مطالعه آزمايشگاهي اثر شيب معكوس و پله منفي ابتدايي بر خصوصيات پرش هيدروليكي

Experimental study of the effect of adverse slope and elementary negative step on hydraulic jump characteristics

حوضچه‌هاي آرامش قسمت كوتاهي از يك كانال كف سازي شده‌اند كه به‌صورت سازه‌اي خاص در انتهاي سرريزها و هر سازه ديگري كه جريان فوق بحراني ايجاد مي‌كند، براي كنترل پرش هيدروليكي ساخته مي‌شوند و هر چه طول آن‌ها كوتاه تر باشداز نظر اقتصادي مناسب تر خواهند بود. هدف از اين تحقيق بررسي خصوصيات پرش هيدروليكي در كانال با شيب معكوس و پله منفي ابتدايي مي‌باشد. در اين پژوهش اثر همزمان سه شيب معكوس و سه ارتفاع پله منفي ابتدايي بر اعماق مزدوج، طول پرش و افت انرژي در اعداد فرود 4.5 تا 9.5 بررسي شده است. نتايج نشان مي‌دهد كه با افزايش شيب معكوس كانال، مقدار نسبت اعماق مزدوج، طول پرش و افت انرژي به ترتيب 12.6، 13.9 و 16.8 درصد كاهش مي‌دهد. در حالي افزايش كه پله منفي نسبت اعماق مزدوج و طول پرش را به ترتيب 5.66 و 6.2 درصد افزايش و افت انرژي را 2.6 درصد كاهش مي‌دهد. تركيب دو عامل شيب معكوس و پله منفي نسبت اعماق مزدوج، طول پرش و افت انرژي را به ترتيب 12.42، 14.8 و 3 درصد كاهش مي‌دهد و نيز تنش برشي 12.56 برابر شرايط بستر صاف به‌دست آمد. بنابراين نقش پله منفي در تثبيت پرش در محل حوضچه آرامش مي‌باشد و شيب معكوس اين تثبيت پرش را سخت‌تر مي‌كند.

Stilling basins are a short section of the floor of channels that are constructed as special structures at the end of the spillways and any other site that creates a supercritical flow to control the hydraulic jump. The shorter their lengths, they will be the more economically viable. One of the most important topics in the study of reducing the destructive energy of the flow is hydraulic jump. Hydraulic jump control is carried out in order to reduce the damage of the downstream structures. A hydraulic jump is one of the most rapidly varied flows occurring when the flow is altered from a supercritical state to a subcritical one in a part of its path. Researchers have always tried to control the hydraulic jump to prevent severe damage to downstream hydraulic structures. The high cost of building recreational pools has encouraged researchers to reduce sequent depth, jump length and reduce energy losses. There are several methods to reduce the sequent depth and length of the hydraulic jump, as well as increasing the energy loss of the hydraulic jump, such as increasing the adverse slope of the bed or using roughness in the channel floor. In addition to reduce the dimensions of the hydraulic jump for the cost-effective construction of the stilling basin, the stabilization of the hydraulic jump at the pool site is also very important. A sudden drop in the bed (negative step) stabilizes a horizontal hydraulic jump of channel adjacent to the step for a wide range of sequent depths. This paper introduced a new solution for the hydraulic jump on adverse slope and negative step to the difference between the upstream and the downstream momentum fluxes. The channel used in this research had a rectangular shape with a length of 8, a width of 0.4 and a height of 0.6 meters. The walls and floor of this channel were made of transparent Plexiglas sheet. This channel was fed by a pump with a maximum discharge of 50 liters per second. In order to increase the initial Froude number before the upstream sliding gate, the tank height is considered to be 1.25 m. To create the stairs at the beginning of the channel, Teflon sheets with thicknesses of 3 and 6 cm were used. Taking into account the zero height, three different heights were considered for the stairs. In this study, three angles of zero, 3 and 6 degrees were used for the adverse slopes. To change the slope of a false plate with a length of 3 meters was applied that its sloping part continued to a distance of 2 meters from the gate, the slope of which was adjustable. This plate was located at the bottom of the channel, just after the initial gate, where the jump occurs completely in this part of the channel. A sliding gate was installed at the inlet of the flume to create the jump. In addition, another valve was used at the end of the flume to stabilize the jump. In the performed experiments, the downstream gate was adjusted so that the start of the jump occurred at the point of change in the height of the channel. In the current study, the total number of the experiments was 81 that performed in the range of 4.5 to 9.5 Froude numbers. In this research, the combined effect of using three inverse slopes and three elementary negative step elevations on hydraulic jump characteristics (including: sequent depths, jump length and energy loss) were studied in Froude numbers of 4.5 to 9.5. The results showed that by increasing the adverse slope of channel reduced the sequent depths, jump length and energy loss by 12.6%, 13.9% and 16.8%, respectively. By increasing the negative step, the ratio of sequent depths and jump length increased by 5.66% and 6.2%, respectively; while, energy loss decreased by 2.6%. The combination of the adverse slope and negative step reduced the sequent depths, jump length and energy loss by 12.42%, 14.8% and 3%, respectively, and also created a shear of 12.56 times equal to the conditions of a smooth bed. Therefore, the negative step only stabilizes the jump in the stilling basin, and the adverse slope makes this jump stabilization difficult.