Hydraulic jump is a complex flow problem and surface discontinuity event that occurs during the transition from supercritical to subcritical regime in free surface flow, its occurrence depends on different hydraulic structures such as threshold, weir, sluice gate, base piers or stilling basin. These hydraulic structures cause the flow depth to increase and the flow to pass through the hydraulic jump process. The transition is an extremely turbulent flow associated with the development of large-scale turbulence, surface waves and spray, energy dissipation and air entrainment, and it is characterized by strong dissipative processes [1]. A hydraulic jump can serve many purposes. For example, to disperse flow energy to prevent bed erosion, to provide ventilation, or to facilitate mixing of chemicals used to purify water [2].
Stilling basins are designed and constructed to dissipate energy and thus reduce erosive power of the high velocity flow downstream of chute spillways. Geometric properties of a stilling basin depend upon the water depth and Froude number of the incoming flow and the required energy dissipation rate. Hydraulic jump stilling basins may include drops, expansion, sills, baffles, blocks and steps, typically used to decrease the basin length and stabilize the jump toe position [1].
An early study on the hydraulic jumps on the sloped channel was presented by Bakhmeteff, Matzke [3]. Kindsvater [4] classified jumps on the sloped channel according to their toe position relative to the channel bottom kink: A-jump for which the toe is at the kink, B-jump is the intermediate of A- and C-jumps, C-jump for which the end of roller is above the kink, and D-jump where the entire roller region is on the sloping channel. After the Kindvater's study, many studies have been conducted on these types of jumps [5–19]. Their main findings can be summarized as follows; (i) The energy dissipation rate decreases from A jump to D jump due to reduction in the force of hydraulic jump with the increasing tail water depth, (ii) The hydraulic characteristics of the classical jump and the A-jump are similar, (iii) Regarding the decay of bottom shear velocity, the sloping and the classical jumps are identical (iv) The roller lengths of C- and D-jumps are almost identical, (v) D-jumps are located between the classical jump and the classical wall jet as regards the decay of maximum forward velocity, (vi) The maximum bottom velocities and maximum surface velocities are near the side-walls and along the centerline of the channel, respectively. Peterka [20] summarized the extensive experimental tests conducted at the United States Bureau of Reclamation (USBR) and presented the general design rules of USBR-type stilling basins. Ohtsu et al. [21] experimentally measured the pressure distributions in the upstream and downstream region of the continuous and vertical sill in order to design a stilling basin for the purpose of creating a force hydraulic jump. By examining the flow characteristics passing over the sill, an experimental approach to the impulse force acting on the sill is proposed. Hager, Li [22], experimentally examined the effect of cross and uninterrupted sill on jump in a rectangular open channel. Based on the classical hydraulic jump results, they reported that the sill controlled hydraulic jump can be shown as an example of a disturbed classical hydraulic jump, especially in the case of the jump pattern. It was also reported that the erosion and hydraulic precision of the tail waterbed should be taken into consideration during the design process. Vittal, Al-Garni [23] presented a new method of design for the type USBR III stilling basin over a major range of discharges passing the spillway structure. The modified new basin is arranged in double rows of the single row of friction blocks in the USBR basin, to set up the velocity distribution of the subcritical flow after the hydraulic jump in the basin. Debabeche, Achour [24] investigated the effect of broad and thin crested sill on both the minimum-B jump and the sill-controlled jump under various inflow conditions in a horizontal symmetrical triangular open channel. Either a thin-crested or a broad-crested sill used to create hydraulic jumps, for the Froude numbers range from 2 to 10. The data obtained from a large number of experimental results were adapted to empirical relations to notice the effect of the inflow Froude number on the different parameters, for instance relative sill height and the non-dimensional toe position of the sill. Izadjoo, Shafai-Bejestan [25] experimentally investigated the effect of trapezoidal shape corrugated bed on flow characteristics of hydraulic jump for different Froude numbers and relative roughness. They found that corrugated bed, namely roughness, decreased the conjugate depth and hydraulic jump length 20% and 50% respectively. Ozbay [26] investigated the energy dissipation ratios of stepped, trapezoidal, T-shaped and wedge type baffle blocks placed the chute channels. From the experiments, it was found to be that the stepped baffle block type has slightly higher values of energy dissipation than the other baffle blocks tested in the study. Alikhani et al. [27] conducted an experiment to interpret the effects of a sill and position of sill on control of length and depth of a forced jump in stilling basin regardless of tailwater depth. The hydraulic properties of the jump measured in different discharges, compared with hydraulic properties of the classical hydraulic jump. As a result of the comparison, they determined that the sill had important effects on energy dissipation. They have developed a new relationship between parameters affecting hydraulic jump, such as sill height, sill distance, stilling basin length and sequential depth ratio. Hamidifar, Omid [28], conducted an experimental study to investigate the effect of a broad-crested sill on controlling the hydraulic jump formed in a horizontal and symmetrical triangular channel. Their results were compared with the previous experimental and theoretical studies and empirical equations were developed to predict the sequent depth ratio and the length of the jump and surface rollers. Ellayn, Sun [29] performed laboratory investigations to appraise the effect of a rough bed on hydraulic jump. The experiments were carried out using an artificially roughened bed with wedge-shaped baffle blocks for the Froude numbers in the range of 3.06 ≤ F1 ≤ 10.95 and a relative bed roughness ranging 0.22 ≤ KR≤1.4. They concluded that in the new stilling basin model, formed with the roughened bed with wedge-shaped baffle blocks, there was a reduction of 30–50% in the hydraulic jump length and 16.5–30% in the sequent depth compared to the smooth bed. Padulano et al. [30] carried out experimental studies on USBSR II type to figure out its hydraulic behavior and dissipation efficiency. The effect of a continuous, transverse sill on the hydraulic jump in a rectangular channel is experimentally analyzed by Hager, Li [22]. They classified submerged hydraulic jumps and hydraulic jump types from A-jump to spray. They provided the drag force and coefficients along with supposes of pressure extreme fluctuations. They also presented an assessment of dissipation efficiency for submerged and non-submerged jumps, and this assessment provided the opportunity to compare between different types of jump and classical hydraulic jump. Nandi et al. [31] experimentally and numerically investigated the hydraulic jump that occurs in flow conditions where the Froude number varies between 2.17 and 7 with three different channel base slopes. Using the data obtained as a result of experimental modeling, they developed an empirical formula to determine the location of the hydraulic jump by regression analysis. As a result of the study, the experimental results were compared with numerical model results and it was determined that the results were quite compatible with each other. Pourabdollah et al. [32], carried out experimentally free and submerged hydraulic jump in different stilling basins. The effects of different adverse slopes, rough beds and positive step heights on hydraulic jump characteristics in the range of 4.56 < Fr < 9.55 were investigated. They stated that the submerged depth ratio, lengths of the free and submerged hydraulic jumps are less than the classical hydraulic jump.
When the studies in which hydraulic jump is controlled using the sill are examined, it has been observed that studies on determining the velocity field of hydraulic jump with LDA is not enough. The LDA system has a great advantage in obtaining the velocity and turbulence characteristics of the flow without any intervention in the flow field. In addition, in the past studies, the effect of sill height on energy dissipation ratio, turbulence intensity, the length of hydraulic jump and roller was not examined in the case of the same Froude number. In addition, while the hydraulic jump occurring after the sluice gate was examined in the previous studies, in this study the hydraulic jump is evaluated in the stilling basin after the spillway chute channel ([33, 34, 12, 35, 36, 21, 37]. For these reasons, this study has differences from the existing literature, and it is thought to contribute to the literature.
It is well known that laser Doppler Anemometry (LDA) provides quantitative information on both instantaneous and time-averaged structures of the velocity field. Using instantaneous data, detailed information on the turbulent flow field could be obtained. In the event of forced hydraulic jump, the determination of the velocity field with the LDA will contribute. In addition, turbulence intensity, hydraulic jump length and characteristics of the roller region are determined in the case of different flows and sill heights. Some experimental study is available in the literature on the hydraulic jump; however, further experimental effort is needed either to confirm the previous findings or to provide new information for different experimental and structural conditions. The aim of the present study is to determine the hydraulic jump characteristics in a stilling basin downstream of the chute channel with different slopes for the Froude numbers in the range of 7 < Fr1 < 12 and relative sill heights in the range of 4 < hs/h1 < 13, where hs is the sill height and h1 is the flow depth at the toe of jump. The results from the measured velocity fields by the LDA, turbulence intensity distributions, flow profiles, energy dissipation ratios and geometrical characteristics of hydraulic jump are presented to provide a detailed evaluation of the jump in a stilling basin.