Numerical study of effective parameters on energy dissipation of baffle blocks in submerged hydraulic jumps

The hydraulic jump takes place in both natural and manufactured systems. As it can be seen in streams, rivers and water distribution and irrigation networks formed downstream of hydraulic structures such as spillways, sluice gates, and drops. Generally, it is necessary to construct special structures downstream of flow in order to prevent damage caused by the high energy of water in supercritical velocities and also to dissipate the extra kinetic energy of hydraulic jumps. Stilling basin is one of these structures which is constructed downstream of spillways or waterfalls. Baffle blocks are often used to stabilize the jump, decrease its length and increase the energy dissipation. In order to make stilling basin with its dissipating equipment effective, the design should be in a way that the tailwater depth becomes greater than or equal to the sequent depth, otherwise, the jump doesn’t occur completely and will be swept out of the basin, resulting in scour of the downstream channel. If the flow rates become more than the design discharge, the tailwater depth will be greater than the one required for a free hydraulic jump. These situations are common in low head hydraulic structures including low diversion dam spillways and gates. Under such conditions, the hydraulic jump will be submerged. For submerged hydraulic jumps with blocks, two different types of flow have been observed, the deflected surface jet regime (DSJ) and reattaching wall jet regime (RWJ). There was also a transition state in which the flow could be changed from one state to the other by some external disturbance. In this article, a numerical study was conducted to investigate the influence of some parameters, consist of block height and shape, Froude number and distance of blocks from the gate, on the performance of submerged hydraulic jumps with blocks as energy dissipators. 3D RANS simulations have been applied by Fluent software. RSM turbulence model was used which illustrated much precise results in verification. In total fifty-four models with different geometrical and hydraulic situation according to the four mentioned parameters have been created and the percentage of dissipating energy is presented in each case to find the most effective condition. It was observed that the Froude number is the most important factor in the study of dissipating energy; such that the percentage of dissipating energy increases almost ten percent per one unit raise in Froude number. Furthermore, the existence of a slope at the back of blocks does not have an effect on energy dissipating, but it can be implemented to avoid cavitation. In addition, the percentage of dissipating energy goes up as the blocks are mounted closer to the gate and also provided the condition which leads to the deflected surface jet regime. The more turbulence in the deflected surface jet regime makes the desirable condition in which baffle blocks perform more efficiently as energy dissipators in comparison to reattaching wall jet regime. Finally, it can be concluded that for effective energy dissipation, block dimensions and all conditions should be provided in a way to form submerged hydraulic jump as the deflected surface jet regime.