بررسي آزمايشگاهي پرش هيدروليكي در حوضچه آرامش با آستانه پلكاني

Experimental Investigation of Hydraulic Jump in stilling basin with Stepped Sill

پرش هيدروليكي جريان متغير سريعي است كه طي آن جريان فوق بحراني به جريان زير بحراني تبديل مي شود. حوضچه هاي آرامش معمولاً در پايين دست سازه هاي نظير شوتها و دريچه ها، براي كنترل پرش هيدروليكي استفاده مي شود. بلوكهاي مياني، پله هاي مثبت و منفي و آستانه انتهايي تجهيزاتي هستند كه معمولاً براي استقرار پرش هيدروليكي درون حوضچه آرامش استفاده مي شوند. در تحقيق حاضر تشكيل پرش هيدروليكي درون نوع جديدي از حوضچه آرامش با آستانه پلكاني مورد بررسي قرار گرفته است. آزمايشات درون كانالي با عرض 60 سانتي متر، ارتفاع 1 متر و طول 12 متر انجام شده است. در اين آزمايشات تاثير آستانه بر خصوصيات پرش هيدروليكي مورد بررسي قرار گرفته است. اثر پارمترهاي مهمي چون عمق پاياب، هندسه آستانه و موقعيت قرار گيري آن مورد ارزيابي قرار گرفته است. پرش هاي شكل گرفته در حوضچه با آستانه پلكاني به پنج نوع A، B، B كمينه، C و C كمينه تقسيم بندي شده است كه پرش ها در تبديل از نوع A به B و نهايتاً C كمينه، بتدريج از حوضچه خارج مي شوند. نتايج نشان مي دهد با وجود اثر مثبت پله اي كردن آستانه شيبدار، در شرايط ثابت جريان (عمق پاياب و عدد فرود اوليه)، افزايش تعداد پله هاي آستانه از 2 به 3 اثر منفي بر حفظ پرش در قبل از آستانه (درون حوضچه) دارد. در انتهاي كار با توجه به نتايج حاصله، روابطي براي محاسبه ميزان خروج پرش از حوضچه ارائه شده است.

Hydraulic jump is a rapid and sudden transition from a highvelocity supercritical flow to a subcritical flow in an open channel flow. Stilling basins are used to control the hydraulic jump at the downstream of chutes, sluice gates,… End Sills, baffle blocks and negative steps are often used to control hydraulic jumps in stilling basins. The present study focuses on the formation of hydraulic jump in the new type of stilling basins with stepped sills. Extensive experiments were conducted in a rectangular flume 0.6 m wide, 12.0 m long and 1.0 m deep, with various discharges from 30 to 120 l/s. Water was pumped from an underground sump into a head tank and the discharge was measured with a ultrasonic flowmeter. At the downstream end of the head tank there was a sluice gate into the flume. The edge of the sluice gate has a streamlined lip in the shape of a halfcylinder of diameter 20 cm to minimize flow contraction and provide a uniform supercritical flow. A point gauge with an accuracy of 0.1 mm was used to measure water depths. In order to visualize the flow field, the dyeinjection method and a high speed camera were employed. A tailgate located at the downstream end of the flume was used to control the tailwater depth. The effects of stepped end sills on hydraulic jumps were investigated experimentally. Firstly, dimentionless parameters affecting the hydraulic jump on stepped sill introduced using Buckingham π theorem. The effect of important parameters such as approach Froude number (Fr1), relative tailwater depth (〖y_t/y〗_2^*) and the end sill geometry (shape and relative height of sill (s/y1)) on hydraulic jump were investigated. The hydraulic jumps over stepped end sills were classified into Ajump, Bjump, minimum Bjump, Cjump and minimum Cjump. By changing the type of flow from Ajump to minimum Cjump, the jump is going to sweepout from basin. Ajump is entirely formed in the basin and at the upstream of sill. In the case of minimum Cjump, most of the surface roller of jump formed at the downstream of sill. The flow types are presented in the form of 6 different diagrams as functions of the relative step height s/y1. By increasing the tailwater depth, sill height, the probability of occurance of hydraulic jump in the stilling basin increased. It was found that the sill with 2 steps have better performance in stabilizing the jump in the stilling basin as compare to sill with 3 steps. By increasing the approach Froude number, the jump began to sweep away from basin. By knowing the initial condition like upstream velocity and upstream froude number, tailwater depth, sill height and its number of steps the toe distance from sill can be found out with desirable accuracy.