مطالعه آزمايشگاهي اثر شكل رويه سازه دفلكتور بر كاهش عمق آبشستگي اطراف پايه پل

Laboratory Study of Deflector Structure’s Shapes on Bridge Pier Scouring

از عوامل مهم تخريب پايه پل ها، آبشستگي موضعي در پنجه آنها مي‌باشد. جهت كنترل اين پديده، محققين مطالعات فراواني را انجام دادهاند، و راهكارهايي جهت كاهش آن پيشنهاد نموده اند. در اين تحقيق، اِلماني به نام سازۀ دفلكتور (منحرف كننده جريان)، كه باعث دور شدن جريان از روي بستر رسوبي مقابل پايه ميگردد، تعريف شد. سناريو آزمايشها، با تثبيت شرايط جريان به ازاي سه سرعت نسبي (U/U_c ) برابر 0/70، 0/83 و 0/97 و زاويه هاي سازه دفلكتور با راستاي جريان، برابر 15، 30 و 45 درجه، به ازاي پايه با مقطع دايره، تعريف گرديدند. در اين تحقيق اثر شكل رويه دفلكتور در دو نوع رويه گرده مثلثي و گرده ماهي بر روي عمق آبشستگي بررسي گرديد. نتايج نشان داد كه، پارامتر بيبعد عمق آبشستگي (d_s/d_(s_max ) ) نزديك آستانه حركت ذراتU/U_c =0/97 براي دفلكتور با زاويه 15 درجه و گرده مثلثي نسبت به نمونه شاهد، برابر 85 درصد و گرده ماهي 77 درصد و رويه مسطح برابر 75 درصد كاهش داشته است. همچنين با تغيير زاويه از 45 درجه به 30 و 15 روند آبشستگي موضعي اطراف پايه كاهش داشته و اثر گذاري سازه دفلكتور افزايش مي يابد.

Introduction One of the main factors in the collapse of the bridge piers in rivers is local scouring. By placing the piers in the direction of the streams, a complex three-dimensional flow is formed around the pier that has been the popular subject of research by many researchers. Methods of reducing the depth of local scouring are divided into two systems: 1. increasing the strength of bed materials around the piers by using more resistant materials, such as riprap, collar and gabion in the riverbed. 2. Reducing the strength of the main factors such as downward flow and horseshoe vortex by the deflector, blade and submerged Vane or changing the geometric shape. In the present study, the effect of the deflector shapes such as triangles and curved surface on the maximum scour depth around the pier under clear water conditions were investigated. General factors of bridge pier scour include down-flows, horseshoe vortex, and wake vortex. In general, the flow impact on the pier and its separation is the main factors that form scour holes around piers. Methodology The experiments were done in the laboratory of Khuzestan water and power authority laboratory (KWPA), equipped with a flume with a length of 10 meters and a height of 500 mm and a width of 310 mm. The flume is equipped with an electromagnetic flow-meter with an accuracy of ±0.1 liters per second and a weir downstream of the flume to adjust the water level. In this study, natural river sand with uniform grain size (δg = 1.36), relative density Gs = 2.64 and the average particle diameter of 0.95 mm. In all experiments, water depth was considered 100 mm. In this research, three different models of PVC deflector surface (the deflector surface shapes such as triangles, curved and simple surface) with angle's face (θ= 15, 30 and 45-degrees). It should be noted that the angle of flow with the deflector head is calculated as α = 90-θ, which used to describe and analyze. The unprotected pier scouring investigated to represent a basis for controlling and comparing with the other scour and bed change conditions. A 12-hour control experiment was also conducted on the control pier to determine the experiment time (equilibrium time), and scour depth changes were recorded in the time unit during experiments. Results and Discussion The horseshoe vortex around the scour hole accelerates digging and transfers the sediments separated from the bed downstream with the main flow. The flow's separation from around the pier also creates perpendicular vortexes on the sedimentary bed known as wake vortexes. These vortexes are active behind the pier, separate the bed particles like a tornado, expose them to the flow, and help move sedimentary particles from the front and sides of piers downstream. The scour hole digging by the horseshoe vortex continues until the water volume inside the scour hole increases and exhausts the vortex energy. In this state, the scour depth changes negligibly over time and reaches equilibrium, and Figure 1 show the scour mechanism. In this study, to reduce the scouring depth pattern around the pier, three shapes of the deflector (the deflector surface shapes such as triangles, curved and simple) with three angles were used. The results showed that by reducing the head slope from 40 to 15 degrees, scouring depth decreases. For all deflectors with 15 degrees in the parameter (U/Uc=0.70), the percentage of the scouring depth reduction is close to 83 to 89 per cent. In the parameter U / Uc=0.96 near inception motion that is the most critical state and the value most comparable to the particle incipient motion, the deflector with triangle surface shows a decrease of 85%, curved surface 77%, and simple 75%. By reducing the angle of the deflector, part of the flow lines didn't deviate towards the bed, which reduced the potential of the high-pressure zone created at the pier. This reduction in compressive potential reduced the flow velocity of the back vortices and, ultimately, reduces their ability to transport sediment downstream. Based on the results, the deflector with a triangle surface shape in all flow conditions had a better and lower scour hole depth than the carved and simple shape. Conclusion This study used a deflector structure to reduce and control the scour depth around bridge piers. The flow effect was investigated by implementing these protections and their impact in various relative velocities (U/U_c=0.97,0.83,0.70). The scouring pattern and sediment point bar created around the pier with The deflectors protection compared with the control pier (without protection) have less scouring depth due to minor deviation of flow streamlines and reduced disturbances around the pier. Finally, the deflector with a triangle surface shape has a better response to reduce the scouring hole. The results state that the deflector at the 15-degree angle significantly affects the flow deflection near the bed, corrects the flow pattern around the pier, and reduces scouring depth.