بررسی نیروی برشی، لایه مرزی و پدیده کاویتاسیون در حوضچه آرامش واگرای ناگهانی با بستر زبر

نوع مقاله : مقاله پژوهشی

نویسندگان

چکیده

پرش‌ هیدرولیکی از نوع جریان‌های متغیر سریع است که در فاصلة کوتاهی، جریان از حالت فوق ‌بحرانی به زیربحرانی تغییر می‌کند. در سال‌های اخیر مشخص شده است ایجاد واگرایی، حضور زبری یا ایجاد شیب در کف حوضچة آرامش، در کاهش ابعاد پرش، مؤثر و از نظر اقتصادی نیز به‌صرفه است. در این پژوهش، تأثیر واگرایی ناگهانی بر مشخصات پرش ‌هیدرولیکی در حضور زبری سنگ‌ریزه‌ای در کانال مستطیلی به طول 8، عرض 0.4 و ارتفاع 0.6 متر بررسی شد. ییی. نسبت واگرایی در این مطالعه در محدودة  و زبری ‌نسبی در محدودة  بود. اعداد فرود در محدودة 4.9 تا 9.5 و دبی 30تا 50 لیتر بر ثانیه انجام شد. ضخامت لایة مرزی بی‌بعد ( ) در حالت بستر واگرا و زبر، 0.51 محاسبه شد که در مقایسه با مقدار  0.16 برای بستر صاف، قابل ملاحظه بود. در صورتی که از هر دو متغیر زبری و واگرایی ناگهانی استفاده شود، مقدار ضریب نیروی برشی در بهترین حالت (در نسبت واگرایی 2 و اندازة زبری 2.2 سانتی‌متر) به‌طور میانگین، 14.7 برابر ضریب نیروی برشی در پرش کلاسیک است. نوسانات آب و تغییرات فشار توسط پیزومترهای نصب‌شدة کف حوضچة ‌آرامش اندازه‌گیری شد. ییی نتایج، نبود کاویتاسیون در این آزمایش‌ها را نشان داد.

کلیدواژه‌ها


عنوان مقاله [English]

Investigation of of shear force, boundary layer and cavitation phenomenon in abrupt expanding stilling basin with rough bed

نویسندگان [English]

  • bashir fallahi
  • Manouchehr Heidarpour
چکیده [English]

One of the most important subjects in reducing the destructive energy of the flow is hydraulic jump. Hydraulic jump control is carried out to reduce the damage to downstream structures. A hydraulic jump is one of the most rapidly varied flows occurring when the flow is altered from supercritical state to subcritical. Most researchers have been tried to provide conditions for more energy dissipation by changing the design of stilling basin. In recent years, it has been determined that divergence, the presence of roughness or the formation of slopes in the basement of the stilling basin, could be effective in reducing jump dimensions and be economically feasible. An appropriate method for controlling the hydraulic jump is to use the bed roughness. In addition to reducing the length and depth and increasing the hydraulic jump energy loss, roughness stabilizes the jump in the position of the stilling basin. Besides, when the required depth for a classic hydraulic jump cannot be provided, or the cost of excavation to lower the bed of the stilling basin is not economical, construction of an expansion in the channel is a way to ensure the formation of the hydraulic jump in the stilling basin. Ead and Rajaratnam (2002) showed that the axial velocity profiles at different sections in the hydraulic jump were similar with some differences from the profile of the simple plane wall jet. Abbaspour, et al. (2009) also indicated that the longitudinal velocity profiles at different sections are similar, but they are a little different from the profiles of classic hydraulic jumps. The results showed that the dimensionless thickness of the boundary layer ( ) for jumps on corrugated beds was about 0.57, while was 0.16 for classic jumps on a smooth bed. The shear force coefficient ) for jumps on corrugated beds is approximately 10 times that for jumps on smooth beds, and the shear force coefficient showed a sharp growth with increasing Froude number.
The experiments were conducted in a flume with Plexiglas bed and walls, which was 8.0 m in length, 0.40 m in width, and 0.60 m in depth. It was connected to a hydraulic circuit allowing for recirculation of discharge. The discharge in the flume was obtained from an overhead tank. The discharge was measured by a calibrated magnetic flow meter located in the supply line, with ±5% accuracy. Water entered the flume under a vertical sluice gate, 1.2 m deep. The supercritical depth was controlled by the upstream sluice gate, while the tailwater depth was controlled by the downstream sluice gate. The point gauge with an accuracy of 1 mm was used for the measurement of the depth of water at various locations in the flume. In each experiment, the flow velocity was measured by using a pitot tube in several sections of the jump length. Twenty-five piezometers were installed on a false floor, at the site of the hydraulic jump. Phenomenon during the jump was investigated by these piezometers. The divergence ratio was considered in the range of , and natural rough bed conditions with 0 ≤ ks/d1 ≤ 0.9. In total, 81 experiments was conducted in the range of Froud numbers of 4.9 to 9.5 and discharge of 30 to 50 . For the rough bed, the reference level was assumed coincident with the plane passing at the top of the particles that the height was assumed equal to the median size (d50).Primary and sequent depths were measured by point gauge and velocity profiles by pitot tube.
By investigating the velocity profiles, it was found that the relative thickness of the boundary layer  was 0.51, while for the classical jump was 0.16.When both roughness and abrupt expansion parameters were used, the value of shear force coefficient (at expansion ratio of 2 and roughness size of 2.2 cm) was 14.7 times the shear force factor in classical jump on average. The water fluctuations, pressure variations the probability of existence cavitation were studied using the piezometers installed at the bottom of the stilling basin. As the expansion ratio and the roughness size increased, the cavitation coefficient decreased. The measurements showed that there was no cavitation in these experiments. In the most critical case, the cavitation coefficient in the structure did not fall below 5.7.

کلیدواژه‌ها [English]

  • Hydraulic jump
  • thickness of the boundary layer
  • abrupt divergence
  • speed profiles
  • cavitation