کوچکی نسبت به جهت جریان به صورت عمود در بستر نصب می‌شوند. در این تحقیق به منظور کاهش آبشستگی موضعی پیرامون پایه پل منفرد مستقر در راس قوس، از صفحات مستغرق استفاده گردید. بدین منظور پایه پل استوانه‌ای شکل به قطر 5 سانتی‌متر در کانال آزمایشگاه با قوس 180 درجه تند، عرض یک متر در شرایط آب زلال با u/u_c =98.0 با دبی70 لیتربرثانیه و عمق 18 سانتی‌متر انجام گردید. صفحات مستغرق نامبرده از جنس پلکسی‌گلس با عرض 1.5 برابر قطر پایه، ضخامت یک سانتی‌متر و کارگذاری در زاویه 25 درجه نسبت به افق با ارتفاع خارج از بستر 4.5 سانتی‌متر استفاده گردید. براساس نتایج این تحقیق کاهش 20 درصدی طول هم‌پوشانی صفحات مستغرق عمق حفره آبشستگی اطراف پایه را 30 درصد کاهش می‌دهند.

The reduction effect of 20% overlapping length of the upstream submerged vanes on topography of the sharp bend with bridge pier

Local scour around bridge pier is one of the important problem in bridge stability against water flow that is created due to hole scour around bridge pier and castrates its stability. So as, providing methods for control and scour reduction around bridge pier have very importance. Yet different methods have been proposed for preventing and reducing the local scouring around the bridge pier. Use of Submerged Vanes is one of the methods that researchers are interested in. Submerged plates are thin rectangular structures that because of having angle with the flow direction, is created high pressure and low pressure region on both sides. These structures create a secondary vortex and change the flow pattern in the river bed and as a result, change the sediment and scour transmission. Submerged vanes performance to reduce the bridge pier scouring the sediment movement around the vanes due to the downward flow in front of them, are affected by the layout of the plates. These vanes are designed in cross section of the channel and fixed with a little angle relative to flow direction perpendicularly on bed. In this research, submerged vanes were used to reduce the local scour around the single bridged pier located in bend head. Effective variables in this study were vanes overlapping length, space of submerged vanes from bridge pier center and space of submerged vanes from each other perpendicular state on flow direction. For this purpose, a cylindrical bridge pier to diameter 5 cm in the laboratory was performed with a sharp bend 180 degree with central radius to channel width ratio equal 2, a width 1 meter and height 90 cm with straight direction length of bend upstream 6.5m and straight direction length of bend downstream 5m in clear water conditions( u) frasl;u_c =0/98 with water discharge of 70 liters per second and depth 18cm. The submerged vanes were Plexi Glass with width 1.5 equal pier diagonal (7.5cm), thickness equal 20% pier diagonal and located in 25 angle relative to horizon with height of out of bed 4.5cm (submergence percent 0.75) with sitting vanes center in the middle of channel (50% channel width from inner shore, with overlapping length (Lcv) 80 and 100% and intervals of submerged vanes from each other perpendicularly on flow direction 5 and 10cm (Lvv/D)=1.2 in position 83 and 79.5 degree from beginning of the bend (interval equal 5 and 7.5 pier diameter toward upstream from located pier site) (Lvp/D=5, 7.5). According to the results of this study, the 20 percent reduction in the overlapping length of the submerged vanes decreases the depth of the scour hole around the bridge pier about 30 percent. Also use of submerged vanes, is transferred location of maximum scour depth to a place near than pier and limited width of scour hole around pier. Furthermore, submerged vanes limit sedimentation region to 012% channel width from inner shore and in 70127 degree from the beginning of the bend. Minimum and maximum scour in hole around pier with submerged vanes is created 0.53 and 0.65 equal flow depth in the beginning of the bend in V8052 and V1007.51 Tests, respectively. The discussion about the results of this research is from the other parts of this article.