كليدواژه :
الگوي جريان , سرعت سنج , ميدان سه بعدي , طوق مشبك نامتقارن
چكيده فارسي :
تاكنون مطالعات مختلفي در خصوص شناخت مكانيزم آبشستگي اطراف سازه‌هاي هيدروليكي و بخصوص پل‌ها انجام شده است. در زمينه آبشستگي اطراف پل‌ها تمركز پژوهش‌ها بيشتر بر روي پايه بوده، ولي به نتايج اثر بخشي در مورد پديده آبشستگي پيرامون پايه‌ها دست نيافته‌اند. بررسي منابع نشان مي‌دهد، كه شكل پايه بر ميزان آبشستگي پيرامون آن تاثير به سزايي دارد، و در بين اشكال مختلف پايه، پايه استوانه‌اي بيشترين ميزان آبشستگي را دارد. بنابراين در اين پژوهش با انتخاب شكل استوانه‌اي براي پايه پل با مقطع دايره‌اي، علاوه بر بررسي تاثير عدد فرود در دو زمان 13 و 5 ساعت، بر ميزان آبشستگي پيرامون آن مورد بررسي قرار گرفت. نتايج نشان داد، كه با افزايش عدد فرود در زمان‌هاي نام برده، عمق آبشستگي افزايش و بر وسعت چاله آبشستگي افزوده مي‌شود. همچنين با افزايش ارتفاع تراز نصب طوقه بر روي پايه پل، عمق آبشستگي و وسعت چاله آبشستگي افزايش مي‌يابد. در ادامه اين پژوهش مولفه‌هاي سه بعدي سرعت جريان پيرامون پايه با استفاده از سرعت‌سنج ADV تعيين و براي رسم ميدان جريان مورد استفاده قرار گرفتند. بررسي آن نشان داد، سرعت بيشينه در نزديكي بستر و بالاي گودال تشكيل مي‌شود، كه دليل آن تقويت جريان‌هاي ثانويه است، و گردابه‌هاي عمودي در نقطه جلوي پايه شكل گرفته كه عامل اصلي آبشستگي و توسعه آن پيرامون پايه پل مي‌باشد.
چكيده لاتين :
Introduction: Investigation of the scour phenomenon is very important. If it is not possible to control the scour around structures such as bridges or bridge supports. Includes irreparable damages. The scour around the bridge piers causes to instability of them, and without applying an appropriate solution, it eventually leads to demolition of the structure. Therefore, study on the mechanism of the occurrence of the scour and the effective parameters on amount of scour are important. So, already various studies have been done on the mechanism of scour around hydraulic structures especially bridges. In the field of scour around bridges, researches are more focused on scour of piers, but no effective results were obtained on the scour phenomenon around the piers. Bed erosion and transport of sand material from its location by a flow called scour. Local scour is a special type of scour that may occur around the bridge piers or bridge abutments. This type of scour is main reason of many bridge failures in the word. Because of this, supply of methods to control and reduce these phenomena is important. One of the methods of scour reduction around the bridge pier or abutment is installation a thin flat rigid plate (collar) on the pier or abutment. There is no comprehensive study to use this method for protecting the pier against scour so far. Therefore, this topic was considered for this research.
Methodology: This study was performed in the flume with a length of 6 m, a width of 0.72 m, a height of 0.6 m and constant bed slope equal to 0 m in Hydraulic Laboratory of Shahid Chamran University. The bed materials were non cohesive sediment with an average diameter equal to 0.73 mm and geometric standard deviation of 1.22. As well as Plexiglas plates with a thickness of 3 mm was used to build the collar. In this study, 27 tests were performed to measured, sediment movement and determine the two-dimensional velocity components. In each section, a series of tests were performed as a control experiment. The tests of sediment were done in the 3 of flow rate equal to 25, 30 and 35 liters per second. In this condition, Froude Number was equal to 0.26, 0.32 and 0.37 respectively. The 3 unsymmetrical collars with different dimensions in four three Numbers were tested. To do so first general non dimensional relationship was developed. Then series of experimental tests were conducted in a physical model using three different Zc (0, 0.25 and 0.5 high).
Results and discussion: Dimensionless plots were obtained regarding effect of dimension of both collars on scour reduction around bridge pier. Different positions of installation of collar were investigated. Dimensionless plots were obtained for determination of collar performance in various heights. Different positions of installation of collar were investigated. Dimensionless plots were obtained for determination of collar performance in various heights. In this section, one of the collars had the best performance, and it was called optimum collar. Local scour around a bridge pier results from the flow and pier interaction and separation of the flow at the sides of the pier. One of the methods used as a local scour countermeasure at bridge piers is collar. Then, we have been check about netted unsymmetrical collar. Also two tests were performed to determine three-dimensional components of velocity in different depth. The results show that the performance of collar in unsymmetrical collar improve by increasing its dimension. The collars located on the bed performance are better than the one located above the bed.
Conclusion: The results show that the collars have important role in retardation of scour development. In section of determination of three-dimensional components of velocity, the results show that the collar act as a shield against down flow. It can control the horseshoe scour around the pier. Literature review shows that cylinders forms, the maximum scour depth occurs in the case of cylindrical pier. Therefore, in this research cylindrical shape for circular bridge pier was selected and effects of Froude number were investigated on scour development. Result shows, that increasing froude numbers will increase the amount scour. Also, as the height of the collar mounting height is increased, the scour depth and the width of the scour hole increase. Following this research, three-dimensional components of velocities were determined with adv. velocimetry and then use for drawing flow pattern. Also result confirms that, maximum velocity occurs near the bed, because the down flow developed the significant secondary currents, and down flow and generated vortex are effective parameters on bridge scour.
Keywords: Flow pattern, Laboratory model, Scour, Bridge pier.
