Fluctuating Hydrodynamic Pressures of Submerged Hydraulic Jumps Stilling Basins

Different types of energy dissipators are used to reduce the excess kinetic flow energy downstream of the spillways. The flow characteristics of submerged jumps differ significantly from those of free jumps. It was observed that the mixing of the flow jet decreases with the increase in the submergence of hydraulic jumps. This leads to reduced energy dissipation compared to free jumps. Pressure fluctuations in energy-dissipator structures are caused by the fluctuating nature of hydraulic jumps, which are caused by strong eddies and water-air mixing. Therefore, the identification and measurement of hydrodynamic pressures on the bottom of stilling basins play a significant role in the design of the thickness of the bottom slab and the determination of effective forces. Most of the research have been carried out in the field of hydrodynamic pressures of free hydraulic jumps in stilling basins with rough bed downstream of sluice gates. However, few studies are found in the literature regarding the effect of chute blocks and the end sill of the stilling basin on the bottom pressure fluctuations downstream of the spillways for different values of the initial Froude number and the submergence ratio in hydraulic jumps. This research aims to understand the mechanism of pressure fluctuations and determine the coefficient of pressure fluctuations intensity (C'P) of hydraulic jump at the bottom of USBR Type I (smooth bed) and Type II stilling basins, including chute blocks and end sill, at the downstream of the spillway.In this paper, the C'P coefficient on the bottom of USBR stilling basins was evaluated based on the experimental data. Several experiments were carried out in the hydraulic laboratory flume of the University of Tabriz to collect data. The geometric dimensions of an Ogee spillway and stilling basins were designed based on USBR criteria. Based on this, the length of Type I (LI) and Type II (LII) basins for the maximum flow discharge (Qmax) was calculated as 200 and 125 cm, respectively. Experiments were performed at different flow discharges varying ratios of submergence (S=Yt/Y2) equal to 1 (for free jump), 1.05, 1.1, 1.2, 1.3, and 1.4, respectively. The values of initial Froude numbers (Fr1) for different flow discharges were calculated in the range of 7.12 to 9.46. Therefore, in the present research, the range of relatively high values of Fr1 was investigated. Pressure transducers were used to measure fluctuating pressure data, which could record instantaneous pressure values at different times and provide the time series of pressure at for each pressure taps. The pressure transducers were calibrated for the pressure load range of ‒100 to +100 cm, and their nominal measurement accuracy is ±0.5%. The data acquisition frequency of the pressure transducers was 20 Hz, and the duration of data collection was 90 seconds for each pressure taps in each experiment. The number of pressure taps was equal to 25 points. Each pressure tap was connected to the corresponding pressure sensor using a hose with a diameter of 3 mm.The results showed that the maximum values of the CʹP coefficient occur in the initial zone of basins. In free jumps, CʹPmax values in the basinI were located in the range of 15 ≤ ΓX ≤ 20, and in the basinII in the range of ΓX ≤ 15. The variation trend of free jumps showed that in the zone of ΓX ≤ 10 of the basinI and basinII, CʹP coefficient values increase by 49% and 36% on average, respectively, by decreasing the Fr1 parameter. The rate of increase in CʹP coefficient values in free jumps with the reduction of the Fr1 parameter in the zone of ΓX > 10 in basinII was somewhat higher than basinI. In the upstream location of submerged jumps, the CʹP coefficient increased as the Fr1 parameter decreased. The increase of the CʹP coefficient with the decline of the Fr1 parameter in submerged jumps for the upstream zone of basinI and basinII was calculated to be equal to 7 and 4.8 percent, respectively. In the area of ΓX > 10 in the basinII, depending on the degree of submergence of the jump, the values of the coefficient CʹP ranged from 62 to 77% with the decrease of the Fr1 parameter. The variations of the CʹP coefficient in the downstream area of basinI were somewhat similar to basinII. The values of the CʹP coefficient in the upstream zone of the basins for higher Froude numbers were higher in submerged jumps than in free jumps. It seems that the turbulence intensity of submerged jumps in this zone was higher than free jumps. The results showed that the position of CʹPmax values of free jumps inside the basins is located in the zone of ΓXmax < 22 and ΓXmax < 16, respectively. The mean values of CʹPmax in free jumps of the basinI and basinII were achieved around 0.039 and 0.062, respectively. The reduction of CʹPmax in basinII compared to basinI in free jumps is 38% on average. The values of the CʹPmax coefficient decreased with the increase of Fr1 in free and submerged jumps. As the submergence ratio (S) increased, CʹPmax values decreased in different Froude numbers (Fr1). CʹPmax for the minimum Froude numbers had higher values in free jumps compared to submerged jumps.