بررسی و ارزیابی تجربی آبشستگی پایین‌دست سرریز کلیدپیانویی ذوزنقه‌ای نوع a در جریان آزاد و مستغرق

در این پژوهش، آبشستگی پایین‌دست سرریز کلیدپیانویی ذوزنقه‌ای نوع a در جریان آزاد و مستغرق، به‌صورت آزمایشگاهی بررسی و ارزیابی شد. برای انجام آزمایش‌ها از فلومی به طول 10، عرض 0.75 و ارتفاع 0.8 متر استفاده شد. آزمایش‌ها با دبی‌های بین 30 تا 70 لیتر بر ثانیه، عدد فرود ذره بین 0.029 تا 0.058، عمق پایاب 0.13 ‌متر برای جریان آزاد و 0.23 و 0.29 متر برای جریان مستغرق انجام شد. جنس بستر در پایین‌دست سرریز، ماسه‌ای با دانه‌بندی یکنواخت، قطر میانگین 2.2 میلی‌متر و به ضخامت 0.425 متر و طول 2 متر بود. نتایج نشان داد در جریان آزاد تغییرات بستر نسبت به جریان مستغرق با سرعت بیشتری انجام می-شود و عمق و طول حفره‌ی آبشستگی بیشتر می‌باشد. عمق آبشستگی نسبی با افزایش 77 درصدی عمق ‌پایاب برای اعداد فرود ذره 0.029، 0.037، 0.045، 0.052 و 0.058، به ترتیب 42، 45، 21، 28 و 27 درصد و با افزایش 123 درصدی عمق پایاب، به ترتیب 95، 92، 90، 75 و 68 درصد کاهش می‌یابد. با افزایش 18 درصدی نسبت استغراق در عدد فرود ذره 0.029، و افزایش 96 درصدی نسبت استغراق در عدد فرود ذره 0.058، بیشینه عمق آبشستگی نسبی به ترتیب 92 و 56 درصد کاهش می‌یابد. محل وقوع بیشینه عمق آبشستگی در عمق پایاب 0.13، 0.23 و 0.29متر به ترتیب در فاصله‌ی 0.18 الی 0.30، 0.18 الی 0.36 و 0.03 الی 0.30 متری از پنجه‌ی سرریز، ایجاد شده است. با افزایش عدد فرود ذره، طول نسبی حفره‌ی آبشستگی افزایش یافته است.

the experimental study of downstream scouring of trapezoidal piano key weir type a under free and submerged flow

introduction: weir is a structure that is made in the body or support of the dam to safe drain the excess volume of water in the tank. weirs are mainly considered for free flow mode, but in some cases there is a possibility of immersion in them. submersion in weirs occurs in two general and local ways. general submersion will occur if the downstream water level is higher than the weir crown level. this is more likely to occur for weirs in canals and rivers and if the weir acts as a diversion dam. local submersion is observed in the downstream part of weir due to local flow conditions. weirs are divided into linear and non-linear weirs based on the shape in the plan. piano key weirs are the newest type of non-linear weirs which recently due to its advantages. non-linear piano key weirs enjoy not only a higher water passage but also a relatively simple and economic structure compared to linear weirs. methodology: all experiments of this research were performed in a channel with a long 10 m, wide 0.75 m and high 0.8 m in tarbiat modares university hydraulic laboratory. a view of the laboratory flume is shown in figure (3). the required water was supplied through an underground reservoir. in this research, trapezoidal piano key weir types a, made of thermoplastic with a thickness 1.2 m, slope of the input and output keys 28 degrees and a height 0.2 m was used. the weir has 6 keys (3 input keys and 3 output keys) with the same slope in the input key and the output key. uniform bed materials with an average diameter 2.2 mm were used. flow discharge was measured with an ultrasonic flowmeter and flow depth and bed level were measured with a laser level gauge. according to the selected discharges, the flow depth upstream of the weir was considered to be more than 3 cm so that the effect of surface tension is not significant. in this study, experiments were performed with five discharges 30, 40, 50, 60 and 70 liters per second. under submerged flow for each discharge, two percent submersion and under free flow for each discharge, the tailwater depth was considered to be 0.13 m.results and discussion: flow characteristics are affected in case of weir submersion. during the test, after the flow hit from the input switch to the tailwater surface, due to the amount of tailwater depth, surface rotations (at low tailwater depth) and surface turbulence (at high tailwater depth) are observed. part of the flow moves downwards and after hitting the bed surface, a weak rotational zone is created in the range of the input switches. the flow enters the downstream in the form of a submerged jet after the output switch and, due to the momentum to the upper fluid, causes a severe rotational zone in the range of the output switches. in the free flow, more turbulence is observed in the range of the output switches due to the intersection of the flow passing through the weir lateral wall and the falling current from the upstream and downstream crowns. less tailwater depth in free flow limits the deep growth and development of the sedimentary hill in downstream of the scouring hole, and sedimentation occurs with longer length and lower elevation. however under the submerged flow, due to the greater tailwater depth and lower fall height, the flow strength is more depleted and the flow strikes the bed with less energy, and result in less scouring. in this case, the flow does not have the power to transfer all the sediments to the downstream and most of the sediments accumulate on top of each other and a sedimentary hill is created as a point. as a result, a sedimentary hill with a higher height and less length is visible under submerged current than free flow. therefore, in the free flow than in the submerged flow, the length and depth of the scour hole occur more, and sedimentation occurs with less height and longer length.