Experimental investigation of the effects of block inclinations on the hydraulic jump characteristics in the stilling basin USBR III

Background and objectives: Hydraulic jump is one of the common methods for dissipation of the kinetic energy of the flow in downstream of the hydraulic structures in stilling basins. Length of the stilling basin, depth ratio and the energy loss are some of the important parameters that should be taken into account for designing an economically efficient structure. Historically, Bidone was the pioneer to conduct investigations on the hydraulic jump in 1818.There after characteristics of this phenomenon have been investigated by many researches using cubic, rhombus and trapezoidal baffle blocks with and without curved upstream edges, sinusoidal bed, trapezoidal corrugation, sills and rectangular corrugations. Despite of the comprehensive researches performed during the last two centuries, there are still unknown issues that warrant more research on this subject. The perpose of this research is investigation of the effects of block inclinations on the hydraulic jump characteristics in the stilling basin USBR III.
Materials and methods: Experiments were conducted in a flume with glass walls and smooth bed with a rectangular cross section 1m wide, 85 cm deep and 15m long. The model consisted of a head tank, a slice gate with constant opening of 3 cm and a tail gate to obtain various flow depth and a flowmeter to measure the discharge. Part of beginning of flume immediately after the gate of the head tank, was used as stilling basin and baffle blocks and end sill were installed according to the criterion of a type ΙΙΙ USBR stilling basin. In this study, baffle blocks were installed in two cases of diverge and converge position relative to the central axis of the flume and in each position 8 placement angle of 10, 20, 30, 40, 50, 60, 70 and 80 degrees were considered for blocks. For each flow discharge, the tailgate was adjusted to establish a non-submerged hydraulic jump so that toe of the jump was formed at the beginning of the flume under upstream gate, then the length of jump, initial and secondary flow depth were measured and burst photos were taken from profile of the jump. Froude numbers and discharges ranged from 4.44 to 8.56 and 39 to 81.7 l/s respectively. In total, 180 tests were carried out for classic, USBRΙΙΙ and diverge and converge position jumps.
Results: In diverge position, increase in the placement angle of blocks up to 30° on convergent position caused significant decrease in jump length and further increase in placement angle resulted in separation of jump front from walls of flume and inclination to the central axis of flume and in turn increase of jump length. Also in diverge position, for all placement angles of blocks increase in jump lengths were observed.
Conclusion: The results showed that, increase in the placement angle of blocks up to 30° on convergent position caused 40 percent decrease in jump length and 10 percent decrease in depth ratio and also increase in relative energy loss to 75 percent. It is believed that the main reason of decreasing the jump length was raising the rate of secondary flows due to inclination of blocks in angle of 30 degrees.