表面涂层对绕水翼空化流动影响的实验研究

本文采用高速全流场显示技术观测了两种涂层对绕Clark-Y型水翼空化流动的影响。研究中,随着空化数的降低,主要观测了绕水翼的初生空化、片状空化、云状空化三个阶段σ=1.50时,绕水翼A的空化流动已经发展至片状空化,绕水翼B的空化流动处于初生空化阶段。σ=1.19时,绕水翼A与水翼B空化流动均处于片状空化阶段,围绕两种模型的空穴长度接近,空泡破裂时形成了大量马蹄涡,水翼A表面的附着型锥状空穴产生了大尺度的涡团。σ=0.7时,绕水翼A和水翼B皆为云状空化阶段,绕水翼A的无量纲空化面积小于水翼B;反向射流导致大尺度涡团周期性脱落,绕水翼A的空化涡团脱落较为整齐,绕水翼B则产生分散的马蹄涡。     

绕水翼云状空化流动特性的研究

采用高速全流场显示技术分别观测了绕超空化水翼和Clark-Y型水翼的云状空化.结果表明:绕超空化水翼和Clark-Y型水翼的云状空化具有相同的变化过程,即:生成、成长、膨胀、脱落和消失溃灭五种状态,两种空化云流动都具有明显的脱落周期和脱落轨迹.在翼型尾部存在的反方向射流,致使空化旋涡脱落;尽管模型尺度、来流速度和空化数基本相同,但由于超空化水翼与Clark-Y型水翼断面形状不同,使翼型尾部的反方向射流强度不同,故与来流相互作用强度不同,导致绕两种水翼的空化云脱落周期不同.在本文实验条件下,绕超空化水翼空化云和Clark-Y型水翼空化云的脱落频率分别为13.5 Hz和19 Hz.     

绕空化器自然空化与通气空化的对比

本文采用高速全流场显示技术和动态应变式测力系统相结合对比了绕圆盘空化器自然空化流场与通气空化流场流动结构和影响因素。结果表明,自然空化和通气空化在流场形态上有着相同的发展过程:初生空化、带状空化、团状空化、半透明超空化和透明超空化;空化数是自然空化主要影响因素之一,通气量是通气空化主要影响因素之一。通气能够促进绕空化器空化流动发展程度。     

绕空化器通气空化流场的实验研究

本文采用高速全流场显示技术和动态应变式测力系统相结合研究了绕圆盘空化器通气空化流动发展过程和动力特性,讨论了通气量对空化流场形态和阻力的影响。结果表明：随着雷诺数增加,绕空化器的通气空化流场经历了初生空化、带状空化、团状空化,阻力系数不断减小,未发生自然空化时,减阻效果明显;随着通气量增加,绕空化器通气空化流场出现了半...     

绕空化器自然空化流动的研究

本文采用高速全流场显示技术、片光源流动显示技术和应变式测力系统相结合研究了圆盘空化器模型的流场。结果表明:随着来流速度增加,绕圆盘流动经过无空化到初生空化的过渡过程后,出现了初生空化、带状空化和团状空化等不同空化阶段;绕圆盘流动阻力系数不断减小,出现了空化后阻力变化梯度降低;绕空化器的流场波动幅度不断加大,但空化器所受阻力主频基本稳定。     

绕Hydronautics水翼空化云流动的实验研究

采用高速全流场显示技术和DPIV对空化水翼近壁处的空化云流动形态和运动机理进行了讨论.结果表明:绕Hydronautics翼型的空化云运动是一个准周期性过程:稳定空泡团初生在翼型前部,向翼型后部发展布满整个翼面,在翼型后缘出现汽泡团旋涡,伴随反向运动,最终向下游脱落.当前条件下空泡团旋涡脱落周期约为74 ms.空化区与主流区的交界面上存在较大的速度梯度,一组对涡出现在翼型尾部处交界面上.此外,采用以空泡为示踪粒子的DPIV能够对空化流动流速分布进行有效测量.     

绕水翼片状空化流动结构的数值与实验研究

采用数值与实验相结合的方法研究了水翼片状空化流动结构.实验采用高速录像技术观察了片状卒泡形态,应用LDV测量了翼型周围的湍动能和速度分布;采用N-S方程和基于空泡动力学方程的空化模型计算了绕水翼片状空化流场.结果表明:在片状空化阶段,翼型吸力面上附着很薄的一层透明空泡,空泡彤态呈现于指状;随着空化数的减少,空泡尾部水汽交界面相互作用增强,并且空泡尾部出现大的旋涡,影响了空泡尾部区域压力和速度分布,片状空泡尾部的水汽混合区出现不稳定现象,同时存在小的空泡团脱落.数值模拟得到的水翼片状空化流动现象和实验观察到的结果基本一致,验证了计算模型和数值方法的可靠性.     

绕不同涂层水翼片状空化运动特性的实验研究

本文采用高速全流场显示技术和粒子图像测速系统(PIV)实验方法讨论了绕不同涂层水翼片状空化流动结构、运动特性的影响。结果表明:涂层的粗糙度和材质共同影响绕水翼片状空化流动结构和运动特性。实验中观察流动结构,发现粗糙表面比光滑表面更易于出现片状空化,氟碳涂层比环氧涂层更能够抑制片状空化;运动特性方面,表面粗糙度与涂层材质相比较,材质对片状空化的运动特性影响更大:表面光滑的氟碳涂层的低速区最小且区内速度波动也最弱,但区内的涡量分布更为集中;不同涂层造成水翼表面速度旋涡的位置不同,导致空穴脱落的位置不一样,空穴发展长度不同。     

绕Hydronautics水翼空化旋涡的高速录像观察

为了解hydronautics超空化水翼的绕流流场特性,采用高速摄像机等流动显示设备,在空化水洞中针对此种超空化水翼进行了一系列绕流流场显示实验。实验结果表明,绕这种超空化水翼的空化形成与发展,均与产生于翼型前端 (吸力面)和尾端(压力面)的旋涡或二者的结合体:空化涡街有关;空化数的大小,决定了空化涡街形成与溃灭的不同特征;当空化数较低时,涡街中空化旋涡的扩张形成了闭合的超空化区域。     

水翼表面粗糙带对空化抑制效果的数值研究

为研究水翼表面粗糙带对空化的抑制效果,在修改的NACA16翼型吸力面施加一粗糙带,对在不同粗糙带条件下翼型的水动力学特性和周围流场进行了数值模拟研究,得到了升阻力系数、吸力面最小压力系数和近壁面湍动能随粗糙带变化的趋势,探讨了水翼表面粗糙带对抑制表面空化的效果。结果表明,随着粗糙带宽度和高度的增大,翼型周围流场的压强显著升高并使吸力面的最低压力高于饱和蒸汽压,从而延缓了初生空化的发生。该结果为认识粗糙带对水翼抗空蚀性能的影响提供了参考。     

基于高速摄像实验的开放腔体圆柱壳入水空泡流动研究

基于高速摄像方法,针对入水空泡流动特征和机理,进行了开放腔体圆柱壳垂直入水实验研究.通过对实验现象的观测,发现开放腔体圆柱壳入水运动会形成波动流动和云化流动两种流动方式,结合影像数据,分别描述了两种流动状态下的空泡形态特征,并获得了空泡波动参数的变化规律;对比不同入水速度实验,分析了入水速度对入水空泡流动方式和流动参数的影响;依据流体力学基本理论,分析了入水空泡波动和云化现象的形成机理.结果表明:随入水速度增加,入水空泡依次呈现波动和云化两种流动状态,波动频率与入水速度无关,闭合发生时间随入水速度增加而减小,与Froude数呈线性关系;入水导致开放空腔内部气体涨缩,引起开放端压力场和速度场周期性扰动,空泡截面扩展程度出现差异,形成空泡波动现象;空泡闭合后尾部形成回射流,回射流触及空泡壁面引起壁面流动转捩,形成空泡云化现象.     

A modified PANS model for computations of unsteady turbulence cavitating flows

A modification to the PANS (partially averaged Navier-Stokes) model is proposed to simulate unsteady cavitating flows. In the model, the parameter f _k is modified to vary as a function of the ratios between the water density and the mixture density in the local flows. The objective of this study is to validate the modified model and further understand the interaction between turbulence and cavitation around a Clark-Y hydrofoil. The comparisons between the numerical and experiment results show that the modified model can be improved to predict the cavity evolution, vortex shedding frequency and the lift force fluctuating in time fairly well, as it can effectively modulate the eddy viscosity in the cavitating region and various levels of physical turbulent fluctuations are resolved. In addition, from the computational results, it is proved that cavitation phenomenon physically influences the turbulent level, especially by the vortex shedding behaviors. Also, the mean u-velocity profiles demonstrate that the attached cavity thickness can alter the local turbulent shear layer.     

Cavitating vortex generation by a submerged jet

The surface geometry of a cavitating vortex is determined in the limit of inviscid incompressible flow. The limit surface is an ovaloid of revolution with an axis ratio of 5: 3. It is shown that a cavitating vortex ring cannot develop if the cavitation number is lower than a certain critical value. Experiments conducted at various liquid pressures and several jet exit velocities confirm the existence of a critical cavitation number close to 3. At cavitation numbers higher than the critical one, the cavitating vortex ring does not develop. At substantially lower cavitation numbers (k ? 0.1), an elongated asymmetric cavitation bubble is generated, with an axial reentrant jet whose length can exceed the initial jet length by several times. This flow structure is called an asymmetric cavitating vortex, even though steady motion of this structure has not been observed.     

Investigation of cavitation noise using Eulerian-Lagrangian multiscale modeling

We have employed the large eddy simulation (LES) approach to investigate the cavitation noise characteristics of an unsteady cavitating flow around a NACA66 (National Advisory Committee for Aeronautics) hydrofoil by employing an Eulerian-Lagrangian based multiscale cavitation model. A volume of fluid (VOF) method simulates the large cavity, whereas a Lagrangian discrete bubble model (DBM) tracks the small bubbles. Meanwhile, noise is determined using the Ffowcs Williams-Hawkings equation (FW-H). Eulerian-Lagrangian analysis has shown that, in comparison to VOF, it is more effective in revealing microscopic characteristics of unsteady cavitating flows, including microscale bubbles, that are unresolvable around the cloud cavity, and their impact on the flow field. It is also evident that its evolution of cavitation features on the hydrofoil is more consistent with the experimental observations. The frequency of the maximum sound pressure level corresponds to the frequency of the main cavity shedding for the noise characteristics. Using the Eulerian-Lagrangian method to predict the noise signal, results show that the cavitation noise, generated by discrete bubbles due to their collapse, is mainly composed of high-frequency signals. In addition, the frequency of cavitation noise induced by discrete microbubbles is around 10 kHz. A typical characteristic of cavitation noise, including two intense pulses during the collapsing of the cloud cavity, is described, as well as the mechanisms that underlie these phenomena. The findings of this work provide for a fundamental understanding of cavitation and serve as a valuable reference for the design and intensification of hydrodynamic cavitation reactors.     

不均匀伴流场中螺旋桨空化的黏性流数值模拟和低频噪声预报

阐述了采用空化黏流CFD瞬态模拟和脉冲球形气泡辐射噪声理论相结合的思路在螺旋桨空化低频噪声谱预报上的应用方法。预报了全附体SUBOFF潜艇标称伴流条件下的NSRDC4383五叶大侧斜桨和某七叶大侧斜桨的片空化低频噪声谱,分析了桨叶负载和空化程度对线谱成分及其谱源级的影响。空化模拟时采用作者提出的且可信性经过验证的改进Sauer空化模型和修正SST湍流模型。噪声谱预报时空化体积由空化特征长度求取,较空泡表面球形等价假设更加合理。计算表明,七叶桨较五叶桨的确具有负载小、空化初生延迟、空化低频线谱噪声低的特征。在相同的基于航速的空化数下,非均匀进流与桨叶相互作用会明显增加线谱成分及其谱源级。在伴流、空化数和转速一定时,随着负载减小,推力、力矩和桨叶空化面积均会减小,但空化体积加速度幅度却变大,离散线谱噪声级增加且由奇次谐频为主转变为以偶次谐频为主;当仅减小空化程度时,谐频线谱成分明显被抑制,且1 kHz频率处谱源级减小2.54 dB。较完整地构建了螺旋桨空化水动力和噪声性能评估的数值平台,可用于指导艇尾低噪声桨的数值设计。      

轴流式水轮机内部空化流动的数值预测

将一种完整空化模型和一种混合流体两相流模型相结合,应用于轴流式水轮机全流道内定常空化流动的数值模拟。根据模拟结果,分别预测了水轮机在正常和较低尾水位情况下运行时,流道内空化发生的部位和程度,重点考察了尾水位的降低对水轮机空化性能的影响,为水轮机在特定运行工况下的空化性能预测提供参考。     

A Rayleigh-Plesset based transport model for cryogenic fluid cavitating flow computations

The present article focuses on modeling issues to simulate cryogenic fluid cavitating flows.A revised cavitation model,in which the thermal effect is considered,is derivated and established based on Kubota model.Cavitating flow computations are conducted around an axisymmetric ogive and a 2D quarter caliber hydrofoil in liquid nitrogen implementing the revised model and Kubota model coupled with energy equation and dynamically updating the fluid physical properties,respecitively.The results show that the revised cavitation model can better describe the mass transport process in the cavitation process in cryogenic fluids.Compared with Kubota model,the revised model can reflect the observed"frosty"appearance within the cavity.The cavity length becomes shorter and it can capture the temperature and pressure depressions more consistently in the cavitating region,particularly at the rear of the cavity.The evaporation rate decreases,and while the magnitude of the condensation rate becomes larger because of the thermal effect terms in the revised model compared with the results obtained by the Kubota model.     

Analysis of Two-Phase Cavitating Flow with Two-Fluid Model Using Integrated Boltzmann Equations

In the present work, both computational and experimental methods are employed to study the two-phase flow occurring in a model pump sump. The two-fluid model of the two-phase flow has been applied to the simulation of the three-dimensional cavitating flow. The governing equations of the two-phase cavitating flow are derived from the kinetic theory based on the Boltzmann equation. The isotropic RNG$k-\epsilon-k_{ca}$ turbulence model of two-phase flows in the form of cavity number instead of the form of cavity phase volume fraction is developed. The RNG $k-\epsilon-k_{ca}$ turbulence model, that is the RNG$k-\epsilon$ turbulence model for the liquid phase combined with the $k_{ca}$model for the cavity phase, is employed to close the governing turbulent equations of the two-phase flow. The computation of the cavitating flow through a model pump sump has been carried out with this model in three-dimensional spaces. The calculated results have been compared with the data of the PIV experiment. Good qualitative agreement has been achieved which exhibits the reliability of the numerical simulation model.