بررسی عددی اثر پارامترهای رسوب و تراز پایاب بر آبشستگی پایین دست دریچه کشویی با استفاده از نرم افزار flow3d

آبشستگی موضعی پدیده ای است که بر اثر اندرکنش جریان آب و خاک در رودخانه ها، مسیل ها و در پایین دست سازه های هیدرولیکی به وجود می آید. عوامل موثر بر میزان تغییرات پروفیل آبشستگی موضعی و نحوه ی کنترل آن در مجاری روباز توسط محققین متعدد به صورت آزمایشگاهی و مدل سازی عددی مورد بررسی قرار گرفته است. در این تحقیق که به صورت مدل‌سازی عددی با نرم افزار flow3d صورت گرفته است، هدف بررسی تغییرات عوامل مختلف نظیر تغییر در میزان بازشدگی دریچه کشویی، اندازه رسوبات بستر، نوع مدل آشفتگی و تعداد سلول های میدان محاسباتی، بر میزان آبشستگی پایین دست دریچه های کشویی می باشد. نتایج نشان داد مدل آشفتگی les با معادله انتقال رسوب van rijn، برای شبیه سازی خصوصیات آشفتگی جریان مناسب تر است. به منظور ارزیابی و صحت‌ سنجی مدل عددی، با استفاده از یک مدل آزمایشگاهی مشخص شد که مدل flow3d از دقت بالایی در شبیه سازی آبشستگی پایین دست دریچه کشویی برخوردار است. علاوه براین مطالعات نشان داد بازشدگی به عنوان یک شاخص، به تنهایی بر روی بیشینه عمق آبشستگی پایین دست دریچه کشویی بدون کف بند، تاثیرگذار نیست؛ بلکه شدت جت عبوری از زیر دریچه اثر غالب بر میزان آبشستگی دارد، هرچقدر جت عبوری از زیر دریچه بیش تر باشد بیشینه عمق آبشستگی نیز به تبع آن بیشتر می گردد.

Investigating the effect of sediment parameters and tailwater el-evation on Scouring downstream of the sluice gate

Extended abstract1 IntroductionThe turbulent jets downstream of hydraulic structures, such as sluice gates, jet nozzles, spillways, or weirs, may cause severe local scour. The scouring process is inherently complex due to the rapid change of turbulent flow characteristics with sediment motion in the scour zone. Excessive scour is undesirable and may be detrimental to the foundations of the structures. Sluice gates are wood or metal sheets that slide vertically and act as a portal to regulate the water passage within the irrigation channels, dams, and some wastewater control plants. The operation of the gates in small channels is traditionally accomplished manually, while in modern channel networks or relatively large channels, an electrically or hydraulically powered mechanism should be considered. The flow passing the gate forms a highvelocity jet with a relatively high erosive force that may develop a scour hole in an erodible channel downstream of the gate. To reduce the destructive action of the jet and regulate the flowing fluid under the gate, a rigid and nonerodible apron is commonly constructed under and downstream of the gate. In this case, the scour forms downstream of the apron, and depending on its shape and depth, it may threaten the stability of the gate foundation. In fact, the apron is generally a thin concrete layer implemented on the canal bed sediments, and its stability depends strongly on the strength of the sediment particles against detachment. The interaction between the flow and the sediment particles may extend the scour hole up to the downstream edge of the apron over a relatively short time. Afterwards, the sediment particles beneath the apron detach and wash away over a longer duration, causing to undermine the apron. Depending on the flow intensity, sediment size, and apron length, this phenomenon may occur in a few hours to several months. A collapsed apron may interrupt the flow passage and threaten the whole structure stability.Recently, several studies have been conducted so as to improve our understanding of the phenomena of scour and sediment transport due to a horizontal jet issuing from below a sluice gate. The highvelocity jet issuing from below the gate produces shear stresses that exceed the critical shear stress for the incipient motion of the bed material. Over a period of time, the scouring of the bed material causes the flow depth downstream from the gate to increase, thereby resulting in a mechanism of reduced bed shear stress, which in turn reduces the rate of scour. Though the flow field is simple, the scour mechanism is complex and dynamic.2 MethodologyThe dimensions and flow conditions of the numerical model were the same as Kells et all., 2001 laboratory model. The geometries of the numerical model were created using flow3D software, and solutions were created out by VOF method. In this study, the effect of grain size on the dynamics of local scour processes is discussed in the context of the erosion that takes place downstream from a submerged sluice gate. Three gradations of noncohesive bed material were used to study the scour process for various tailwater depth and sluice openings. The sand gradations included three sizes of uniformly graded. A total of 18 model was simulated, each for a period of 2 h. An equilibrium scour condition was attained over this time period for all of the models, although a sense of similarity in the bed profiles is observed in the region close to the sluice gate. 3 Results The present results indicate that the depth and the area of scour are highly dependent on the bed grain size, both increasing as the grain size is reduced. Moreover, the maximum scour depth increases with increases in the tailwater depth.Finally, it was found that the location of the point of maximum scour depth, as measured from the upstream end of the erodible sand bed, moved downstream with an increase in either the opening or tailwater depth and upstream with an increase in the grain size. 4 Discussion ConclusionsThe primary purpose of this study was to evaluate the effect of grain size and, to a lesser extent, of grain size distribution on the dynamics of local scour below a sluice gate. In short, it was found that the grain size has a significant influence on the extent of scour, which occurs, with more scour occurring for smallersized material. As well, it was found that less scour occurred for a graded sand than a uniform one having a similar median grain size. Of particular interest, however, is that for any given grain size, the greater is the tailwater depth, the greater is the depth, extent, and volume of scour. It appears that the tailwater serves to slow the rate of jet expansion, thus increasing the length of bed exposed to high velocity, hence high shear stress, conditions.