الگوي جريان و آبشستگي سري سه تايي آبشكن هاي سرسپري در قوس تند

Flow Field and Scouring around Triplex Shielded Spur Dikes in Sharp Bend

آبشكن‌هاي رودخانه‌اي، از سازه‌هاي مهم ساماندهي رودخانه به شمار مي‌آيند كه سبب انحراف جريان از كناره‌ها و نواحي بحراني شده و جريان را به سمت محور مركزي رودخانه هدايت مي‌نمايند. نتيجه‌ي انحراف جريان، توسعه يك ناحيه چرخشي با تلاطم شديد در پيرامون آبشكن است. فرايند هيدروليكي اين جريان، توسعه حفره آبشستگي پيرامون آبشكن و ته‌نشيني بار رسوبي در پايين‌دست و كناره رودخانه است. در حالي كه آبشستگي يك خطر جدي براي سازه آبشكن و در نتيجه براي رودخانه محسوب مي‌شود، ضرورت تحقيق در اين زمينه را آشكار مي سازد. در اين مقاله سه آبشكن سرسپري دافع (رو به بالادست) با فاصله 3.5 برابر طول موثر آبشكن در قوس خارجي كانال 90 درجه تند قرار گرفته‌اند و آبشكن اول در موقعيت 30 درجه از قوس خارجي قرار دارد. نتايج حاصل از يك آزمايش ميدان جريان در بستر هموار و آزمايش آبشستگي آن ارائه شده است. ميدان جريان با استفاده از دستگاه سرعت سنج Vectrino II برداشت شده است. نتايج آبشستگي نشان داد ميزان آبشستگي آبشكن دوم 33% و آبشستگي آبشكن سوم 81% بيشترين ميزان آبشستگي است كه در بالادست آبشكن اول رخ مي‌دهد. در اين مقاله الگوي جريان و مكانيزم ايجاد آبشستگي و جريان‌هايي كه در اين محدوده به وقوع مي‌پيوندند به تفصيل آمده است.

Spur dike is one of the river training structures that is considered to deviate the river from critical and erodible areas and the flow from the sides and towards the central axis. As a result of flow is developing a circular area with high turbulence around the spur dike. The hydraulic process results development of the scour hole on the upstream of the spur dike and settlement of sediment in the downstream and sides of the river. While scouring in spur dike structures results a serious threat to the river so it is needed to be researches in this field. This paper describes triplex repellent shielded spur dikes (directed to the upstream) with a distance of 3.5 times of the effective length of the spur dike in the outer bank of the channel. The first spur dike is located at 30 degree from the start of bend. The experimental channel is a 90º channel with rectangular section. The radius of curvature to the channel width is 2, which is classified as a sharp bend. Materials used are sands with uniform grains and its mean diameter is 1.28mm and its standard deviation coefficient is 1.3 and the relative density of sediment is 2.35. The results of flow field on flat bed and a scouring experiment are presented. Discharge was 25 l/s and All scour tests were done in 24 hours and in the moving threshold conditions (U/Uc =0.98) and clear water condition. Flow field is recorded using the Vectrino II velocimeter that can profile water in a 3cm column. It was found that in the levels upstream of the first spur dike in an adjacent to bed, stream lines are deviated to the inner bank. While in the middle levels, flow lines upstream of the spur dike is almost parallel to the channel walls and approached the spur dike, resulting deviation in the separation zone. In the scouring experiment it was obvious that at the beginning of the experiment, thus creating the down flow upstream of the spur dikes scouring initiates near the wing of each spur dike and it develops by the horse shoe vortex. But with the time sediment had been washed from upstream of the first spur dike, and moved to the foot of the spur dike until it reaches upstream of the former to the latter. Then scour hole upstream of the second spur dike starts to form. Results showed that the amount of scour upstream of second spur dike is 33 % and upstream of third spur dike is 81 % the maximum amount of scour that occurs upstream of the first spur dike. Mechanism causing scour and flows occurring within this range detailed in this paper.