水洞实验中空泡图象的一种修正方法

在水洞实验中，由于光线在空气、水和观察窗面板中发生折射，准确实验图象的获取一直是一个难题。本文以水洞模型超空化实验中的超空泡外形测量为例，基于计算机图形学提出了一种水洞实验图象的处理方法，该方法可以对水洞实验中所拍摄的图象边界进行检测与定量标注；基于光学原理，借助实验模型与其图象，获得了实验空泡边界坐标的修正函数。该图象处理与修正方法，不仅可以消除摄取图象因不同介质引起的折射误差，而且可以获得图形的定量结果。经多次应用表明，具有较高的精度，取得了很好的应用效果。     

细长体空泡流型试验研究

在水洞中对细长体通气附体空泡流型和控制途径进行了系列模型实验研究；获得了细长体通气附体空泡五种对称基本空泡流型与三种非对称基本空泡流型；在此基础上，从水下超空化超高速自主航行体的需求出发，提出了三种可用的空泡流型：非对称双空泡流型、尾部开式穿透非对称空泡流型和尾部半开式非对称空泡流型。并分析讨论了这三种空泡流型下航行体的重力平衡、运动控制与稳定性问题。同时，通过对有关参数的设计与控制，在水洞模型试验中实现了水下超空化超高速自主航行体的这三种空泡流型。     

通气超空泡水洞试验阻塞效应影响研究

水洞阻塞效应对于通气超空泡流动有重要影响,会导致水洞试验获得的相关参数与在无限流场中存在较大差别。为此,该文采用基于欧拉两流体多相流模型的数值模拟方法,对水洞试验阻塞效应机理进行了研究。首先,数值方法的有效性通过水洞试验数据进行了验证;其次,分别研究了水洞壁面黏性效应和尺度效应对于通气超空泡形态的影响规律,分析了水洞阻塞效应的产生机理。研究结果表明:水洞阻塞效应将会导致空泡长细比变大,仅通过增加空化器直径来增加空泡最大直径效果不明显;水洞阻塞效应主要受到壁面黏性效应和尺度效应的影响,且随来流速度增加而愈加明显;水洞试验设计应该对试验水速和模型尺度进行权衡考虑。以上研究成果对于通气超空泡水洞试验设计具有一定的指导意义。     

攻角对轴向约束模型加速过程超空泡影响的试验研究

本文研究了模型攻角对轴向约束模加速运动中超空泡形态的影响。研究结果发现,对于小攻角或零攻角情形,合适的通气量可以在研究的自然空泡数范围内逐步形成超空泡。在一定的攻角范围内,攻角越大,要达到同样超空泡长度所需的通气量越多。当攻角大于某个值(本试验模型的此攻角为2.5度),无论通气量多大都无法形成超空泡。攻角对超空泡的影响和定常水洞试验结果相似。     

空泡在管道中的动力学行为研究

该文将实验与数值计算相结合研究了空泡在管道中的动力学行为,使用高速相机记录了空泡在受限制管道空间内的成长及溃灭过程,采用BEM的数值仿真方法模拟了空泡在管道中的成长及溃灭过程,并将仿真结果与实验结果进行了对比。分析了空泡的演化过程及对应的压力分布情况,最后讨论了管道对空泡周期和体积的影响。     

空泡两相流场PIV测试技术研究

基于图像分析和TR-PIV建立了一套空泡周围非定常两相流场的PIV测试技术,并对分析方法进行详细的描述与试验验证.利用本文针对空泡现象选择的图像处理方法可以同时获取空泡的轮廓边界和空泡周围流场信息.三维水翼空泡流场试验表明,该方法适用于空泡现象的研究和分析,可获取高时间和空间分辨率的量化空泡发展过程以及空泡生长、脱落和溃灭过程中空泡壁面附近和周围流场的量化信息.     

大攻角下轴对称航行体空化流动特性试验研究

为掌握大攻角条件下轴对称航行体的空泡形态以及表面压力分布特征,本文对不同攻角,不同空泡数条件下轴对称航行体的流体动力特性进行了水洞试验研究,获取了不同试验条件下空泡的不同形态和压力分布.试验结果分析表明,在大攻角状态下,航行体表面的空泡分布不对称性加剧,迎背水面压差作用范围加大,导致航行体法向受力加大.研究结果在工程设计中有重要意义.     

双锥头型回转体空化特性实验研究

该文以双锥头型回转体为研究对象,开展了攻角0°–6°、空化数0.3–1.0范围内共计20个工况的水洞实验,获得了空泡形态和回转体表面压力分布;根据实验观测结果,提出了四种类型的流型划分,研究了各类型空泡流状态下的表面压力特征,给出了不同空化数和攻角下的空泡流流型图;对空泡长度进行了定量判读,分析了小空化数下重力对空泡长度的影响,获得了空泡长度、迎背流面空泡长度差随攻角、空化数的变化规律。     

带空泡轴对称细长体水动力脉动的实验研究

为了研究带空泡轴对称细长体定常来流下的水动力脉动特性，本文对四种头体模型进行了不同攻角下的空泡水洞实验，记录了水动力系数的时间序列，并对其进行了小波分析。本文运用小波分析方法从水动力脉动频率特征的角度揭示空泡流的脉动特性，是对何友声等(1997)用高速摄影方法研究空泡脉动的补充。     

锥头航行体非匀速垂直出水数值研究

在航行体高速出水过程中,空化现象不可避免,并会对航行体受到的载荷产生显著影响。该文采用数值模拟的方式,耦合VOF多相流模型和自然空泡模型,计算了不同加速度下锥头航行体垂直出水过程。通过肩部空泡体积与局部空化数的关系,将整个过程分为一次成长阶段、二次成长阶段和溃灭阶段,得到了二次成长阶段具有的准定常特征。分别给出了空泡外形在三个阶段中的变化过程,分析了速度、加速度和局部空化数对空泡发展过程以及航行体上压力分布的影响。     

UNSTEADY SUPERCAVITATING FLOW PAST CONES

1. INTRODUCTION As a natural phenomenon, cavitations would occur when the flow pressure is close to vapor pressure. The early research on cavitation flow mainly focused on destruction and protection of cavitation phenomenon to power equipment. It was late…     

Reverse motion characteristics of water-vapor mixture in supercavitating flow around a hydrofoil

The supercavitation has attracted a growing interest because of its potential for high-speed vehicle maneuvering and drag reduction. To better understand the reverse flow characteristics of a water-vapor mixture in supercavitating flows around a hydrofoil, a numerical simulation is conducted using a unified supercavitation model, which combines a modified RNG k-? turbulence model and a cavitation one. By comparing the related experimental results, the reverse motion of the water-vapor mixture is found in the cavitation area in all supercavitation stages. The inverse pressure gradient leads to reverse pressure fluctuations in the cavity, followed by the reverse motion of the water-vapor two-phase interface. Compared with the water-vapor mixture area at the back of the cavity, the pressure in the vapor area is inversely and slowly reduced,a higher-pressure gradient occurs near the cavity boundary.     

Three-dimensional large eddy simulation and vorticity analysis of unsteady cavi- taring flow around a twisted hydrofoil

Large Eddy Simulation (LES) was coupled with a mass transfer cavitation model to predict unsteady 3-D turbulent cavitating flows around a twisted hydrofoil. The wall-adapting local eddy-viscosity (WALE) model was used to give the Sub-Grid Scale (SGS) stress term. The predicted 3-D cavitation evolutions, including the cavity growth, break-off and collapse downstream, and the shedding cycle as well as its frequency agree fairly well with experimental results. The mechanism for the interactions between the cavitation and the vortices was discussed based on the analysis of the vorticity transport equation related to the vortex stretching, volumetric expansion/contraction and baroclinic torque terms along the hydrofoil mid-plane. The vortical flow analysis demonstrates that cavitation promotes the vortex production and the flow unsteadiness. In non-cavitation conditions, the streamline smoothly passes along the upper wall of the hydrofoil with no boundary layer separation and the boundary layer is thin and attached to the foil except at the trailing edge. With decreasing cavitation number, the present case has σ= 1.07, and the attached sheet cavitation becomes highly unsteady, with periodic growth and break-off to form the cavitation cloud. The expansion due to cavitation induces boundary layer separation and significantly increases the vorticity magnitude at the cavity interface. A detailed analysis using the vorticity transport equation shows that the cavitation accelerates the vortex stretching and dilatation and increases the baroclinic torque as the major source of vorticity generation. Examination of the flow field shows that the vortex dilatation and baroclinic torque terms increase in the cavitating case to the same magnitude as the vortex stretching term, while for the non-cavitating case these two terms are zero.     

Hydrodynamics and modeling of a ventilated supercavitating body in transition phase

Compared to other underwater vehicles, supercavitating vehicles can attain a high speed because they eliminate drag by creating a large cavity, thus establishing the so-called "supercavitating condition." Such a cavity is difficult to develop under normal conditions, hence, ventilation is used to attain the supercavitating condition in the initial phase of flight. In this paper, we focus on the hydrodynamic characteristics of a ventilated supercavitating vehicle. First, dynamic modeling of the supercavitating vehicle is performed to calculate the hydrodynamic force/moment acting on the vehicle for a given size of cavity. We then define the relationship between the ventilation rate and the cavitation number based on an air entrainment model of the ventilated cavity. Numerical simulations were performed to analyze the physical feasibility and characteristics of the modeling. The results show that the cavity length/radius increases with the ventilation rate, proving that ventilation can be used to attain the supercavitating condition.     

绕水翼超空化流发展及其旋涡特性的实验研究

为深入了解超空化流场结构特性,利用数字粒子图像测速(DPIV)和高速全流场显示技术,观测了绕hydronautics水翼的超空化流动涡量场。采用空化流场中的空化泡作为示踪粒子来显示流动结构。结果表明:在超空化的不同发展阶段,流动涡量场呈现不同的分布特征:随着空化数的降低,上下涡带逐渐靠拢,并向后延伸、拉直;下涡带随空化区域内汽相和水汽混合相分布的不同而发生明显变化:空穴形成阶段的下涡带基本稳定;在两相共存阶段,汽相和水汽混合相的频繁转换使得下涡带的起始位置和形状发生剧烈的变化;而在超空化完全发展阶段,下涡带又趋于稳定,且起始位置移至水翼后部。在整个超空化阶段,汽相和水汽混合相之间存在着较大的速度梯度;与来流方向相比,垂直于来流方向的速度脉动要大得多。     

Numerical analysis of the unsteady behavior of cloud cavitation around a hydrofoil based on an improved filter-based model

The unsteady cavitation evolution around the Clark-Y hydrofoil is investigated in this paper, by using an improved filter-base model(FBM) with the density correction method(DCM). To improve the prediction accuracy, the filter scale is adjusted based on the grid size. The numerical results show that a small filter scale is crucial for the unsteady simulations of the cavity shedding flow. The hybrid method that combines the FBM and the DCM could help to limit the overprediction of the turbulent viscosity in the cavitation region on the wall of the hydrofoil and in the wake. The large value of the maximum density ratio ? l /? v, clip promotes the mass transfer rate between the liquid phase and the vapor phase, which results in a large sheet cavity length and the vapor fraction rise inside the cavity. The cavity patterns predicted by the improved method are verified by the experimental visualizations. The time-average lift, the drag coefficient and the primary oscillating frequency St for the cavitation number ?= 0.8, the angle of attack, ?= 8o, at a Reynolds number Re= 7 ?105 are 0.735, 0.115 and 0.183, respectively, and the predicted errors are 3.29%, 3.36% and 8.93%. The typical three stages in one revolution are well-captured, including the initiation of the sheet/attached cavity, the growth toward the trailing edge(TE) with the development of the re-entrant jet flow, and the large scale cloud cavity shedding. It is observed that the cloud cavity shedding flow induces the vortex pairs of the TE vortices in the wake and the shedding vortices. The positive vorticity vortex of the re-entrant jet and the TE vortices interacts and merges with the negative vorticity vortex of the leading edge(LE) cavity to produce the shedding flow.     

Determination of frequencies of oscillations of cloud cavitation on a 2-D hydrofoil from high-speed camera observations

A method is presented to determine significant frequencies of oscillations of cavitation structures from high-speed camera recordings of a flow around a 2-D hydrofoil. The top view of the suction side of an NACA 2412 hydrofoil is studied in a transparent test section of a cavitation tunnel for selected cloud cavitation regimes with strong oscillations induced by the leading-edge cavity shedding. The ability of the method to accurately determine the dominant oscillation frequencies is confirmed by pressure measurements. The method can resolve subtle flow characteristics that are not visible to the naked eye. The method can be used for noninvasive experimental studies of oscillations in cavitating flows with adequate visual access when pressure measurements are not available or when such measurements would disturb the flow.     

超空化水翼流场的DPIV实验研究

该文采用DPIV粒子成像测速系统对绕2D超空化水翼(Tulin翼)的空化流场进行了测量,研究了初生空化阶段,附着型片状空化阶段,空化云以及超空化四个阶段典型流场的速度分布及涡量分布。实验结果表明:在空化发展的四个阶段,相对于主流区域,空化流区域速度明显偏小;在整个流动区域,有两个速度梯度较大的区域,一个是空化区和水体主流区的交界面,一个是翼形表面的边界层区域。在空化发展的四个阶段邻近翼型尾部处均会产生一个大小近似相等的正向涡和一个反向涡,并且正向涡总是处于反向涡的下方,形成一组正向对涡。     

3-D NUMERICAL SIMULATION OF FLOW THROUGH AN ORIFICE SPILLWAY TUNNEL

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