【作者】 吴福生；

【导师】 姜树海；

【作者基本信息】 南京水利科学研究院 ， 水力学及河流动力学， 2009， 博士

【摘要】 基于河流生态修复、河道管理与水资源可持续发展的要求,提出了含植物明渠水动力特性研究的课题,并强调了这一研究具有重要的实用价值与学术意义。建立了变底坡玻璃水槽含植物明渠流的实验及测试系统,采用了先进的三维超声多普勒流速仪(ADV)、标准二维粒子图像测速仪(PIV)以及二维高帧频Mini-PIV,并配合传统的测试手段如水位测针、量水堰及测压管等,实验测量了含植物水流的二维时均流速场、时均涡量场、瞬时涡量场,研究分析了含植物水流的阻力特性、紊流特性,进而从涡动力学观点出发研究了含植物水流的能量耗散与紊动耗散特性。本文的主要创新成果和研究结论如下:1、用二维标准PIV测量研究了含淹没植物明渠流在不同植物密度与不同流量时的二维时均流速场分布特性。研究表明:由于淹没植物的存在,影响了植物区明渠水流的流速分布,植物冠层以下的流速分布受植物的影响很大,且与离植物断面的距离有关。植物冠层处流速梯度最大。与淹没柔性植物相比刚性植物冠层处的流速梯度更大。而植物冠层以上区域,水流受植物的影响程度逐渐减小。2、在恒定均匀流条件下,实验研究了含柔性植物水流在不同淹没度时曼宁糙率系数与柔性植物的偏转角度的变化规律;对比分析了含淹没刚、柔性植物明渠水流的曼宁糙率系数变化规律。分析了含非淹没刚性植物明渠流的阻流因素,在一定前提条件下推导出拖曳力系数及当量曼宁糙率系数公式,实验研究了不同植物密度与水深条件下的阻流特性,得出拖曳力系数及当量曼宁糙率系数随不同因素如水深、植物密度以及V/V.等的变化规律。3、从不可压缩流体涡运动方程可知,流场中涡量取决于流速梯度与流体的粘滞性。在高Re数条件下的含植物明渠的紊流流场中,涡量主要取决于流速梯度;流体中能量耗散率是与拟熵直接相关的,即与流体中涡量绝对值的平方成正比。因此,流场中特征拟熵的分布直接反映了能量耗散率的分布规律。采用二维标准PIV测量研究了含淹没刚、柔性植物的垂向二维时均流速场与涡量场的分布。研究表明:流速梯度是影响含植物水流涡量场的关键因素,刚、柔性植物冠层附近水流受到的干扰大、流速梯度大;涡量最大值出现在淹没刚、柔性植物冠层以下约30%～50%处。植物的刚度对涡量场的大小和分布特征有一定的影响。由于植物的存在使得特征拟熵明显比无植物时高得多,特别是植物冠层处的顶端附近出现峰值。含淹没刚性植物冠层处水流的特征拟熵明显高于柔性植物,相同流量下其峰值约为柔性植物的2～5倍。可见,植物冠层处水流的能量耗散大,是造成水流能量损失的重要原因。与柔性植物比较,刚性植物冠层处的水流的能量损失明显要高于柔性植物。由此可见,植物的弹性模量大,造成水流的动能损失越大。4、为进一步认识含植物水流的阻力机制和紊流结构,研究了含植物水流结构的紊动特性。在高雷诺数Re条件下,不可压缩流体的紊流耗散率ε可近似等于(?)。通过试验实测的紊流拟熵的变化规律,以探讨紊流耗散的规律。采用三维ADV逐点测量研究了含淹没刚、柔性植物段垂线紊流流速特性。结果表明:含淹没刚、柔性植物明渠紊流流速具有各向异性特性;在植物冠层处,水流紊动交换强烈,该处紊流强度、紊动能及雷诺应力均出现最大值。与柔性植物相比较,刚性植物顶层处紊流强度更大。采用高帧频二维Mini-PIV测量了含淹没植物水流的瞬时流速场、瞬时涡量场,并进一步计算分析了含淹没植物明渠水流的紊流拟熵,得到了含淹没刚、柔性植物水流紊流拟熵的分布规律。研究结果表明:受植物的影响,在其冠层处,水流的紊流拟熵明显增大,紊动能的耗散增大。而在冠层以下区域,水流的紊流拟熵相对较小。在植物间断面,随植物扰动影响的减小,紊流拟熵分布逐渐坦化。更多还原

【Abstract】 Such a research topic, the hydrodynamic characteristics of open channel flow with vegetation, was proposed. It was in use of flood management, river restoration and water resources for sustainable development. And it was stressed that the importance of this research topic has important practical value and academic significance.In this doctoral dissertation, a complex test system was established. It was made up of a slope alterable glass flume containing experimental vegetation. The accessorial monitoring instruments consists of a three-dimensional ultrasound Doppler Velocimeter (ADV), a standard two-dimensional particle image velocimetry (PIV), a two-dimensional high frame rate Mini-PIV, and some other traditional monitoring instruments, such as water level measurements needle, weir and pressure tubes, etc.. In use of the new established test system, the vegetation water flow resistance characteristics, turbulent flow characteristics, two-dimensional velocity field, two-dimensional vorticity field, the instantaneous vorticity field were studied. Then from the point of view of vortex dynamics study of vegetation flow with energy dissipation and the turbulent dissipation characteristics were studied by physical experiment. In this dissertation, such innovation and research findings were shown as follows:1. In constant gradient flow conditions, the two-dimensional flow velocity distribution of open channel flow was studied with submerged vegetation in different vegetation density and different discharge in use of the standard two-dimensional PIV.Research result shown that the existence of submerged vegetation had notable effect on the velocity distribution of open channel flow. And the distribution of velocity was the anti-S-shaped. The effect goes greater as the distance from the cross-section of the vegetation goes smaller. The largest velocity gradient occurs near the vegetation canopy. The velocity gradient increased with the flow discharge increasing, and the vegetation density increasing. The flow velocity gradient near the vegetation canopy with rigid submerged vegetation is greater than that near the flexible. The effect of vegetation to velocity gradient decreased in the area above the vegetation canopy.2. The Manning roughness coefficient of flexible vegetation changes with the flexible deflection was studied under the steady flow condition. The variation of Manning roughness coefficient was studied by comparative experimental of submerged rigid vegetation and flexible vegetation.Analysis with non-submerged rigid vegetation in open channel flow resistance factors, the drag coefficient and equivalent roughness coefficient of Manning formula was derived in a certain supposed condition. Fixed at the end of the slope, the experimental study of the different vegetation density and depth of flow under the conditions of the resistance mechanism to draw the drag coefficient and equivalent roughness coefficient of Manning with the different factors such as water depth, vegetation density and V/V_* such as changes in the law. 3. It was induced from incompressible fluid from the vortex equation of motion that the vorticity of the flow field depends on the velocity gradient and fluid viscosity. The vorticity mainly depends on the velocity gradient in open channel flow with vegetation. The fluid energy dissipation rate was directly related to entropy, that is, fluid vorticity and the square of the absolute value proportional. Therefore, the distribution of characteristic entropy was a direct reflection of the rate of energy dissipation distribution in flow field.The distribution of velocity field and vorticity field of the flow with submerged rigid and flexible vegetation were studied using a standard two-dimensional PIV measurement. The study shows that velocity gradient was a key element in vorticity of flow with vegetation. The water flow was disrupted near the vegetation canopy. And the larger the velocity gradient occurs the more close to the vegetation canopy. The maximum vorticity value appeared in the 30%～50% parts below the submerged vegetation canopy. The velocity gradient and absolute vorticity value near the vegetation canopy were affected by the stiffness of the vegetation.The existence of vegetation intended the characteristic energy entropy was much higher than entropy as no vegetation. Particularly, the peak value appeared near the vegetation canopy. And with the flow increased, the peak value went multiply increasing. The characteristic energy entropy of flow near canopy of submerged rigid vegetation was significantly higher than that of the flexible vegetation. The peak value was about 2 to 5 times of that near flexible vegetation at the same flow discharge. Which shown that vegetation canopy layer flow of energy dissipation was the main reason for the current energy loss, and was also one of the main reasons of resistance to flow. Compared with the flexible Vegetation, energy loss of water flow was obviously much higher near the canopy of rigid vegetation. That shown that the bigger elastic modulus value of vegetation the greater energy loss caused by flow.4. The turbulent flow characteristics with vegetation was studied to make a better understanding to the mechanism of vegetation resistance to water flow and turbulence structure. Under conditions of high Reynolds value, incompressible fluid turbulence dissipation rateεcan be approximately equal to . The data from turbulent entropy measured through the test could explore the law of turbulence dissipation.The velocity distribution and turbulent characteristics of open channel flow with submerged rigid and flexible vegetation were studied using the three-dimensional ADV. The results show that there had anisotropic characteristics of open channel turbulent flows with rigid and flexible submerged vegetation. The maximum value of the turbulence intensity, the turbulence and the Reynolds stress appeared near the plant canopy. And the exchange of turbulent flow was strongly. The turbulence intensity was higher obviously near the top-level part of vegetation which was rigid than flexible. The instantaneous flow velocity field and the instantaneous vorticity field were measured in the open channel flow test with submerged vegetation using the high frame rate two-dimensional Mini-PIV. And further analysis of calculating the turbulent entropy in open channel flows with submerged vegetation was done. A distribution law was proposed that was the turbulent entropy in open channel flows with submerged rigid and flexible vegetation.All the research results were shown that the turbulent entropy was small below the vegetation canopy. The vegetation canopy layer had significantly disturbance to the flow that the turbulent entropy and the turbulent energy dissipation near the vegetation canopy increased significantly. At the vegetation cross-section, the distribution of turbulent entropy became gradually homogenizing.更多还原