| Energy dissipation has been one of the main problems to be solved during hydraulic design for a long time in low head hydro projects. With the quick development of native hydro-constructions and the higher demand in water resource exploitation, the released flow from reservoirs tends to be large discharge per unit width and with low Froude numbers when using underflow energy disspatior. If there is no enough space to build spillway sections according to the releasing discharge, there should be serious bed scour and even dam broken down. Therefore, it’s quite necessary to solve the problem of energy dissipation in low Froude number flow. At present the domestic and international research in this area can be mainly divided into two parts. Domesticly, the research is mainly aimed at presenting special energy dissipation structures to meet the needs of actual engineering, as well as the quantitative relationship between them. While the foreign researches focuse on detailed flow pattern in lab model rather than energy related sections. In such context, this thesis starts from practical needs, with no limitation to specific engineering backgroud, seizes the main characteristics of water-air two-phase flow, introduces some new concepts, puts forward main factors which fundamentally influence energy dissipation efficiency through the generalized model test for hydraulic jumps at low Froude numbers and finally attemps to provide some new, valuable ideas for design consideration.In this study, the hydraulic jumps of low Froude numbers (Fr1<4.5) is taken as the research object and the generalized model test is carried out. Firstly, the basic flow the basic flow characteristics of hydraulic jumps is studied and the fluctuation characteristics of both internal, external flow structure is obtained. Then the overall energy dissipation efficiency of different jumps is estimated. This part of content is the basis for the further study and the main conclusions include:1. The hydraulic jump is a kind of flow pattern with periodic variations. Its external strcuture is mainly characterized with the change with time which shows some volatility and regularity. The experiment results show weak impacts on jump profiles from location under low Froude numbers and the gap between the minium and maximum value of water level is relatively small. The range of the gap will gradually expand with the increase of Froude numbers and can be formed a certain function with the turbulence intensity of the flow near surface region. Similarly, the formulation to calculate the conjugate water depth ratio is provided by flow force analysis inside jump length scope. The range of fluctuation under impacts of momentum correction factor a is obtained and the test results match well to it. Besides, the "Bubbles Escaping Position" method has been used to determine the jump length, which results a wide range of values. After comparison between the test results and empirical fromula, it can be found that the test values of length of jumps agree well with Chen’s formula at low Froude number condition. Moreover, during the research process, the relationship between jump location and the depth of contraction section has been builded and regularity has been found that with the increase of Froude numbers, under the same amplification in h1/hc, there need higher downward water level to hold the location of the hydraulic jump.2. In this thesis, the hydraulic jump has been divided into four main regions according to the energy dissipation effect and flow pattern, and both the time average and turbulent related parameters have been measured in the turbulent shear layer and the core area of water jet. Due to the existance of air, the flow rate in turbulence shear layer can reach about1.8times as that of inflow section. Bubbles will be torn into smaller ones under the effect of large velocity gradient, thus the turbulence shear layer becomes the main flow region where water bubbles interact. The diffusion pattern of the flow in core jet region is as nealy the same as the typical wall jet flow, but still there is some difference. The mean velocity of the flow in this region will gradually decrease along the flow distance and the location of the maximum value of section mean velocity tends to move towards water surface. But the location of the maximum value of section mean velocity is close to the bottom (Fr1=1.93, y/h1<1) under low Froude numbers, while Fr1=4.44, at a distance of0.75times the length of jump, the position of the largest section mean velocity keeps a distance from the bottom elevation of y/h1>2and the velocity distribution is more close to natural river flow.3. The needle type measuring instrument has been used to measure the distribution of void fraction in hydraulic jumps. The results demonstrate that the inflow section is the main region of aeration function, where the void fraction keeps the maximum value and will increase with Froude numbers. The changing rate of both the average and the maximum void fraction under different Froude numbers has been compared and conclude that the void fraction will reach its maximum value at section of0.1-0.3times of jump length. But the average void fraction will not be more than10%, while the maximum not more than20%. Finally the maximum energy dissipation rate is less than40%when considering complementary energy in outflow section, which confirms the fact that the energy dissipation efficiency of low Froude numbers jumps is relatedy low.The following content can be divided into two parts, according to the mechanism of energy dissipation in hydraulic jumps, one part starts from energy dissipation effect by water itself and the other part from the work done by bubbles existing in water. The main results can be concluded as follows.1. From the perspective of coherent structures within the detail of the hydraulic jump and vortex flow characteristics relationship between the vortex and the energy, which show jump in the water near the wall and the free jet region structure and horizontal strips both coherent vortex structures and study of these structures with the variation of position and time, and gives the burst period, distance between characteristic quantities are calculated and found that such a large-scale coherent structures in the flow field characteristics of the whole movement has a greater impact while the flow field in this discrete particles bubble movement also has some leading role, and according to the different bubble size, the degree of influence of this leading role also changes the Stokes number can be used to evaluate this degree of size, was found by experiment that there are two thresholds, respectively St=1.0and St=2.0.2. MicroADV is used to test the hydraulic jump the flow field spectroscopy. turbulence dissipation rate can be calculated by the assumed isotropic and Taylor frozen hypothesis. The study found that the same area within a hydraulic jump dissipation rate and the average dissipation rate of overall hydraulic jump will increase as the Froude number increased significantly. Meanwhile, the study found dissipation rate distribution in the cross section of obedience the law of "decrease-increase-decrease again". Studies indicate that the same kind of "anti-S" type of distribution patterns that exist in two inflection points for the hydraulic jump the special role of energy dissipation physical meaning of the core area of the jet were identified with the demarcation point and the bottom boundary layer and the core area of the jet and the surface area of the cut-off point roll rotation, these two regions are large changes in velocity gradient position, which shows the shear stress velocity gradient flow turbulence dissipation is causing a major factor.3. According to internal hydraulic jump dissipation rate calculation, results were obtained by small-scale coherent structures-Kolmogorov scale vortex distribution, the study found in different Froude number, the Kolmogorov scales ranging from0.02mm to0.11mm, and the Froude number compared to jump into the water after the jump inside the overall scale eddy dissipation region are relatively small, but due to the larger number of hydraulic jump Froude has a higher rate of change of the Kolmogorov scale, thus making smaller scale eddies in the same distance can be increased the same level of large-scale, and finally at a distance of2.5times the cross section before jump long jump position to stabilize. This paper also analyzes the size of the viscosity of the control force Kolmogorov scale of the main factors, and local Reynolds number using the average Reynolds number of the flow field of a specific position evaluate the viscous force extent size, in particular through the distribution of local Reynolds number found on the bottom border layer close to the surface near the rotary scroll area and location will have greater value, which clearly locate the two main areas of energy dissipation, and it consists with the foregoing findings.4. For the study of energy dissipation effect of air bubbles in the water, in this paper, Froude number of the first test had jumped into the water in each region bubble size, velocity distribution, which range in size between1-1Omm, and gathered in a large number of small-scale bubbles close in front section of jump water. Correspondingly, in this size range, bubble rise velocity was generally not more than0.5m/s.5. In the research process of the bubble consume water energy, by distinguishing bubble to change the water energy in several ways and focuses on the bubble formation and transport two process consumes water role of energy, to simplify the calculation ignores the coupling effect between the bubble and the water body and made a few assumptions, eventually found the Froude number of the work done under the bubbles and bubble diameter variation between basically the same, only in a different air concentration area, the speed of its growth varies when c(y) is larger, the growth is significantly faster.6. In the bubble quantitative calculation process of the bubble energy consumption, the first with a single bubble formation and transport processes in water bodies are acting in quantitative basis to calculate a single bubble in a specific area of total energy consumption, and with the local turbulent dissipation establish the functional relationship between the rate at low Froude number of conditions can be directly calculated using the formula proposed under different Froude number of single bubble of energy consumption. Secondly, through the hydraulic jump velocity within the bubble size and some approximation to estimate a Froude number of the entire wavelength range of hydraulic jump total energy dissipation of the bubble, the results showed that the bubbles in the hydraulic jump two different movement of the energy consumed in the process is very close, the overall5%deviation is not checked, and bubble two movement together the role played by energy dissipation increases as the Froude number decrease Fr1=1.93when the bubbles disappear total amount of water can be the total energy dissipation of36.97%, while in Fr1=4.44when the ratio was only4.58%, according to the results the paper also presents a direct calculation of the number of bubbles through Froude overall energy dissipation ratio of the empirical formula, the correlation coefficient was0.9977, can basically meet the general computing needs. |
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