旋涡空化流动结构及抑制机理研究

Vortex cavitation is a common phenomenon which can be observed in many flow situations such as the flow around the marine propellers, hydro-turbines and pumps. It often causes serious problems in the engineering applications including the increase of noise, the vibrations of machine set, as well as the cavitation erosions. In the naval warships the noise increase caused by vortex cavitation implies the stealth decrease of the ship. The suppression of vortex cavitation and reducing its damages has strong demand in engineering applications. So understanding the flow structures of vortex and vortex cavitation is not only the basement to solve the problem, but also the academic requirement for fluid mechanics. However, up to now the mechanism of vortex cavitation inception, the flow structures of vortex cavitation and the evolution features of vortex cavitation are still open problems in the hydrodynamics research area. .The present proposal will study systemically the vortex flow structures and reveal the effects of turbulence fluctuation in the vortex flow, water quality including free nuclei and the dissolved air in the water, as well as the ventilation on vortex cavitation. The focus of the study will be in the flow details of vortex cavitation and the quantitative relations with these effect parameters. The tip vortex cavitation in marine propeller is conducted as the main research object. The vortex flow around different hydrofoils and vortex cavitations will be studied by experimental methods in the cavitation tunnels with advanced measurement techniques, such as high speed camera, Laser Doppler Velocimetry (LDV), Particle Image Velocimetry (PIV) and Time Resolved Particle Image Velocimetry (TR-PIV) etc. At the same the high resolution numerical method will be developed to simulate the flow details of the vortex flow and vortex cavitation. The water quality will be carefully controlled in the experiments by high efficiency degassing system and nuclei seeding system, as well as measurement system to establish the quantitative relations with vortex cavitation. From the analysis of these experiments and numerical simulations, new vortex flow model and prediction model of vortex cavitation inception will be proposed. The relations of inception and evolution features of vortex cavitation with the effect of hydrofoil shapes, incoming flow conditions and water qualities will be summarized. The mechanism of suppression of vortex cavitation and methods will be presented.

旋涡空化是船舶推进器和水泵、水轮机等水力机械中常见的空化类型，其发生和发展直接影响我国大型、高速水面舰船声隐身性能及机组的安全运行，精确预报和有效抑制旋涡空化具有迫切需求。旋涡流动的复杂性，及旋涡空化中包含的水-气-汽混合复杂流动结构和相变过程，使得旋涡空化的产生机理、流动结构及演化过程成为水动力学空化研究关注的热点和难点，但缺乏精确、定量的成果。本项目以船舶推进器旋涡空化为主要研究对象，以现代试验测试技术为主要手段，结合高分辨率数值模拟，系统揭示旋涡空化流动中旋涡流动本身的涡流结构、涡内部的湍流脉动、水中含气量和气核及通气等因素对旋涡空化起始及其演化的影响规律，重点研究这些影响因素与旋涡空化之间的定量关系，提出更为全面科学的旋涡空化起始判别准则，同时探索抑制旋涡空化的机理并提出原理方案。研究成果将提高我国在旋涡空化研究方面的系统性和认识水平，为低噪声推进器和其他水力机械设计提供技术支撑。

本项目以船舶推进器旋涡空化为主要研究对象，以提高我国大型高速水面舰船声隐身性能为背景，以精确预报和有效抑制旋涡空化为应用目标，采用了实验和数值模拟相结合的研究方法。主要研究内容包括，1）典型水翼旋涡全湿流场和旋涡空化流场精细测量和高分辨率数值模拟；2）空化与流场涡流动结构相互作用；3）气核对旋涡空化初生影响研究； 4）旋涡空化初生特性及预报方法；5）梢涡空化形态演化与噪声特性；6）梢涡空化抑制研究。本项目取得的主要研究成果有，1）揭示了梢涡形成及流动结构与梢涡空化起始的关系，试验和数值模拟发现全湿梢涡涡核中心存在强烈的湍流脉动，空化起始除了与涡心平均压力有关外，还与涡核中心的湍流脉动有关，解释了梢涡空化起始点与最小涡心压力点间的偏差，并为建立新的梢涡空化起始模型提供的试验依据。2）在空化与旋涡之间的交互影响规律研究方面取得实质性进展，引入涡动力学方程揭示了涡拉伸项与膨胀项是空化湍流中旋涡生成与演化的主导因素，而斜压扭转项主要发生在空化的交界面，对应着空化的溃灭；成果获得国际同行的关注和认可，相关成果论文被引已超过100多次。3）国内首次在实验室重现梢涡空化中的“涡唱”现象，获得了可靠再现梢涡空化涡唱的临界参数和实验条件。通过对柱状汽泡动力学和高速摄像实验结果的分析，确认涡唱主频为空化柱状泡的本征频率，是一种外界激励诱导的共振现象，研究成果发表已在国际知名杂志发表。4）建立考虑水质影响的梢涡空化起始预报模型，推进了梢涡空化尺度效应的研究。在气核运动数值模拟研究的基础上，通过解析方法建立了快速筛选涡流中临界气核的准则，该准则筛选的气核与数值模拟中筛选的气核一致。以临界气核抗拉强度作为水质的理论表征，建立了包含气核谱分布的梢涡空化初生预报模型，经初步实验检验，该预报模型能够定量预报不同水质条件下的起始空化数。