Abstract

Hydraulic jump is a rapidly varied flow phenomenon that the flow changes suddenly from supercritical to subcritical. Hydraulic jumps are frequently observed to exist in natural river channels, streams, coastal water, and man-made water conveyance systems. Because of a sudden transition of flow regime, hydraulic jumps result in complex flow structures, strong turbulence, and air entrainment. Accordingly, they are two-phase flow, with air being the gas phase and water being the liquid phase. Consequences of the occurrence of hydraulic jumps include: unwanted fluctuations in the water surface with unstable waves and rollers, undesirable erosion of channel sidewalls and channel bottom, and reduced efficiency for water conveyance systems. Thus, it is important to study various aspects of the phenomenon.
So far, knowledge of the phenomenon is incomplete. The main objective of this research is to improve our understanding of the complex flow structures and distributions of air entrainment in a hydraulic jump. Previously, both experimental and computational studies of the phenomenon have typically suffered a scale problem. The dimensions of the setup being used were unrealistically too small.
In this research, we took the computational fluid dynamics (CFD) approach, and simulated hydraulic jumps at relatively large and practical dimensions. This would help reduce artificial scale effects on the results. On the basis of Reynolds averaged continuity and momentum equations, CFD simulations of hydraulic jumps were performed for four different cases in terms of the approach flow Froude number Fr1, ranging from 3.1 to 5.1. The Reynolds number is high (between 577662 and 950347), which ensures turbulent flow conditions. The CFD model channel is discretized into 2,131,200 cells. The mesh has nearly uniform structures, with fine spatial resolutions of 2.5 mm. The volume of fluid method provides tracking of the free surface. The standard k-ε turbulence model provides turbulence closure.
For each of the simulation cases, we carried out analyses of time-averaged air volume fraction, time-averaged velocity, time- and depth-averaged (or double averaged) air volume fraction at a series of locations along the length of the model channel (Note that the terms air volume fraction and void fraction are used interchangeably in this thesis). We compared the CFD predictions of air volume fraction with available laboratory measurements. It is important to note that these measurements were made from laboratory experiments that corresponded to essentially the same values of Fr as this CFD study, but used a channel of smaller dimensions, in comparison to the CFD model channel. The CFD results of time-averaged air volume fraction are reasonable, when compared to the experimental data, except for the simulation case with Fr1 = 3.8. For all the four simulation cases, the predicted variations in air volume fraction show a trend in consistency with the experimental results. For the three simulation cases (with Fr1 = 3.1, 3.8 and 4.4), the time-averaged air volume fraction in the hydraulic jumps is larger at higher Reynolds number. However, for the simulation case with Fr1 = 5.1, it is smaller at higher Reynolds number. This implies that the amount of air being entrained into a hydraulic jump depends on not only Fr1 but also the depth of the approach flow. In future studies of the hydraulic jump phenomenon, one should consider using approach flow of realistically large dimensions at various values of Fr1, for realistic predictions of air entrainment in hydraulic jump rollers.