بررسي آزمايشگاهي اثر زبري بستر جريان بر مشخصات پرش هيدروليكي

Experimental investigation of the effect of bed roughness on hydraulic jump characteristics

پرش هيدروليكي يكي از پديده هاي مهم جريان آب در كانالها بوده كه به تناسب شدت پرش، با استهلاك انرژي محسوسي روبرو خواهد شد. استفاده از بسترهاي زبر، عامل موثري در كاهش هزينه هاي حوضچه هاي استهلاك انرژي ميباشد. بنابراين هدف محققين، به كارگيري بسترهاي زبر در كف حوضچه هاي آرامش و بررسي تاثير آن بر مشخصات پرش هيدروليكي بوده است. در اين پژوهش، به بررسي آزمايشگاهي اثر زبري بستر بر مشخصات پرش هيدروليكي در فلومي به طول 12، عرض 0.5 و ارتفاع 0.7 متر در محدوده اعداد فرود 5/5 تا 9 پرداخته شد و از 24 بستر زبر مكعبي با آرايش زيگزاگي و رديفي، رديف ها و زاويه هاي مختلف استفاده گرديد. نتايج نشان داد كه عمق و طول نسبي (نسبت عمق ثانويه به اوليه و نسبت طول به عمق اوليه پرش) در آرايش زيگزاگي در مقايسه با رديفي نسبت به بستر صاف كاهش بيشتري دارد. ضريب نيروي برشي كه عامل اصلي كاهش عمق و طول نسبي پرش ميباشد در هر دو آرايش 6 درصد بيشتر از بستر صاف محاسبه شد. در نهايت روابطي براي برآورد عمق نسبي و طول نسبي به ازاي اعداد فرود، استخراج گرديد. همچنين اثر آرايش ها در سطح معني داري 5 درصد براي همه بسترها بر عمق و طول نسبي به اثبات رسيد.

The hydraulic jump is one of the important phenomenon of water flow in channels that in proportion to the intensity of jump, will face a significant energy dissipation. True hydraulic jump form for Froude numbers greater than 4.5. Position of steady jump is the least sensitive type to fluctuations in the tail water elevation and forms steadily at the same location. The change of flow state from supercritical to a subcritical state is accompanied by discontinuity in depth of flow, formation of a surface roller, visible air entrainment and two phase flow, significant turbulence, kinetic energy dissipation, generation of spray and sound. The basic characteristics of a hydraulic jump, such as conjugate depth, location of jump, the energy loss in the jump, efficiency of jump, the length of the jump. From the designer's standpoint, hydraulic jump is a powerful energy dissipation mechanism and due to its erosive potential, is the centroid of scour. In order to take full advantage of the hydraulic jump in apron design, the erosive effect of jump should be confined to the limits of the heavily protected bed surface, hence intensive scour energy would not cause any harm to apron. This is why; hydraulic jump should be in a stilling basin with a fixed location. Turbulent flow over a rough surface is an important problem and it has been subject of diverse fields. Using roughness elements at the bottom of the flow causes increase in resistance to the flow hence accelerate the upstream flow development and increase in amount of kinetic energy, converted into the turbulent energy. Using of rough beds are effective factor in reducing costs on energy dissipation basins. So researchers’ next purpose have been the use of roughness beds in the stilling basins bed and investigation the effect it on hydraulic jump characteristics. In this research, experimental investigation effect of bed roughness on hydraulic jump characteristics in a flume with length of 12 meters, width of 0.5 meters and height of 0.7 meters was done in range of Froude numbers 5.5 to 9 and used 24 cubic rough beds with staggered and strip arrangement, different rows and angles. Discharge was measured by a triangular weir with a 90° angle, placed at the end of channel and size of cubic blocks was 0.05 meters. Point gauges with a sensitivity of ±0.1 mm used to measure the depths of flow. To evaluate the effect of roughness, both staggered and strip arrangement were used in the experiments. For creating a hydraulic jump, sharp sluice gate has been used in a condition which places the rough beds in downstream. Relative depth and relative length was obtained as a function of Froude number. The relative depth values on smooth beds obtained in the present study are in good agreement with the data of Belanger classic jump. Relative depth, relative length and shear force coefficient increase for staggered arrangement, strip arrangement and smooth bed with increasing Froude number. The value of relative depth, relative length in compare to smooth bed in the same Froude number on rough beds is less and shear force coefficient is greater than smooth bed. Equations obtained with SPSS software for jump relative depth. Result shows that relative depth and length for staggered arrangement in compare strip have more reducing ratio smooth bed. Also showed staggered arrangement produced better performance compare to strip arrangement in ratio secondary to primary depth and ratio length to jump primary depth. Both types arrangement a better performance compared to jumps formed on smooth beds. For staggered and strip arrangement the average of length ratio to jump primary depth was obtained 1.9 and 1.6 times less than smooth bed respectively. In the experiments the ranges of relative for the staggered and strip arrangement were 20.47 to 39.04 and 46.66 to 24.28, respectively. Shear force coefficient that main factor is in reducing of depth and jump length calculated 6 percent more of smooth bed in both arrangement. On the other hand, comparing of this experimental results with the other similar researches on different types of rough beds expresses match results. Finally, equation for estimate of relative depth and relative length for Froude numbers extracted. Also the effect of arrangements in the level of 5 percent were proofed for all beds on relative depth and relative length.