مطالعه آزمايشگاهي تغييرات زماني بستر در قوس 081 درجه تند با زوج پايه همگرا

Effect of Time Duration on Bed Topography With Twin Convergent Bridge Piers at 180 Degree Steep Bend

تخريب پل ها به دليل فرسايش بستر مساله اي است كه اگر به آن به درستي پرداخته نشود مي تواند خسارات جبران ناپذيري به بار آورد. هدف از ارائه اين تحقيق بررسي آبشستگي حول زوج پايه كج تحت تاثير پارامتر زمان و نقش آن در توپوگرافي بستر مي باشد. در اولين مرحله آزمايش زمان تعادل انجام گرفته شد. در مرحله بعد به منظور شناخت شرايط آبشستگي در قوس آزمايش بدون نصب پايه در زمان تعادل نسبي انجام شد، با استناد به نتايج به دست آمده از اين آزمايش موقعيت 60 درجه براي اسقرار پايه ها انخاب گرديد. آزمايش هاي بعدي به ترتيب در 20، 50 و 100 درصد زمان تعادل نسبي و با استقرار زوج پايه همگرا در موقعيت 60 درجه از ابتداي قوس و در صفحه عمود بر جريان انجام شدند. كانال مورد استفاده در آزمايش ها با عرض يك متر و مسير قوس تند با نسبت شعاع مركزي به عرض كانال برابر با 2 و زاويه 180 درجه مي باشد. نتايج بدست آمده نشان مي دهد در زمان تعادل نسبي چاله تشكيل شده در قوس بيروني 20 درصد عميق تر از چاله آبشستگي اطراف پايه مي باشد. اين چاله در تقارن با محل احداث پايه در زاويه 123 درجه شكل مي گيرد. بررسي پارامتر زمان در آزمايش ها نشان مي دهد در ابتداي آزمايش پايه نزديك به ساحل خارجي فرسايش بيشتري دارد ولي بعد از گذشت 20 درصد زمان تعادل نسبي سرعت آبشستگي در هر دو پايه يكسان مي شود، همچنين كاهش زمان تعادل از 100 درصد به 50 درصد باعث كاهش 20 درصدي عمق چاله آبشستگي حداكثر مي شود.

The destruction of the bridges because of the erosion of the bed is a question that if is not addressed properly it can’t be compensated. The aim of this research reviews the scour around the twin bridge piers affected by parameter of time and its role in the bed topography. In this research, the equilibrium time test was done to determine the equilibrium time. After that a test without the establishment of bridge pier was done. The aim of this test is to know the effect of steep bend flume to the bed topography and scour pattern. The next tests were done at 20, 50 and 100 percent of the relative equilibrium time with the establishment of the twin bridge piers. The experiments were performed at the Advance Hydraulic Laboratory of Persian Gulf University of Bushehr in Iran. The channel used in this study has 1 m wide and bend routh with the 180 degrees angle flume with the relative curvature of 2. The upstream routh has the length of 6.5 m and the downstream path is 5 m long. The condition was clear water in all test and live bed using sediment with average diameter of 1 millimeter and standard deviation equals to 1.3. Flow rate was fixed at 70 litter per second with depth of 18 centimeter at straight upstream rought. The piers had the diameter of 5 centimeter and making the angle of 21 degrees with the vertical axis and also placed at the perpendicular plane to the flow stream. Due to maximum scouring at 60 degree of the flume in preliminary tests without the establishment of the piers, for the rest of the tests the piers were installed at 60 degrees angle of the channel bend. At the end of each test channel was gradually drained and after drying the bed topography was harvested with the use of laser device called bed topographer with the accuracy of 1 millimeter. For the best result according to the test more than 4500 points were measured. The most important results achieved is that by the relative equilibrium time the second scour hole is 12 percent deeper than the main scour hole around the piers. In addition the second scour hole is created at the 123 degrees along the outer wall of the flume. Studying the parameter of time indicated that at the beginning of the experiment the second pier which is closer to the outer wall has more scouring depth , but after the relative balance time of 20 percent both pier has the same scouring rate. Reducing the time of the test by 100% to 50% of the relative equilibrium time reduces the maximum scouring depth of the main hole by 20 percent. In all test a scour hole at the middle of the channel bend was seen which deeper at 50 percent of the equilibrium time compared to the 100 and 20 percent of the relative equilibrium time. Advanced discussion and analysis about the results achieved from the tips are outlined in this paper.