基于掺气作用的低Froude数水跃消能问题研究
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摘要
低水头大下泄流量水利枢纽的消能防冲问题一直是水利设计中需要解决的主要问题之一,随着我国水利建设的速度加快以及对水资源开发要求的提高,坝体下泄水流在采用底流消能形式时越来越趋向于大单宽流量、低Froude数的水流形态,当江河坝址上没有足够的空间建设与下泄流量相适应的泄洪坝段时,势必将会造成下游河道严重冲刷的不良结果,甚至将威胁坝体安全。为此,对于低Froude数水流消能问题的研究显得十分必要。目前国内外在这方面的研究主要分为两个方向：国内主要是针对实际工程提出特殊的、具体的消能防冲结构以满足工程需要,专注于指定枢纽布置背景下的各种消能型式与消能效率之间量化关系的研究；而国外却是以室内概化模型为主,专注于水流细部结构而对能量方面研究略显不足。本文即是在这样的研究背景下,希望从实际工程需要出发,但不拘泥于特定的工程背景,通过概化模型试验对低Froude数水跃进行消能机理方面的研究,抓住水跃的水气两相流动这一主要特征,并引入一些新的观念,试图从根本上提出影响水跃消能效率的主要因素,为实际工程中低Froude数水流消能防冲措施的设计提供一些新的、有价值的思路。
本文以低Froude数水跃为研究对象(Fr1<4.5),采用概化模型试验的方式,首先对水跃的基本流动特性进行了研究,得到了水跃内部、外部流动结构的时均和脉动特征,并估算了水跃整体的消能效率,这部分内容是为后面的深入研究作基础,所得到的主要结论包括：
1)水跃是一种周期变化着的水流流动形式,其外部结构具有明显的随时间变化的特征,并展现出一定的波动性和规律性,如试验发现水跃水面线在低Froude数下受到断面位置的影响很小,并且水面高度的最大值与最小值相差较小,这种变化的幅度将随着Froude数的增大而逐渐增加,并且可以将其与近水面区域水体紊动强度联系起来,形成一定的函数关系。与此相似的是共轭水深比在各Froude数下的大小分布,文中通过水跃跃长段受力分析推导了共轭水深比的计算式,并给出在考虑动量修正系数α影响下的波动区间,试验所测得的值恰好分布在该区间内,表征时均值的分布情况。而水跃长度的确定也由于采用气泡逸出位置定位跃后断面的方法,导致所测数值在某个范围内变化,取其时均值并与前人的经验公式作对比,发现在低Froude数条件下跃长试验值与陈椿庭公式计算得到的数值吻合较好。研究过程中还通过将水跃发生的位置与收缩水深断面之间的距离和hl/hc’建立关系,发现随着Froude数的增加,在hl/hc’的增幅相同的条件下,下游水位要高于较低Froude数同等条件才能稳定住水跃位置,否则水跃将发生前后摆动甚至变化为淹没或远驱水跃。
2)文中根据消能作用和水流形态将水跃分为四个主要区域,并测试了紊动剪切层和射流核心区中水体时均和紊动相关的量。紊动剪切层由于与空气的存在,其流速最高可以达到跃前流速的1.8倍左右,较大的流速梯度将把气泡撕裂成更小尺度的气泡,这是水体与气泡相互作用的主要流动区域；而射流核心区水体扩散的形态与经典的贴壁射流相类似又略有不同。不同Froude数下该区域内的时均流速沿程都逐渐减小,并且断面时均流速最大值的位置也表现为向水面方向移动的趋势,但当跃前Froude数较低的时候,断面最大时均流速位置偏于水槽底部(Fr1=1.93,y/h1<1),而相比Fr1=4.44时,在距离跃前断面0.75倍跃长的位置处断面最大时均流速距离槽底高程y/h1>2,更接近天然河道水流“上大下小”的流速分布情况。
3)试验中通过针式测量仪测得了水跃内掺气浓度的分布情况,测试结果表明跃前位置是水跃掺气的主要区域,该区域内掺气浓度都为最大值并随着Froude数的增加而增大。文中还通过对比不同Froude数下不同断面的平均掺气浓度和最大掺气浓度分布,给出其沿程变化规律,并提出在0.1～0.3倍跃长位置掺气浓度的均值将达到最大值,但低Froude数下平均掺气浓度不超过10%,最大掺气浓度不超过20%。在这样的掺气条件下,考虑跃后断面余能计算得到的水跃整体消能效率最大值不到40%,且在Fr1=1.93时仅为2%左右,印证了超低Froude数水跃消能效率低的事实。
在接下来的内容中对于低Froude数水跃消能问题的探讨可分为两个部分,根据水跃中消能机制的不同,分别从水体本身所产生的消能作用以及气泡的存在导致水体做功从而达到消能的目的出发,来对水体消能机制进行进一步的研究,主要结论如下：
1)从拟序结构的角度探讨了水跃内细部流动特性以及涡体与能量之间的关系,其中展现了在水跃近壁区和自由射流区的条带结构和横向涡这两种拟序结构,并研究这些结构随位置和时间的变化规律,给出了猝发周期和间隔距离等特征量的计算方式,并发现这种大尺度的拟序结构对流场的整体运动特性有着较大的影响,同时对流场中气泡这种离散颗粒的运动过程也有着一定的主导作用,而且根据气泡尺寸的不同,这种主导作用的影响程度也随之变化,可以用Stokes数来评价这种程度的大小,通过试验结果发现存在两个临界值分别为St=1.0和St=2.0。
2)试验采用micro ADV测试了水跃内的流场能谱,并通过各向同性和泰勒冻结假定计算得到了紊动耗散率的分布情况,研究发现水跃内相同区域的耗散率以及水跃整体平均耗散率都将随着Froude数的增加而明显增大。同时,研究发现耗散率在断面上的分布服从“减小-增大-再减小”的规律,研究指出了这种类似“反S”型的分布型式中存在的两个拐点对于水跃消能作用的特殊物理意义,期分别标识着射流核心区与底部边界层的分界点以及射流核心区与表面旋滚区的分界点,这两个区域都是流速梯度变化较大的位置,这就说明流速梯度产生的剪切应力是导致水流紊动耗散的主要因素。
3)根据水跃内耗散率的计算结果,得到了小尺度拟序结构——Kolmogorov尺度涡的分布情况,研究发现在不同的Froude数下Kolmogorov尺度从0.02mm到0.11mm不等,并且各Froude数下水跃内部相比跃后区域耗散涡的整体尺度相对都较小,但由于较大Froude数水跃拥有着较高的Kolmogorov尺度变化速率,进而使得较小尺度涡在相同的距离内可增大到同等的尺度水平,最后在距离跃前断面2.5倍跃长的位置达到稳定。文中还分析了控制Kolmogorov尺度大小的粘性作用力这一主要因素,采用平均雷诺数和局部雷诺数评价了流场特定位置的粘性力作用程度大小,特别通过局部雷诺数的分布情况发现在底部边界层附近以及靠近水面旋滚区位置会有较大值,这清晰地定位出了两个主要消能的区域,和前文的研究结果保持一致。
4)为研究气泡在水体中的消能作用,文中首先测得了各Froude数下水跃各个区域中气泡尺寸、运动速度的分布情况,其尺寸范围在1-10mm之间,并且小尺度气泡大量聚集在接近跃前断面(x/L,≈0.2)的位置。相对应地,在该尺寸范围内,气泡时均上升速度一般不大于0.5m/s。
5)在对气泡消耗水体能量的研究过程中,通过区别气泡改变水体能量的几种途径,重点提出了气泡在形成和输运两个过程中消耗水体能量的作用,为简化计算忽略了气泡与水体之间的耦合作用并作了几点假设,最终发现各Froude数下气泡所做的功与气泡直径之间的变化规律基本相同,仅在不同的掺气浓度区域中,其增长的速度快慢不一,当c(y)较大时增长的速度明显加快。
6)在气泡消能量的定量计算过程中,首先以单个气泡在形成和输运过程中水体所做功的量化为基础,计算了单个气泡在特定区域内的总消能量,并与当地紊动耗散率之间建立的函数关系,在低Froude数条件下可以直接利用提出的公式计算出不同Froude数下的单个气泡消能量。其次,通过对水跃内部气泡尺寸和运动速度的一些近似处理,估算出了各Froude数下整个水跃跃长范围内气泡的消能总量,结果表明水跃内的气泡在两个不同的运动过程中所消耗的能量十分接近,总体偏差不查过5%,而气泡两个运动过程综合起来所起的消能作用随着Froude数的增加而有所减少,在Fr1=1.93时气泡的消能总量占水体消能总量的36.97%,而在Fr1=4.44时该比例仅为4.58%,根据计算结果文中还提出了通过Froude数直接计算气泡整体消能比例的经验公式,其相关系数为0.9977,可基本满足一般计算需要。
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.
本文以低Froude数水跃为研究对象(Fr1<4.5),采用概化模型试验的方式,首先对水跃的基本流动特性进行了研究,得到了水跃内部、外部流动结构的时均和脉动特征,并估算了水跃整体的消能效率,这部分内容是为后面的深入研究作基础,所得到的主要结论包括：
1)水跃是一种周期变化着的水流流动形式,其外部结构具有明显的随时间变化的特征,并展现出一定的波动性和规律性,如试验发现水跃水面线在低Froude数下受到断面位置的影响很小,并且水面高度的最大值与最小值相差较小,这种变化的幅度将随着Froude数的增大而逐渐增加,并且可以将其与近水面区域水体紊动强度联系起来,形成一定的函数关系。与此相似的是共轭水深比在各Froude数下的大小分布,文中通过水跃跃长段受力分析推导了共轭水深比的计算式,并给出在考虑动量修正系数α影响下的波动区间,试验所测得的值恰好分布在该区间内,表征时均值的分布情况。而水跃长度的确定也由于采用气泡逸出位置定位跃后断面的方法,导致所测数值在某个范围内变化,取其时均值并与前人的经验公式作对比,发现在低Froude数条件下跃长试验值与陈椿庭公式计算得到的数值吻合较好。研究过程中还通过将水跃发生的位置与收缩水深断面之间的距离和hl/hc’建立关系,发现随着Froude数的增加,在hl/hc’的增幅相同的条件下,下游水位要高于较低Froude数同等条件才能稳定住水跃位置,否则水跃将发生前后摆动甚至变化为淹没或远驱水跃。
2)文中根据消能作用和水流形态将水跃分为四个主要区域,并测试了紊动剪切层和射流核心区中水体时均和紊动相关的量。紊动剪切层由于与空气的存在,其流速最高可以达到跃前流速的1.8倍左右,较大的流速梯度将把气泡撕裂成更小尺度的气泡,这是水体与气泡相互作用的主要流动区域；而射流核心区水体扩散的形态与经典的贴壁射流相类似又略有不同。不同Froude数下该区域内的时均流速沿程都逐渐减小,并且断面时均流速最大值的位置也表现为向水面方向移动的趋势,但当跃前Froude数较低的时候,断面最大时均流速位置偏于水槽底部(Fr1=1.93,y/h1<1),而相比Fr1=4.44时,在距离跃前断面0.75倍跃长的位置处断面最大时均流速距离槽底高程y/h1>2,更接近天然河道水流“上大下小”的流速分布情况。
3)试验中通过针式测量仪测得了水跃内掺气浓度的分布情况,测试结果表明跃前位置是水跃掺气的主要区域,该区域内掺气浓度都为最大值并随着Froude数的增加而增大。文中还通过对比不同Froude数下不同断面的平均掺气浓度和最大掺气浓度分布,给出其沿程变化规律,并提出在0.1～0.3倍跃长位置掺气浓度的均值将达到最大值,但低Froude数下平均掺气浓度不超过10%,最大掺气浓度不超过20%。在这样的掺气条件下,考虑跃后断面余能计算得到的水跃整体消能效率最大值不到40%,且在Fr1=1.93时仅为2%左右,印证了超低Froude数水跃消能效率低的事实。
在接下来的内容中对于低Froude数水跃消能问题的探讨可分为两个部分,根据水跃中消能机制的不同,分别从水体本身所产生的消能作用以及气泡的存在导致水体做功从而达到消能的目的出发,来对水体消能机制进行进一步的研究,主要结论如下：
1)从拟序结构的角度探讨了水跃内细部流动特性以及涡体与能量之间的关系,其中展现了在水跃近壁区和自由射流区的条带结构和横向涡这两种拟序结构,并研究这些结构随位置和时间的变化规律,给出了猝发周期和间隔距离等特征量的计算方式,并发现这种大尺度的拟序结构对流场的整体运动特性有着较大的影响,同时对流场中气泡这种离散颗粒的运动过程也有着一定的主导作用,而且根据气泡尺寸的不同,这种主导作用的影响程度也随之变化,可以用Stokes数来评价这种程度的大小,通过试验结果发现存在两个临界值分别为St=1.0和St=2.0。
2)试验采用micro ADV测试了水跃内的流场能谱,并通过各向同性和泰勒冻结假定计算得到了紊动耗散率的分布情况,研究发现水跃内相同区域的耗散率以及水跃整体平均耗散率都将随着Froude数的增加而明显增大。同时,研究发现耗散率在断面上的分布服从“减小-增大-再减小”的规律,研究指出了这种类似“反S”型的分布型式中存在的两个拐点对于水跃消能作用的特殊物理意义,期分别标识着射流核心区与底部边界层的分界点以及射流核心区与表面旋滚区的分界点,这两个区域都是流速梯度变化较大的位置,这就说明流速梯度产生的剪切应力是导致水流紊动耗散的主要因素。
3)根据水跃内耗散率的计算结果,得到了小尺度拟序结构——Kolmogorov尺度涡的分布情况,研究发现在不同的Froude数下Kolmogorov尺度从0.02mm到0.11mm不等,并且各Froude数下水跃内部相比跃后区域耗散涡的整体尺度相对都较小,但由于较大Froude数水跃拥有着较高的Kolmogorov尺度变化速率,进而使得较小尺度涡在相同的距离内可增大到同等的尺度水平,最后在距离跃前断面2.5倍跃长的位置达到稳定。文中还分析了控制Kolmogorov尺度大小的粘性作用力这一主要因素,采用平均雷诺数和局部雷诺数评价了流场特定位置的粘性力作用程度大小,特别通过局部雷诺数的分布情况发现在底部边界层附近以及靠近水面旋滚区位置会有较大值,这清晰地定位出了两个主要消能的区域,和前文的研究结果保持一致。
4)为研究气泡在水体中的消能作用,文中首先测得了各Froude数下水跃各个区域中气泡尺寸、运动速度的分布情况,其尺寸范围在1-10mm之间,并且小尺度气泡大量聚集在接近跃前断面(x/L,≈0.2)的位置。相对应地,在该尺寸范围内,气泡时均上升速度一般不大于0.5m/s。
5)在对气泡消耗水体能量的研究过程中,通过区别气泡改变水体能量的几种途径,重点提出了气泡在形成和输运两个过程中消耗水体能量的作用,为简化计算忽略了气泡与水体之间的耦合作用并作了几点假设,最终发现各Froude数下气泡所做的功与气泡直径之间的变化规律基本相同,仅在不同的掺气浓度区域中,其增长的速度快慢不一,当c(y)较大时增长的速度明显加快。
6)在气泡消能量的定量计算过程中,首先以单个气泡在形成和输运过程中水体所做功的量化为基础,计算了单个气泡在特定区域内的总消能量,并与当地紊动耗散率之间建立的函数关系,在低Froude数条件下可以直接利用提出的公式计算出不同Froude数下的单个气泡消能量。其次,通过对水跃内部气泡尺寸和运动速度的一些近似处理,估算出了各Froude数下整个水跃跃长范围内气泡的消能总量,结果表明水跃内的气泡在两个不同的运动过程中所消耗的能量十分接近,总体偏差不查过5%,而气泡两个运动过程综合起来所起的消能作用随着Froude数的增加而有所减少,在Fr1=1.93时气泡的消能总量占水体消能总量的36.97%,而在Fr1=4.44时该比例仅为4.58%,根据计算结果文中还提出了通过Froude数直接计算气泡整体消能比例的经验公式,其相关系数为0.9977,可基本满足一般计算需要。
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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