绕栅中水翼空化流动的数值和实验研究

采用数值计算和实验研究的方法研究了绕水翼和栅中水翼的非定常空化流动. 实验采用高速录像技术分别观察了绕水翼和栅中水翼云状空化形态随时间的变化, 测量了升阻力, 并对测量数据进行了频率分析. 计算时空化模型选用了能比较准确描述旋涡空化非定常特性的Kubota模型, 湍流模型采用能准确捕捉流场非定常特性的FBM模型. 计算模型的可靠性用实验结果进行验证. 结果表明, 计算与实验的结果基本一致, 相比绕单个水翼的空化流动, 绕栅中水翼的空穴厚度比较薄, 翼型近壁处的逆压梯度较小, 反向射流的速度较小, 且水汽混合区速度梯度较小, 空穴的脱落周期变长, 平均升阻力系数较小      

考虑当地涡旋运动修正的湍流模型在非定常空化湍流流场计算中的应用

为了更精确地模拟当地涡旋运动对复杂非定常空化湍流流动的影响,考虑当地涡旋运动特性对标准k-ε湍流模型进行了修正;并采用该修正模型对二维Clark-Y水翼周围非定常空化湍流场进行了计算;通过与实验结果进行对比,分析并验证了该修正模型计算结果的有效性。研究表明:该修正湍流模型根据当地涡旋效应对湍动能输运方程中的生成项进行修正,可以有效解决标准k-ε模型对湍动能的过度预测问题,得到的空化核心区域时均湍动能和湍流黏性的预测结果降低了约30%,与实际值更为接近;标准k-ε模型计算所得的时均空泡长度不大于0.6倍弦长,而修正模型计算所得的时均空穴可以覆盖水翼吸力面;修正模型可以准确预测水翼吸力面表面空泡的非定常脱落过程,包括附着空穴的拉伸和断裂,以及脱落空泡的逐渐消失。     

质量传输模型在考虑热力学效应空化流动计算中的应用评价

通过二次开发技术，将Merkle、Kunz、Kubota、Singhal四种不同的质量传输模型和液氮物质属性随温度变化函数等引入了计算软件CFX。在考虑热力学效应的条件下，对绕翼型的液氮空化流动进行了数值计算，并与实验结果进行对比分析。结果表明：由于传输方程的物理机制不同，造成各模型的计算结果尤其是对热力学效应影响的描述存在明显的差别。Merkle模型计算得到的压力和温度分布与实验最为接近，质量传输过程能较好地反映温度场变化的影响，从而能较好地反映热力学效应对空化发展的影响；Singhal模型计算得到的结果与实验数据差距最大，其模拟得到的质量传输过程不能很好地反应温度流场的变化的影响。     

空化可压缩流动空穴溃灭激波特性研究

为深入研究空化可压缩流动中空泡/空泡团溃灭过程中激波产生、传播及其与空穴相互作用规律,本文采用数值模拟方法对空化可压缩流动空穴溃灭激波特性展开了研究.数值计算基于OpenFOAM开源程序,综合考虑蒸汽相和液相的压缩性,通过在原无相变两相可压缩求解器的控制方程中耦合模拟空化汽液相间质量交换的源项,实现了对空化流动的非定常可压缩计算.利用上述考虑汽/液相可压缩性的空化流动求解器,对周期性云状空化流动进行了数值模拟,并重点研究了空穴溃灭激波特性.结果表明:上述数值计算方法可以准确捕捉到空穴非定常演化过程及大尺度脱落空泡云团溃灭激波现象,大尺度脱落空泡云团溃灭过程分为3个阶段:(1) U型空泡团形成;(2) U型空泡团头部溃灭;(3) U型空泡团腿部溃灭.在U型空泡团腿部溃灭瞬间,观察到激波产生,并向上游和下游传播,向上游传播的激波与空穴相互作用,导致水翼吸力面新生的附着型片状空穴回缩,直至完全溃灭.并且空穴溃灭激波存在回弹现象,抑制了下一周期的空化发展.     

空化可压缩流动空穴溃灭激波特性研究

为深入研究空化可压缩流动中空泡/空泡团溃灭过程中激波产生、传播及其与空穴相互作用规律,本文采用数值模拟方法对空化可压缩流动空穴溃灭激波特性展开了研究.数值计算基于OpenFOAM开源程序,综合考虑蒸汽相和液相的压缩性,通过在原无相变两相可压缩求解器的控制方程中耦合模拟空化汽液相间质量交换的源项,实现了对空化流动的非定常可压缩计算.利用上述考虑汽/液相可压缩性的空化流动求解器,对周期性云状空化流动进行了数值模拟,并重点研究了空穴溃灭激波特性.结果表明:上述数值计算方法可以准确捕捉到空穴非定常演化过程及大尺度脱落空泡云团溃灭激波现象,大尺度脱落空泡云团溃灭过程分为3个阶段:(1) U型空泡团形成; (2) U型空泡团头部溃灭; (3) U型空泡团腿部溃灭.在U 型空泡团腿部溃灭瞬间,观察到激波产生,并向上游和下游传播,向上游传播的激波与空穴相互作用,导致水翼吸力面新生的附着型片状空穴回缩,直至完全溃灭.并且空穴溃灭激波存在回弹现象, 抑制了下一周期的空化发展.      

采用分段湍流模式研究绕水翼的空化流动

结合空化流动特点,建立了一个包含空间尺度信息的分段湍流模式。计算中,应用基于质量传输空化模型,分别采用三种湍流模型计算了绕Clark-y型水翼云状空化流动,得到了随时间变化的空泡形态以及升、阻力等流场和动力特性。通过与实验结果的对比,表明这三种湍流模型均能捕捉云状空化区域的空泡形态和空泡脱落的非定常细节。分段湍流模式能够更好地调整流场内的湍流黏性,更精确地预测空穴长度和空穴尾部水汽分布,与实验结果吻合较好。     

收缩扩张管内液氮空化流动演化过程试验研究

本文基于低温空化试验平台研究了收缩扩张流道内液氮非定常空化流动的演化过程. 试验采用高时空分辨率的高速摄像机对77 K液氮在不同空化数σ下空穴结构的演变进行了精细化的分析和研究. 利用试验得到的空穴长度和面积等数据, 定量分析了液氮空化流动的非定常特性与时空演变规律. 研究结果表明: (1)在相似来流速度和温度条件下, 随着空化数的减小, 液氮空化流动呈现四种典型流型, 空穴长度在2.5 h以内为初生空化、空穴长度在2.5 h ~ 7.5 h之间为片状空化、空穴长度在7.5 h ~ 15 h之间为大尺度云状空化, 空穴长度超过15 h为双云状空化, 且在大尺度云状空化和双云状空化阶段均捕捉到了回射流现象; (2)液氮空化流动从初生空化到双云状空化, 脱落空穴的尺度逐渐增大, 空穴面积脉动的幅值和准周期均有所增加. 同时, 在大尺度云状空化与双云状空化阶段, 喉口处堵塞效应对空化流动的影响显著增强; (3)相比于初生空化, 片状空化、大尺度云状空化以及双云状空化中脱落空穴的移动距离依次增加了0.97倍、2.65倍与2.68倍, 溃灭时间依次增加了1.18倍、3.59倍与4.47倍, 但溃灭速度依次减小了0.10倍、0.20倍与0.30倍. 除此之外, 对于双云状空化阶段, 存在两种显著不同的脱落空穴演化过程.      

绕水翼空化流动及振动特性的实验研究

空化是发生在水力机械内部的一种水动力现象,其发展具有显著的非定常特性.空化流动中空穴的脱落以及溃灭会诱发结构振动,对水力机械的效率、噪声、安全性等造成影响. 研究空化流动中结构的振动特性具有重要的工程意义. 采用实验的方法研究了绕NACA66 水翼空化流动的空穴形态和水翼振动特性. 实验在一闭式空化水洞中进行. 采用高速摄像技术观测不同空化阶段的空穴形态,应用多普勒激光测振仪测量水翼的振动速度,并通过一套同步系统实现了高速相机和多普勒激光测振仪的同步触发和测量. 采用小波分析方法对不同空化阶段下的空穴形态和水翼振动数据在时域和频域中的特性进行了分析.对云状空化阶段的同步测量结果进行了研究,分析了振动与空穴发展过程的联系. 结果表明,随着空化数的降低,流场经历了无空化、初生空化、片状空化和云状空化4个阶段,水翼的振动强度呈逐渐增大趋势. 在片状空化和云状空化阶段,空穴脱落导致水翼振动,诱发的振动频率与空穴脱落频率相同. 对于云状空化,在附着型空穴生长阶段水翼发生高频小幅度振动,在空穴脉动和断裂脱落期间水翼表现为低频大幅振动.      

云状空化非定常脱落机理的数值与实验研究

结合数值计算和实验技术研究了云状空化的非定常脱落机理. 实验采用高速录像技术观察了绕Clark-y型水翼云状空化形态随时间的变化. 数值计算采用了汽-液两相的当地均相流模型,湍流封闭采用了一种修正的RNG k-\varepsilon方程,利用商业软件的二次开发技术,引入了一种与空化区域水汽相密度相关的系数,对湍流模型进行了修正. 计算和实验结果均表明:云状空化尾部存在着准周期性的涡状空化团的脱落;局部压强的增加是引起空穴断裂进而脱落的直接原因;压强升高是由于近壁处的反向射流引起的;空穴尾部的大规模的旋涡运动引起的近壁处的逆压梯度是引起反向射流的主要原因.      

可压缩空化流动空穴演化及压力脉动特性实验研究

空化流动具有高度的压缩性,空化流动非定常特性及其流体动力与压缩性密切相关.为研究可压缩空化流动空泡脱落的回射流和激波机制下周期性空穴结构演化及其诱导流体动力特性,本文采用多场同步测试方法对典型云状空化流动进行了实验研究,获得了文丘里管扩张段内部云状空化空穴形态演化及其诱导同步壁面压力脉动信号.并基于数字图像处理技术,对附着型片状空穴和脱落型云状空穴结构演化进行了精细化定量分析.结果表明:可压缩空化流动回射流机制下,空穴演化呈现附着型空穴生长-回射流产生及发展-附着型空穴失稳断裂及大尺度空泡云团产生脱落的非定常过程,激波机制下空穴演化具有附着型空穴生长-激波产生及传播-附着型空穴失稳断裂及大尺度空泡云团脱落的非定常特征,激波传播时间占空穴脱落周期小于回射流推进.激波与空穴相互作用导致空穴内部含气率瞬间大范围大幅度下降,诱导复杂流体动力.激波传播过程中,空泡内部压力脉动大幅增加,激波前缘诱导压力脉冲,而回射流推进过程中,壁面压力脉动相对平稳,回射流头部存在小幅增加.不同机制下空穴结构存在显著差异,具有不同的相间质量传输过程.     

Interfacial dynamics‐based modelling of turbulent cavitating flows,Part‐2: Time‐dependent computations

The interfacial dynamics‐based cavitation model, developed in Part‐1, is further employed for unsteady flow computations. The pressure‐based operator‐splitting algorithm (PISO) is extended to handle the time‐dependent cavitating flows with particular focus on the coupling of the cavitation and turbulence models, and the large density ratio associated with cavitation. Furthermore, the compressibility effect is important for unsteady cavitating flows because in a water–vapour mixture, depending on the composition, the speed of sound inside the cavity can vary by an order of magnitude. The implications of the issue of the speed of the sound are assessed with alternative modelling approaches. Depending on the geometric confinement of the nozzle, compressibility model and cavitation numbers, either auto‐oscillation or quasi‐steady behaviour is observed. The adverse pressure gradient in the closure region is stronger at the maximum cavity size. One can also observe that the mass transfer process contributes to the cavitation dynamics. Compared to the steady flow computations, the velocity and vapour volume fraction distributions within the cavity are noticeably improved with time‐dependent computations. Copyright © 2004 John Wiley & Sons, Ltd.     

Time‐dependent turbulent cavitating flow computations with interfacial transport and filter‐based models

Turbulent cavitating flow computations need to address both cavitation and turbulence modelling issues. A recently developed interfacial dynamics‐based cavitation model (IDCM) incorporates the interfacial transport into the computational modelling of cavitation dynamics. For time‐dependent flows, it is known that the engineering turbulence closure such as the original k–ε model often over‐predicts the eddy viscosity values reducing the unsteadiness. A recently proposed filter‐based modification has shown that it can effectively modulate the eddy viscosity, rendering better simulation capabilities for time‐dependent flow computations in term of the unsteady characteristics. In the present study, the IDCM along with the filter‐based k–ε turbulence model is adopted to simulate 2‐D cavitating flows over the Clark‐Y airfoil. The chord Reynolds number is Re=7.0 × 10⁵. Two angles‐of‐attack of 5 and 8° associated with several cavitation numbers covering different flow regimes are conducted. The simulation results are assessed with the experimental data including lift, drag and velocity profiles. The interplay between cavitation and turbulence models reveals substantial differences in time‐dependent flow results even though the time‐averaged characteristics are similar. Copyright © 2005 John Wiley & Sons, Ltd.     

A numerical method for simulation of attached cavitation flows

A new numerical algorithm for attached cavitation flows is developed. A cavitation model is implemented in a viscous Navier–Stokes solver. The liquid–vapour interface is assumed as a free surface boundary of the computation domain. Its shape is determined with an iterative procedure to match the cavity surface to a constant pressure boundary. The pressure distribution, as well as its gradient along the wall, is taken into account in updating the cavity shape iteratively. A series of computations are performed for the cavitating flows across three kinds of headform/cylinder bodies: conic, ogival and hemispheric heads. A range of cavitation numbers is investigated for each headform/cylinder body. The obtained results are reasonable and the iterative procedure of cavity shape updating is quite stable. The superiority of the developed cavitation model and algorithm is demonstrated. Copyright © 2006 John Wiley & Sons, Ltd.     

Evaluating cavitation regimes in an internal orifice at different temperatures using frequency analysis and visualization

Experiments on a water cavitating orifice were conducted to investigate the influence of pressure and temperature on flow regime transition due to cavitation. The thermal effects could be important in cases with cryogenic cavitation or hot fluid injection. The investigations were based on CCD observations and a pressure fluctuations frequency analysis.The high-speed photographic recordings were used to analyze the cavitation evolution and individuate the frequency content of the two-phase flow by processing the pixel-intensity time-series data.The cavitating structures showed different behaviors and characteristics with variations in operating conditions, as the pressure inside the orifice and the flow temperature .The flow regime map for the cavitating flow was obtained using experimental observations to analyze the occurrence of the different two-phase flow regime transitions at various operating conditions.As the pressure at the orifice inlet increased, at the same downstream pressure, cavitation inception occurred. The decrease of the cavitation number brought a significant increase in cavitation zone extension. As the pressure drop inside the orifice increased, the cavitation was characterized by an evident increase in cavitation zone length to the outlet of the orifice. With a further cavitation number decrease, the transition to jet cavitation was evident.The temperature influenced both the cavitation intensity and the cavitation number at which different two-phase flow regime transitions occurred, which tended to increase with temperature.The vapor fraction was estimated using an image processing algorithm.The frequency content given by the pressure fluctuations was analyzed and compared with the frequency spectra obtained from the visual observations. The behavior of the different cavitating flows could be correlated to the frequency spectrum of the pressure fluctuations measured upstream and downstream of the orifice. The cavitation number reduction and consequent increase in cavitating area width were related to a corresponding significant increase in the amplitude of typical frequency components. The transition to jet cavitation was characterized by a significant increase in the first peak in the frequency spectrum; weaker spectral peaks were also present at high cavitation numbers.     

Experimental evaluation of numerical simulation of cavitating flow around hydrofoil

Cavitation in hydraulic machines causes different problems that can be related to its unsteady nature. An experimental and numerical study of developed cavitating flow was performed. Until now simulations of cavitating flow were limited to the self developed “in house” CFD codes. The goal of the work was to experimentally evaluate the capabilities of a commercial CFD code (Fluent) for simulation of a developed cavitating flow. Two simple hydrofoils that feature some 3D effects of cavitation were used for the experiments. A relatively new technique where PIV method combined with LIF technique was used to experimentally determine the instantaneous and average velocity and void ratio fields (cavity shapes) around the hydrofoils. Distribution of static pressure on the hydrofoil surface was determined. For the numerical simulation of cavitating flow a bubble dynamics cavitation model was used to describe the generation and evaporation of vapour phase. An unsteady RANS 3D simulation was performed. Comparison between numerical and experimental results shows good correlation. The distribution and size of vapour structures and the velocity fields agree well. The distribution of pressure on the hydrofoil surface is correctly predicted. The numerically predicted shedding frequencies are in fair agreement with the experimental data.     

Numerical simulation of multiphase cavitating flows around an underwater projectile

The present simulation investigates the multiphase cavitating flow around an underwater projectile. Based on the Homogeneous Equilibrium Flow assumption, a mixture model is applied to simulate the multiphase cavitating flow including ventilated cavitation caused by air injection as well as natural cavitation that forms in a region where the pressure of liquid falls below its vapor pressure. The transport equation cavitating model is applied. The calculations are executed based on a suite of CFD code. The hydrodynamics characteristics of flow field under the interaction of natural cavitation and ventilated cavitation is analyzed. The results indicate that the ventilated cavitation number is under a combined effect of the natural cavitation number and gas flow rate in the multiphase cavitating flows.     

空泡流非稳态现象的流动控制

处于跨临界阶段的空泡流必然导致强烈的周期性冲击和振动,空泡流的激振来源于空泡云的周期性大规模脱落,空泡云的形成和发展与流动的边界层效应有着强烈的相关性,且空泡末端的局部流动直接影响空泡流的整体稳定性,本试验在NACA16012水翼表面粘附一条展向1mm厚10mm宽的挡流条,尝试以干扰水翼上表面局部流动的方法来影响整个空泡流的形态及其流动稳定性,最终在一定的空泡数范围内抑制了空泡流激振现象,并从试验研究的角度探索了空泡云脱落的机理。