مدل‏سازی عددی پارامترهای موثر بر جریان از روی هواده سرریز‌ شوت و غلظت هوا در پایین‌دست آن

برای حفاظت سازه‌های هیدرولیکی نظیر سرریز‌ها، شوت‌ها و تخلیه‏کننده‌های تحتانی در مقابل کاویتاسیون، به‏طور معمول مقداری هوا به جریان در نواحی با شاخص کاویتاسیون کمتر از مقدار بحرانی، اضافه می‌گردد. با استفاده از هوادهنده‌ها می‌توان از فرسایش‌های ناشی از کاویتاسیون بر سطوح سرریز، جلوگیری نمود. معمولاً هوادهنده‌ها روی کف و گاهی بر دیواره‌های جانبی سرریز نصب و باعث جدایی جریان‌های با سرعت بالا از سطح سرریز شده و با وارد کردن مصنوعی هوا به جریان، از فرسایش در مرزهای صلب جلوگیری می‌کنند. بیشتر آزمایش‌های انجام شده ورود و خروج هوا بر غلظت متوسط هوای جریان متمرکز بوده و نیاز به اندازه‌گیری مقدار و نحوه خروج هوا از جریان می‌باشد. لذا در تحقیق حاضر با استفاده از داده‌های آزمایشگاهی فیشر (2007) جهت شبیه‌سازی عددی جریان عبوری از هواده، به بررسی تغییرات غلظت هوا در طول بستر شوت پرداخته شده است. بدین منظور جهت مدل‌سازی جریان دوفازی هوا–آب از نرم‌افزار fluent استفاده شده و طول پرش جت جریان به عنوان عاملی مهم و تاثیرگذار در ورود هوا به جریان ملاک واسنجی قرار گرفته است. با توجه به اهمیت نقطه برخورد جریان به بستر شوت، نقطه مذکور به عنوان مرجع محاسبات در رابطه قرار گرفت. به طور کلی نتایج نشان داد که غلظت هوای بستر پایین‌دست هواده‌ها با افزایش عدد فرود، ارتفاع رمپ، ارتفاع پله و زاویه رمپ افزایش و با افزایش ارتفاع آب بالادست هواده، کاهش می‌یابد و با افزایش شیب شوت ورود هوا به جریان افزایش یافته و تغییرات غلظت هوا کاهش می‌یابد، لذا نیاز به حضور هواده کمتر می‌گردد.

Numerical modeling of parameters affecting the flow over the chute spillway aerator and the air concentration in downstream of it

Introduction Dams have played an important role in the development of human civilization, the simplest of which is the provision of water resources in agriculture, industry and drinking. The share of earthen and gravel dams, which often have tunnel overflows and shoots, is significant. In dams with this type of overflow, increasing the height of the dam increases the flow velocity on the shot and increases the probability of cavitation. To protect hydraulic structures such as overflows, shoots, and lower discharges from cavitation damage, some air is typically added to the flow in areas with a cavitation index below the critical value. By using aerators, erosions that occur due to cavitation on overflow surfaces can be prevented. Aerators are usually installed on the floor and sometimes on the side walls of the overflow, separating the highvelocity currents from the surface of the overflow and preventing erosion at the rigid boundaries by artificially introducing air into the flow. Methodology Most air inlet and outlet experiments focus on the average concentration of air in the stream and require the measurement of the amount and manner of air out of the stream. Computational fluid dynamics is a relatively new method and a review of studies in numerical modeling of overflows shows that the use of this tool as a research tool in research institutes began and gradually accepted by the hydraulic engineering community. Using computational fluid dynamics alongside physical models is a good way to reduce costs and save time. Due to the high accuracy of this method in determining the jump length of the flow jet, its results can be used to determine the geometric parameters of aerators such as the width of the air distribution duct. As the slope of the shot increases, the entry of air into the stream increases and also the changes in air concentration decrease, so the need for the presence of aerator decreases. Therefore, in the present study, using Fisher (2007) laboratory data to numerically simulate the flow through the aerator, the changes in air concentration along the shoot bed have been investigated. For this purpose, FLUENT software was used to model the twophase airwater flow and the length of the flow jet jump was used as an important and effective factor in the entry of air into the flow. . Although aerators have been proposed since 1970’s but today there is no any reliable design guideline for determinate aerator spacing. Results and Discussion By determining the trend of changes in bed air concentration, the distance between two aerators can be determined. The air in the stream causes the stream to condense and dampens the shocks caused by the explosion and bursting of the bubbles, thus reducing the damage caused by cavitation; On the other hand, if more than necessary to prevent cavitation, air enters the stream, causing the flow to become bulky, and higher walls should be considered for the shot, which is not economically appropriate. Therefore, it is important to determine the minimum air concentration required to prevent cavitation damage. Determining the location of the second aerator can be determined according to the minimum required air concentration and the length of the flow jet for the height and landing number upstream of the first aerator. And the ramp angle increases and decreases as the water level above the aerator increases. Determining how the air concentration changes downstream of the shot aerator is important for calculating the distance of the aerators from each other, and FLUENT models the process of these changes well. The comparison between numerical and laboratory results shows a very good agreement between the laboratory and numerical model. Finally, a relation for the distribution of air concentration in the substrate was presented, which has a good fit with laboratory data. Due to the importance of the point of impact of the current to the shooting bed (sudden outflow of air due to the impact), the point was used as a reference for calculations. Conclusion The comparison between numerical and laboratory results shows a very good agreement between the laboratory and numerical model. In general, the results showed that the air concentration of the lower bed of the aerators increases with increasing landing number, ramp height, step height and ramp angle, and decreases with increasing water height upstream of the aerator, and increases with increasing slope of the air flow and also changes. The air concentration decreases, so the need for aeration is reduced. Keywords Minimum air concentration, Cavitation, Computational Fluid Dynamics, Overflow, Aeration.