CFD ANALYSIS OF HYDRAULIC JUMP UNDER VARYING CHANNEL CONDITIONS

Abstract

Hydraulic jumps are commonly observed phenomena in open channels, including rivers, canals, spillways, and weirs, where they serve a significant function in dissipating excess energy. The observed phenomenon is characterised by the interaction between a high-velocity water flow and a comparatively slower water flow, leading to an abrupt increase in water depth and the subsequent release of a substantial amount of energy. Hydraulic jumps are significant part of various hydraulic processes, as most often they are used not only as energy dissipation tool but also as tool for water aeration and, thus, as chemical mixing agents or as hydraulic jumpers which can lift water levels downstream. Consequently, much research has been conducted with a view of understanding and improving hydraulic jumps. Hydraulic jump type stilling basin came into knowledge to reduce a large amount of kinetic energy of the flowing fluid in the downstream of the hydraulic structure. This design feature is important in order to exclude the possibility of bed erosion and to achieve the rational use of protective aprons. Within these basins, the water's kinetic energy undergoes a process of transformation, resulting in the generation of turbulence. This turbulence, over time, is dissipated as both heat and sound energy. In order to accomplish this objective, stilling basins are outfitted with various components, including baffle blocks and chutes. Prior research has investigated the behaviour of hydraulic jumps over uneven surfaces in order to improve their characteristics downstream. The present investigation is oriented around examining the influence of different strip macroroughness shapes, their aspect ratios, and arrangements on the characteristics of hydraulic jumps using numerical simulation. This particular area of study has not been extensively explored in previous research activities. This computational study applied the Computation Fluid Dynamics (CFD), which allowed for an understanding of both types of hydraulic jumps, namely, free and viii submerged hydraulic jumps. Key characteristics, such as tail-water depth, sequent depth, jump length, roller length, velocity profiles, flow patterns within the cavity region, Turbulent kinetic energy (TKE), and energy loss are examined over both smooth and macrorough beds. CFD involves predicting fluid flow by solving mathematical equations through simulation. This study takes the advantage of Ansys Fluent software where Reynolds-averaged Navier-Stokes (RANS) equations and standard k-ε, re-normalization group (RNG) k-ε, realizable k-ε, and shear stress transport (SST) k-ω turbulence models are used to predict the mean flow characteristics in turbulent flows. The interface between two immiscible fluids is represented using Volume of Fluid (VOF) model. To ensure the accuracy of the numerical models, sensitivity analyses of the mesh and turbulence models are performed, and the results are compared with experimental findings for three channel conditions: smooth bed, triangular strip macrorough bed, and trapezoidal strip macrorough bed. Among the various turbulence models RNG k- ε model outperforms for predicting both free and submerged hydraulic jumps. Additionally, the standard deviation between the numerically derived results and experimental findings for basic hydraulic jump parameters, such as sequent depth ratio and tailwater depth ratio, is reported below 6%. These findings suggest that Ansys Fluent is a reliable tool for predicting complex phenomena like hydraulic jumps over smooth and rough beds. To numerically study the hydraulic jump phenomenon, it is crucial to optimize the combination of flow domain and boundary conditions to simulate real-world fluid flow phenomenon cost-effectively, a topic not fully addressed in previous research. In this regard, two approaches are tested for forming hydraulic jumps in a horizontal rectangular smooth channel numerically. The first approach considered the effect of a reservoir just upstream of the sluice gate, while the second approach ignored the reservoir entirely. The results indicated no significant differences in the hydraulic jump's flow behaviour between the two approaches. However, the second approach reduced computational time by up to 50% due to the smaller computational flow domain. ix Further investigations were conducted in a prismatic rectangular channel with strip macrorough elements on the downstream bed. Eight distinct macrorough shapes, namely triangular, rectangular, trapezoidal, semicircular, and four new non-regular shapes were compared for their effect on free and submerged hydraulic jump characteristics. It is reported that rough beds generally improve jump characteristics compared to smooth beds. While the shape of the macroroughness has a minor influence on jump characteristics, the triangular strip macrorough shape is the most effective in enhancing hydraulic jump characteristics. Given the effectiveness of the triangular strip macroroughness, further studies are carried out to explore how the height-to-base width ratio and height-to-wavelength ratio, as well as the arrangement of the strips, affected jump characteristics. The results indicated that varying the height-to-base width ratio does not significantly impact the jump characteristics. However, better energy dissipation is achieved in submerged hydraulic jumps by altering the arrangements of the macrorough elements. Additionally, increasing the spacing between consecutive macrorough elements or decreasing the height-to-wavelength ratio improved the jump characteristics. Overall, strip type macrorough beds are found effective in enhancing the jump characteristics and a well-designed model can improve the efficiency of hydraulic jump type stilling basins. Further, the numerical simulation technique is well able to predict the complex fluid flow phenomenon and can be beneficial in hydraulic designs.