水翼空泡流剥蚀定量评估方法研究

Erosion induced by cavitation is always one of the most important issues which imposes limitation to the development of ship propulsion and hydraulic systems. Due to the complexity of cavitation erosion, current investigation often gives a qualitative assessment of cavitation erosion risk by model tests and/or numerical simulations, and few quantitative analyses are developed only for 2D domain. Staring from the study of the influence of periodic evolution process of unsteady cloud cavitation on the local flow field, this research aims at proposing a method for the quantitative assessment of cavitaiton erosion which is induced by 3D unsteady cavitating flow, where material response is taken into account. The investigations mainly focus on the following contents:1) To experimentally investigate unsteady cloud cavitation and erosion by high-speed video, paint test and acoustic signal capture or impact loads measurement; 2) To numerically investigate capabilities of cavitation models and turbulence models for better simulation of unsteady cavitation evolution and turbulence details, and to explore the correlation between unsteady characteristics of cloud cavitation and high erosive potential; 3) To evaluate the application prospect of current assessment methods on qualitative and quantitative analyses of cavitation erosion,and to propose a quantitative assessment method on the basis of an effective mathematical model for erosive aggressiveness of cavitating flows and analyses of material response during erosion incubation stage, as well as to validate the newly proposed method with the aid of experimental analysis.

空化诱导产生的空泡剥蚀一直是制约船舶推进系统以及水利机械发展的重要问题之一。鉴于空泡剥蚀现象的复杂性，现有的研究多通过模型试验和/或数值模拟定性评估空泡剥蚀的风险，为数不多的定量分析也都是基于二维流域而展开，本研究拟从云状空化非定常周期性演变过程对当地流场的影响入手，结合材料的响应特性，最终提出可对三维非定常空泡流诱导的空泡剥蚀进行定量评估的方法，主要研究内容包括：1）通过高速摄像、漆面试验及声信号或冲击载荷测量等手段对水翼非定常云状空化及其剥蚀进行试验研究；2）通过空泡模型和湍流模型的研究着重提高非定常空化演变及湍动细节的模拟精度，揭示云状空化非定常特性与高剥蚀风险的关联性；3）评价现有的空泡剥蚀预估方法定性乃至定量分析空泡剥蚀的应用前景，提出有效的衡量空泡流侵蚀性的数学模型，分析材料在空蚀孕育初期的响应特性，发展定量评估的方法，并通过试验分析进行验证。

随着船舶向大型化、高速化发展，空化现象及其诱导产生的空泡剥蚀越来越为大家所关注。空泡剥蚀的产生是空化破坏作用和材料抗蚀能力综合作用的结果，影响因素较多，其内在机理迄今尚未充分阐明。复杂空泡流动的数值模拟方法及其侵蚀性的定量评估方法是该领域的研究热点和难点。本项目以具有集聚效应的绕水翼非定常云状空化现象为研究对象，针对如何提高非定常云状空化演变过程、空泡精细结构、升阻力特性、瞬态流场以及空泡流湍动特性的模拟精度展开研究，结合高速摄像机记录的空泡结构形态变化及其溃灭时的瞬态特性、同频率传感器记录的压力脉动、水听器测量的噪声信号及软面试验测量的破损区域，着重分析云状空化演变过程中尤其是溃灭末期的全局及当地流场信息，建立衡量空泡流侵蚀性的数学模型，寻求定量评估空泡剥蚀的方法。.首先，采用大涡模拟（LES）方法进行了绕NACA0015水翼空化现象的数值模拟研究，发现LES方法能够捕捉到非定常雷诺平均（URANS）方法未能观测到的次级漩涡结构，Smagorinsky-Lilly亚格子模型所预报的片空泡范围、马蹄形空泡云团的脱落位置及脱落频率与试验观测结果更为接近；其次，结合试验结果重点探讨马蹄形云空泡发展及溃灭过程中的瞬态特性，剖析流场信息与高剥蚀风险的关联，发现片空泡破裂过程中生成的次级漩涡结构以及马蹄形空泡云团的溃灭与高剥蚀风险密切相关，翼型表面瞬态压力突增区域与试验观测得到的剥蚀区域最为接近，最终选定瞬态压力场的时间导数作为衡量高剥蚀风险区域的有效特征参数；在此基础上，累计周期内所有超过由材料响应特性决定的极限阈值的压力时间导数，所预报的高剥蚀强度区域与试验测得的破损区域较为接近。这一方法可以实现对高剥蚀风险区域及空泡流侵蚀性的定量评估，但未计入尺度效应及软面材料所带来的影响，今后可进一步尝试构建数值模型判断不同材料的极限阈值，为脱离开试验纯粹依靠数值方法进行复杂空泡流侵蚀性的研究打下基础。