自由液面附近空化泡的溃灭特性

研究自由液面附近空化泡的溃灭特性对认识水利水电工程中的空蚀破坏机理具有重要意义。为进一步认识自由液面附近空化泡的溃灭特性,开展了电火花激发空化泡试验。结果表明:(1)空化泡与自由液面距离较近时总是背离自由液面溃灭,背离的临界条件为空化泡与自由液面的距离约是空化泡最大半径的5倍。空化泡尺寸越大或空化泡与自由液面距离越近,其背离现象越明显。(2)自由液面附近双空化泡相向溃灭融合,且融合体背离自由液面移动。(3)在自由液面和空气泡的共同影响下,空化泡的溃灭方向是二者分别对空化泡作用的矢量和。通过本文研究,对自由液面附近空化泡的溃灭有了定量认识,可以在一定条件下预测空化泡的溃灭方向。     

针状突体空化结构的高速摄影观察

应用高速摄影技术在直流式水洞中对针状突体的空化结构进行了试验研究。试验流速为8～40m/s，高速摄影机拍摄速率为20000fps，曝光时间为4μs。观察了不同流速下针状突体后空化云的形态以及随流动的演变情况，及不同掺气量对空化云的影响。此外，还给出针状突体空化区包络线（自由流线）的表达式。     

High-speed experimental photography of collapsing cavitation bubble between a spherical particle and a rigid wall

The synergetic effects between cavitation bubbles and silt particles on the damages of materials are essential problems in fluid machineries. For studying the underlying microscopic mechanisms, in the present paper, the dynamic behaviors of a single cavitation bubble between a spherical particle and a rigid wall are experimentally investigated with a high-speed camera. The results indicate that the existence of the particle can affect the bubble shape during collapse and significantly accelerate the collapse velocity of the bubble. The influences of the particle size, the distance between the bubble and the particle and the distance between the bubble and the rigid wall on the phenomena are qualitatively and quantitatively analyzed. These parameters can prominently affect the collapse velocity of the bubble(especially its maximum value).     

基于伪势模型的表面张力对空化泡溃灭的影响

采用可调节表面张力的大密度比、大黏滞系数比格子玻尔兹曼伪势空化模型模拟了近壁区空化泡溃灭过程,并进一步分析了表面张力和初始空化泡内外压差对空化泡溃灭过程中流场分布的影响,探究了表面张力变化对空化泡溃灭时产生的微射流和溃灭压力演化的影响。结果表明:在近壁区空化泡溃灭过程中,随着表面张力减小,加剧了气液交界面的变形,导致微射流更为集中。同时空化泡在溃灭过程中蓄积的表面能减小,在溃灭时刻迅速释放后,减弱了空化泡溃灭强度,空化泡溃灭最大微射流流速和最大压力均随着表面张力的减小而减小,导致空化泡溃灭时间缩短,增加了壁面引起的Bjerknes力,加剧空化泡朝向壁面溃灭的趋势。     

CAVITATION BEHAVIOR OF A NACA16-012 HYDROFOIL

Numerical and experimental study is carried to investigate the cavitation behav-ior in the turbulent flow around a NACA16-012 hydrofoil.The Navier-Stokes Equations forcompressible fluid are adopted to simulate the overall motion and dynamical characteristics of thecavity flow,while the bubble dynamics is used to calculate the motion and growth of nuclei in-side the cavity.Cavitation experiment for the hydrofoil is carried out in a water tunnel of CSS-RC,the phenomenon in the experiment is recorded and analyzed with high speed photographictechnique.     

AN EXPERIMENTAL STUDY ON INCIPIENT CAVITATION EROSION OF FLOOD DISCHARGE TUNNEL WITH ORIFICE

Serial grades of cement mortar specimens have been tested on the model of con-crete-lining flood discharge tunnel with orifice,The results show that incipient cavitation erosionnumber is much smaller than incipient cavitation number.Based on the principle of bubble dy-namics,the flow speed effect on incipient cavitation erosion number for cement mortar is pre-sented.     

高水头深孔平板门门槽水力特性研究

平板闸门门槽内的水流流态十分复杂,容易因空化空蚀而破坏门槽。以卡基娃水电站放空洞工程为实例,采用水工模型试验探讨门槽附近水流流态、压力、空化数的变化规律。研究结果表明:有压式门后连接段能较好地改善门槽及门后连接段水流的流态;闸门全开时,各个区域没有负压出现,水流发生空化的可能性也较低;闸门局开时,门槽及门后连接段会出现负压,最大负压出现在门槽的底板处,水流发生空化的可能性也较大;水流的脉动表现出明显振幅大、频率低的强紊流脉动压力的特点。研究成果对于高水头深孔闸门门槽的设计具有一定的借鉴意义。     

基于MATLAB的泄洪洞气泡运动轨迹检测

针对高水头泄洪洞气泡运动轨迹难以测定的情况,设计并制造了比尺为1∶100的雅砻江锦屏一级水电站泄洪洞模型,首次采用图像测量方法对泄洪洞内气泡沿程运动特性进行测量。使用高速摄像机拍摄到清晰的气泡运动图像序列,并利用MATLAB图像处理方法实现了对流场中气泡的分割识别,得到气泡的面积、形心等参数,并进一步获取了泄洪洞流场中单个气泡运动轨迹。     

南水北调西线阿安引水隧洞水力学试验研究

南水北调西线阿安引水隧洞具有水头高、流速急、定流量、变闸门开度、水位变幅大等特点,借助于水工模型试验对输水洞在不同工况运用下的水流流态、泄流能力、压力特性及关键部位的空化特性、闸门区水流特性、消能工体型等进行了较系统的研究、分析,并对洞内消能工体型进行优化。在工作闸门区突扩跌坎处,选择合适的体型并严格控制表面不平整度,可达到防止或减小空化的目的。     

EXPERIMENTAL STUDY OF EFFECTS OF AIR CONTENT ON CAVITATION AND PRESSURE FLUCTUATIONS

This article studies the effects of air content on propeller cavitation and pressure fluctuations. The cavitation is observed while the pressure fluctuations on the hull are measured. When adjusting the air content, the sheet cavitation range does not change distinctly, but the pressure fluctuations see obvious differences. The amplitudes of the pressure fluctuations increase with the decrease of the air content. The results indicate that the air content has little effect on the sheet cavitation range but has an important effect on the bubble cavitation and the tip vortex cavitation. When the air content decreases, the water tensile force increases, which results in the instability of the bubble cavitation and the tip vortex cavitation and the increase of the pressure fluctuations. To minimize the scale effects, the experiments should be run at a high Reynolds number with a high nuclei content. The high Reynolds number is often realized by increasing the flow velocity and the propeller rotation speed, and the high nuclei content is often made by increasing the dissolved air content.     

A HIGH-SPEED PHOTOGRAPHIC STUDY OF ULTRASONIC CAVITATION NEAR RIGID BOUNDARY

This article investigated an existing steady pattern of collapse and rebound （disintegration and aggregation） of cavitation bubbles near rigid boundary in acoustic field. A deformation process of cavitation bubble was accomplished in two acoustic cycles, namely, a spherical bubble collapsed towards the boundary to its minimum volume and then rebounded and grew into a toroidal bubble （or two individual bubbles） in one acoustic cycle, and the toroidal bubble （or two individual bubbles） collapsed towards the center of ring to its minimum volume, and then rebounded into a spherical bubble in the next acoustic cycle. Inertia force plays a key role in the transition between these two states. The microjet produced during the collapse of spherical bubble and the shock wave produced during the collapse of toroidal bubble （or two individual bubbles） impacts the boundary alternately. A cavitation bubble operating in this pattena can thus be an effective corrosion mechanism of rigid boundary.     

汽液黏滞系数对空化泡溃灭过程影响的数值模拟

利用格子玻尔兹曼方法(LBM)伪势模型模拟了近壁区空化泡溃灭过程,分析了不同汽相黏滞系数和液相黏滞系数对空化泡溃灭过程中的最大微射流流速、最大溃灭压力、最大溃灭时间的影响。结果表明,相同液相黏滞系数条件下,改变汽相黏滞系数,空化泡溃灭时产生的最大微射流流速、最大溃灭压力和最大溃灭时间不变;保持汽相黏滞系数不变,空化泡溃灭时最大溃灭压力、最大微射流流速均随着液相黏滞系数的增大而减小,但最大溃灭时间随着液相黏滞系数的增大而增大。     

超空泡实验中的数字图像检测

在水洞实验中，高速摄像机以数十赫兹至数十千赫的速度记录空化过程的系列影像，是研究水中空化现象机理的有力工具。如何从众多的数字图像中准确获取空泡的外形、空泡波动周期等信息，同时克服背景干扰和光线在空气、水以及水洞观察窗面板中的折射效应，是进行超空泡非定常实验要解决的重要课题。文中应用现代数字图像分析理论，通过对图像的滤波、灰度拉伸、图像校准、边缘检测等操作，总结出一套适合在超空泡水洞实验中应用的自动图像获取技术。使用该方法能够自动获取高速摄像图像中的空泡外形和振动周期等参数，并形成数据文件。实验表明，该方法自动化水平高，并具有较好的精度，在实际应用中获得了良好的效果。     

LES NUMERICAL SIMULATION OF CAVITATION BUBBLE SHEDDING ON ALE 25 AND ALE 15 HYDROFOILS

A cavitation calculation scheme is developed and applied to ALE 15 and ALE 25 hydrofoils, based on the Bubble Two-phase Flow (BTF) cavity model with a Large Eddy Simulation (LES) methodology. The Navier-Stokes equations including cavitation bubble clusters are solved through the finite-volume approach with a time-marching scheme. Simulations are carried out in a 3-D field with a hydrofoil ALE 15 or ALE 25 at an angle of attack of 8° and cavitation number σ = 2.3 with a 2 × 10⁶, meshing system. With the time-marching, the cavitation bubble gradually grows to a steady lump shape and then produces an irregular small bubble behind the main cavitation bubble, finally shedding from the leading edge of the cloud cavitation structure. The calculated results including velocity field and pressure field are consistent with experiment data at the same Reynolds number and cavitation number. The vortex and reverse flow are observed on the hydrofoil surface.     

渐扩式泄洪洞出口水流模拟及空化 特性预测研究

为了获得泄洪洞出口段在高速水流情况下的水流特征，分析空化及其对结构可能存在的空蚀破坏影响，本文采用了VOF（Volume Of Fluid）方法，并结合Realizable k-ε紊流模型，对泄洪洞水流进行了三维数值计算。计算获得了压强、流速及水深的三维分布参数，并将部分计算结果与设计值验证，吻合较好。通过引入自定义函数方式计算了整个流场的空化数，获得了泄洪情况下泄洪洞高速水流空化数分布规律及易发生空化空蚀的区域。     

考虑液体压缩性影响下空泡溃灭冲击波对固体颗粒的作用

本文讨论了无限大水域中考虑液体压缩性影响下空泡溃灭冲击波对固体颗粒的作用,求得了固体颗粒所获得的最大速度;该速度的颗粒足以打坏或嵌入固壁(混凝土)中,引起更严重的空化、空蚀,形成空蚀和磨蚀的恶性循环,以致大规模剥落过流固壁面等。     

HYDRAULIC RESEARCH OF AERATORS ON TUNNEL SPILLWAYS

The selection of the configuration and size of an aerator was of importance for a tunnel spillway under the conditions of high speed flows. Experimental investigations were conducted on the effects of entrained air on the tunnel spillway in the Goupitan Project, based on the criterion of gravity similarity and the condition of aerated flow velocity of over 6 m/s, with physical models. The configurations of the aerators were presented of a larger bottom air concentration, to protect the tunnel spillway from cavitation as well as to see no water fills in the grooves.     

明渠自掺气水流气泡形成过程的试验研究

采用高速摄像技术对陡槽内自掺气水流进行了观测，拍摄到了掺气起始区气泡在水流内部形成的完整过程，对气泡形成的过程进行了概化分析水面附近涡体运动产生的脉动压强和水面相互作用，导致水面发生上凸下凹的变形当这种变形达到一定程度后，受顺水流向或横向脉动压力 横向切应力的作用，深入水体凹坑的边壁在某一部位闭合下部包裹空气形成气泡而发生自掺气随着紊动强度进一步增大，水面变形加剧，表现为高高跃出水面的柱状突起和水滴这些水柱的倾倒、聚并也会造成空气进入水中形成气泡明渠水流自掺气是水体内部紊动与水面相互作用的结果。     

砂浆孔板洞临界空化荷载的实验测量及数值计算

本文介绍了一套用于测量空化荷载的自制仪器；给出了与砂浆孔板洞初生空蚀相应的临界空花荷载的测量结果；根据空泡动力方程；利用格尔莫方法数值模拟了与实验条件相应的空泡运动过程，并将计算的临界空化荷载与实验结果进行了比较。     

INTERACTION OF A CAVITATION BUBBLE AND AN AIR BUBBLE WITH A RIGID BOUNDARY

The motion of a spark-induced cavitation bubble and an air bubble near a rigid boundary is experimentally studied by using high-speed photography. Several dimensionless parameters are used to describe the geometrical configuration of the bubble-bubble-boundary interaction. The bubble-bubble interaction can be considered in two different conditions. The cavitation bubble will collapse towards the air bubble if the air bubble is relatively small, and away from the air bubble if the air bubble is relatively large. The two zones are identified in the bubble-boundary interaction, and they are the danger zone and the safety zone. The relative position, the bubble-boundary distance and the bubble-bubble distance play important roles in the bubble-bubble-boundary interaction, which can be considered in several conditions according to the responses of the bubbles. Air jets are found to penetrate into the cavitation bubbles. The cavitation bubble and the air bubble (air jet) move in their own way without mixing. The motion of a cavitation bubble may be influenced by an air bubble and/or a rigid boundary. The influence of the air bubble and the influence of the boundary may be combined, like some thing of a vector.