بررسي آزمايشگاهي تاثير پايه‌ي استوانه‌‌يي‌شكل در جريان‌هاي ناشي از شكست سد

EXPERIMENTAL INVESTIGATION OF CYLINDRICAL PIER SHAPE EFFECT ON FLOW INDUCED BY DAM BREAK

در اين نوشتار، مشخصات جريان به‌دست‌آمده از نتايج مدل آزمايشگاهي شكست سد با درنظرگرفتن پايه‌ي پل استوانه‌‌يي‌شكل واقع‌شده در ميان كانال مستطيلي‌شكل بررسي شده است. تغييرات سطح آب توسط سنسورهاي آلتراسونيك و دوربين با سرعت بالا و مولفه‌هاي سرعت با دستگاه سرعت‌سنج (ADV) برداشت شده است. مشاهدات جريان، نشان‌دهنده‌ي تغيير مسير جريان براي عبور از پايه‌ي پل و ايجاد جريان‌هاي عرضي در پايين‌دست است؛ به طوري‌كه پشت پايه‌ي پل تا فاصله‌يي به اندازه‌ي 13 برابر قطر آن، ناحيه‌ي برخاستگي جريان مشاهده مي‌شود. همچنين مقايسه‌ي تغييرات سطح آب به‌دست‌آمده با مقادير حل تحليلي ريتر، بيانگر وجود درصد اختلاف‌هاي كمتر (4 الي 12 درصد) در محدوده‌ي بين مخزن سد تا محل پايه‌ي پل است؛ در حالي‌كه در فاصله‌ي پشت پايه‌ي پل تا 13 برابر قطر آن به‌علت وجود گردابه‌هاي برخاستگي، مقادير آزمايشگاهي از حل تحليلي ريتر فاصله مي‌گيرد.

Although dam failure rarely occurs, flooding caused by dam-break flow usually results in considerable property damage, as well as, in some cases, loss of life. Induced flow by a dam break occurrence is affected very much by natural and artificial obstacles, such as bridge piers, in the downstream of a dam. Understanding the basic characteristics of dam-break flow, such as maximum water level, maximum discharge and the arrival time of the wave front around the obstacle, such as the bridge pier, at assigned locations, is fundamental to a reliable engineering design. Therefore, it is necessary to determine the extent of damage in cases of possible dam failure around obstacles. In the present study, experimental results of a cylindrical bridge pier with a diameter of 4cm have been modeled in a rectangular flume. The mentioned pier was located 108cm from the gate and made of plexiglass in the downstream. The experiments were carried out to assess flow propagation along the flume and around the pier under dry bed conditions in the downstream reach. Also, effects of an initial upstream depth on flow propagation were investigated. Dam failure was simulated by sudden removal of the gate that separates the reservoir from the channel, by means of a pneumatic jack. The gate was made of plexiglas plate in order to keep gate-water interference at a minimum level. Variations in water surface were recorded using ultrasonic sensors and a high speed camera. In addition, an ADV sensor has been used for recording the velocity components at different locations. Flow results show the significant effect of the bridge pier on the flood wave induced by the dam break. It caused a change in flow direction and made a slight increase in water depth. Also, the wake zone is observed behind the bridge pier at a distance of 13 times the pier diameter. In addition, experimental data are compared with the analytical solution proposed by Ritter. This comparison shows a very small difference (4-12%) between experimental results and the Ritter solution in the distance between the dam reservoir and the bridge pier location. However, at the distance of 13 times pier diameter behind the bridge pier, due to the existence of the wave zone, the difference percentage is increased.