Laboratory Investigations on Hydraulic Jump Characteristics Using Submerged Vanes with a Certain Angle of Attack.

Hydraulic jump is a rapid energy dissipation phenomenon that falls under the sub-branch of hydraulic science. It involves a transformation of water flow from a shallow and high-velocity state to a deep and low-velocity state, leading to substantial energy dissipation. This energy dissipation primarily occurs through the conversion of kinetic energy into thermal energy, resulting in a significant reduction in destructive effects (Chanson, 2011). To control hydraulic jumps, energy dissipaters, such as baffles are used in the stilling basins. One possible alternative to the use of baffles is the implementation of artificial roughness on the channel bed. This approach involves introducing roughness elements that disrupt the water flow, which leads to unequal momentum at the inlet and outlet. Specifically, the resistance forces exerted by the roughness elements reduce the momentum at the outlet (Beirami & Chamani, 2010). Ead and Rajaratnam (2002) studied the effect of wavy beds on hydraulic jump characteristics. Similarly, Abbaspour et al. (2009) found that the use of sinusoidal-shaped roughness elements resulted in a decrease in the length and depth of hydraulic jumps. Soltani (2013) investigated the impact of submerged vanes with different heights and placements in the channel bed. The findings indicated that increasing the height of the vanes resulted in a decrease of up to 9% in secondary depths and up to 18.6% in the length of the hydraulic jump. In another study by Parsamehr et al. (2020), the influence of bed slope and inverse bed slope was examined. Additionally, Pourabdollah et al. (2020) examined the effect of inverse bed slope and positive end sill on hydraulic jump characteristics. Both laboratory studies demonstrated a reduction in length and secondary depth, with the inverse bed slope having a greater impact on reducing hydraulic jump characteristics.

The experiments were conducted at the Hydraulic Laboratory of Isfahan University of Technology in Isfahan, Iran. A Plexiglas flume measuring 8 meters in length, 0.4 meters in width, and 0.6 meters in depth was used. Submerged vanes with a fixed attack angle of 30° and varying arrangements (d=0.2, 0.3 meters) were employed as roughness elements on the channel bed. The flow rate and initial Froude number were controlled by adjusting the upper and lower initial gates. Furthermore, the position of the hydraulic jump could be regulated along the channel using the lower outlet gate. The channel consisted of two reservoirs: one located at the end of the channel and beneath it, and the other situated at the beginning of the channel and in front of it. Water was pumped from a reservoir below the channel to the upstream reservoir and then directed into the channel.To start the experiment, the pump was turned on, and both the initial and outlet gates were kept closed until the water level rose above the first gate. Then, the desired initial Froude number for the experiment was achieved by adjusting the opening of the initial gate to create the hydraulic jump. The jump could be controlled and fixed at the desired location using the outlet gate. The depths of the initial and secondary hydraulic jumps were measured using a depth gauge, while the flow rate was measured using an electromagnetic flow meter. The research involved 27 experiments with flow rates of 30, 40, and 50 l/s. Parameters such as flow rate (Q), upstream water depth (H), initial depth of hydraulic jump (D1), secondary depth of the jump (D2), and length of hydraulic jump (Lj) were measured.

The research and experiments were divided into three stages. Initially, we examined variations in the depths of hydraulic jump in the presence of submerged vanes. These vanes had an attack angle of 30 degrees and were arranged at two different distances: 0.2 and 0.3 meters between each vanes groupes. The results showed that the presence of submerged vanes caused a reduction in the secondary depth of the hydraulic jump. This reduction was due to the obstruction of flow and the trapping of air bubbles in the protrusions created by the vanes. As a result, there was a decrease in energy dissipation, leading to a decrease in the secondary depth of the jump. To compare the relative length of the hydraulic jump, it was non-dimensionalized with respect to the initial depth. Then corresponding points for each experiment were plotted against the initial Froude number. The presence of submerged vanes in the channel bed was found to cause a decrease in the relative length of the hydraulic jump. On average, the relative length of the hydraulic jump decreased by 15.5% and 14.4% for experiments with attack angles of 30 degrees and vane distances of 0.2 and 0.3 meters, respectively. The reduction in the hydraulic jump was also greater compared to the classical condition. Additionally, the corresponding equations for each section were derived and presented.