褶皱幅度对仿生翼型滑翔气动特性的影响

蜻蜓翅膀具有独特的褶皱状形貌．研究者们致力于利用仿生学原理,设计在低雷诺数条件下具有更优气动性能的褶皱翼型．本文采用计算流体力学方法,求解二维不可压Navier-Stokes方程组,探讨了四种翼型（平板翼型、流线翼型、小幅度褶皱翼型和大幅度褶皱翼型）的气动表现．在低雷诺数条件下得到以下结果：(1) 较小幅度的褶皱结构有利于增加升力和减小阻力．(2) 雷诺数变化时褶皱翼型的升力系数呈非线性变化;在特定雷诺数区间,幅度相近的褶皱翼型会发生相对气动优势的转变．(3) 褶皱结构内的回流区通过减小粘性阻力,使得翼型总阻力下降．(4) 翼型前缘的极小区域会产生脉冲高升力,对升力表现产生较大影响．这些结果表明,调整褶皱幅度是实现褶皱翼型气动优化的有效方案．     

基于相变松弛法则的水下爆炸空化模型研究

水下爆炸过程中存在着大量的空化现象,空化的产生、演化及其溃灭过程对于水下冲击波传播、爆炸气泡运动以及水下结构物冲击损伤都会产生重要影响。本文基于多相可压缩流体理论模型,考虑空化发生过程中汽-液两相流体亚平衡状态下两相之间发生的热力学-化学平衡机制,分析汽-液两相介质之间的质量和热量交换,从而实现对相变过程的自动捕捉。该系统的控制方程采用分步法处理,首先利用二阶MUSCL-Hancock格式和HLLC黎曼求解器来求解齐次双曲型方程,再采用牛顿迭代法求解相变方程。数值测试结果表明,本文的计算模型对于空化相变过程具有较好的捕捉能力。最后将该模型应用到水下近水面爆炸空化的数值模拟当中,研究发现空泡的溃灭压力峰值约为冲击波压力峰值的15%,有效作用时间是冲击波载荷有效作用时间的2倍以上。本文的空化相变模型能够为水下爆炸空化现象的机理研究提供重要支撑。     

多孔翼型流场的数值模拟研究

空化和空蚀是影响水力机械能否安全、经济运行的主要因素之一.为了研究新的翼型结构——多孔翼型对于水力机械的空化及空蚀现象的影响,利用PROE建立具有不同微孔数的翼型模型,再借助Fluent软件对多孔翼型的三维绕流流场进行了定常数值模拟计算.基于伯努利定律,对具有不同孔数的翼型绕流流场进行流态分析.根据模拟结果显示的不同翼型结构的速度场和压力场的分布特征,从能量转换的角度出发,揭示了多孔翼型绕流流场的内在特性,即绕流翼型的流场(速度场、压力场)是随着孔数而变化的.结合空化发生的条件,说明采用多孔翼型结构对改善水力机械内部空化现象是有一定的作用的,通过数值模拟方法来分析研究水力机械内部的空化空蚀现象,可以作为实验与理论分析的参考.从一个新的角度,为改善水力机械的空化和空蚀现象提供了新思路.     

并列弹性双圆柱在均匀流场中的流体激励力的研究

实验研究了并列弹性双圆柱体在均匀流场中的水动力特性，其中包括柱表面稳态及脉动压力系数，稳态阻力及升力系数，脉动阻力和升力系数，结果表明：Ｖ＿ｒ＞Ｖ＿（ｒｃ）时，柱的振动对其流体激励力有明显影响。     

基于涡粘性模型的翼型湍流流场熵产率计算

利用有限体积法实现了基于非正交同位网格的SIMPLE算法。基于熵分析方法,采用涡粘性模型求解湍流熵产方程,系统研究了湍流模型对二维翼型绕流流场熵产率的影响。通过计算NACA0012翼型在来流雷诺数为2.88×106时,0°攻角~16.5°攻角范围内的翼型表面压力系数分布和升阻力特性,验证了算法及程序的正确性。结果表明,选择不同湍流模型时,翼型流场熵产的计算结果存在差异,湍流耗散是引起流场熵产的主要原因;翼型流场的熵产主要发生在翼型前缘区、壁面边界层和翼型尾流区域,流场熵产率与翼型阻力系数线性相关;当产生分离涡时,粘性耗散引起的熵产下降。     

基于表面设计减阻的水下航行体模型实验与数值分析

针对水下航行体高速航行时提高航速的需要,开展近壁面湍流边界层减阻的研究。进行表面设计使旋涡流动加强导致局部空化从而产生壁面附着的微气泡,以降低流体粘性系数,伴随近壁面湍流结构的改变来实现一定幅度的减阻。设计并进行轴对称体模型在高压循环水洞中的空化现象观测和阻力测量并做了相应条件下的数值计算。。实验和数值结果分析的基本结论比较一致:某种特征表面形貌在湍流中有一定幅度的减阻,局部空化效应在中低流速下牺牲压差阻力而导致航行总阻力升高,在高流速条件下则降低总阻力。     

气泡夹带对壁面油膜流动特性的影响

油-气润滑系统工作过程中,润滑油膜受微油滴冲击和压缩空气扰动影响易形成气泡夹带现象,气泡夹带行为将对壁面润滑油膜层的形成及流动过程产生重要影响。基于VOF数值模拟方法,对含气泡油膜沿倾斜壁面的流动行为进行研究,考察了气泡的存在对油膜形态和流动速度的影响规律,以及气泡破裂阶段空腔邻域内流体压力变化特性。研究表明,油膜夹带气泡的形变和迁移诱发气泡周围微流场的速度扰动现象,导致气液界面处产生非均匀速度梯度分布,进而引发油膜表面的形态波动。气泡发生破裂时,油膜空穴部位发生明显的正负压力波动现象,气泡附近壁面将承受一定的交变载荷作用。     

DERIVATION OF LIFT,DRAG AND THE KUTTA--JOUKOWSKI THEOREM FOR AIRCRAFTS 1)

本文介绍了空气动力学中几个基本概念与定律的起源。其中,升力与阻力分别是空气对物体作用力的两个方向上的分量,它们均是由空气与物体的相对运动而产生的,并与该运动速度的平方成正比。库塔儒可夫斯基升力环量定理给出了翼型升力与翼型绕流之间的关系,开启了20世纪早期各国对翼型性能的研究。同时,鉴于理想流体圆柱绕流无阻力的理论结果与实验观察存在的矛盾开始激发人们对黏性流体运动的研究兴趣,并由此诞生了纳维斯托克斯方程组。而后普朗特提出边界层概念,巧妙解决了局部流动与整体流动的关系问题。针对大展弦比直机翼,普朗特又提出了基于升力线假设的升力线模型,并根据翼型气动数据得到三维机翼的气动性能。     

飞行器的升力、阻力及升力与环量定理的起源

本文介绍了空气动力学中几个基本概念与定律的起源。其中,升力与阻力分别是空气对物体作用力的两个方向上的分量,它们均是由空气与物体的相对运动而产生的,并与该运动速度的平方成正比。库塔儒可夫斯基升力环量定理给出了翼型升力与翼型绕流之间的关系,开启了20世纪早期各国对翼型性能的研究。同时,鉴于理想流体圆柱绕流无阻力的理论结果与实验观察存在的矛盾开始激发人们对黏性流体运动的研究兴趣,并由此诞生了纳维斯托克斯方程组。而后普朗特提出边界层概念,巧妙解决了局部流动与整体流动的关系问题。针对大展弦比直机翼,普朗特又提出了基于升力线假设的升力线模型,并根据翼型气动数据得到三维机翼的气动性能。     

Level Set方法在地面效应数值模拟中的应用

利用Level Set方法隐式捕捉界面并能处理复杂的物质界面及其拓扑结构变化（如合并、交叉、破碎等）情况的优势,基于可压缩N-S方程的预处理技术,采用基于界面两侧介质黎曼关系的修正虚拟流体方法对介质边界进行处理;将只能应用于可压缩流动界面模拟的修正虚拟流体方法推广应用于不可压缩流动问题的模拟,数值模拟了气-水运动界面与近水面飞行器的相互影响规律。通过与将水面视为对称面的简化模型对比发现：利用自由运动界面模型计算的NACA6409翼型升力较小,能更准确地反映飞行器流场对水面形状的影响,其结果更为符合实际;文中还对比了翼型距水面不同高度时的结果,得到了翼型受水面影响的强弱与翼型距水面的高度成反比的结论。     

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      

绕三维水翼云状空化现象的实验研究

为了研究云状空化阶段空穴发展和脱落的机理,采用实验的方法对绕三维水翼云状空化流动进行了研究.实验在高速水洞中进行,采用高速摄像技术研究了不同空化阶段的空穴形态,并测量了翼型所受的升阻力,并对上述数据进行了频谱分析.结果发现:在云状空化阶段,观测到空穴的产生-发展-脱落-溃灭的准周期性变化;并捕捉到空泡脱落时附着在翼型前...     

Design approach for cavitation tolerant hydrofoils and blades

Cavitation inception and growth on conventional shape hydrofoils and blades leads initially to a jump of their flow-induced noise, further to an amplification of flow-induced vibration with frequently assisted erosion and finally, to a lift/thrust decrease combined with the drag increase. These undesirable cavitation effects can be mitigated or even suppressed for stable partial cavities experiencing no tail pulsations. A design approach enhancing performance of cavitating hydrofoils/blades by maintaining stable partial cavities is described. Experimental data manifesting an increase of hydrofoil lift with reduction of its drag and of force pulsations by such design are provided. Application of this design approach to propeller/turbine blades and advantages of its employment for blades operating in non-uniform incoming flows are analyzed. The possibility of an increase of the lift to drag ratio and of a reduction of the cavity volume oscillation in gust flows for blade sections is numerically manifested.     

主动射流控制水翼空化的数值模拟与分析

为了改善高速流动工况下水翼吸力面上流场的空化特性,提出了水翼表面主动射流对绕水翼周围流动加以控制的方法.基于密度分域滤波的FBDCM混合湍流模型联合Zwart-Gerber-Belamri空化模型,分析了来流空化数为0.83,来流攻角为8°,射流位置距水翼前缘为x=0.19c时,主动射流对于水翼吸力面上流动的空化特性和水动力特性影响.对回射流的强度进行了量化分析,以探究回射流与流场空化特性的关系.数值分析结果表明,在射流水翼吸力面上的时均空泡体积为原始水翼的1/15,使得流场内空化流动由云空化状态转变为较为稳定的片空化状态,显著地削弱了云空化的发展.此外,射流极大地改善了水翼的水动力性能,使得水翼的升阻比较原始水翼提高了22.9%,空泡的脱落频率减少了26.2%,空泡脱落所引起的振幅减小了9.1%.射流大幅降低了水翼吸力面上低压区面积,水翼吸力面上流体的逆向压力减小,回射流强度降低;同时,射流使水翼吸力面上的边界层减薄,增强了流动的抗逆压梯度能力,一定程度上阻挡了回射流向水翼前缘的流动,这也从机理上分析了主动射流抑制空化的原因.      

Influence of cavitation on the hydroelastic stability of hydrofoils

This work numerically examines the effect of turbulent and cavitating flow on the hydroelastic response and stability of a hydrofoil. A cantilevered, rectangular, chordwise rigid hydrofoil is modeled as a 2-degrees-of-freedom structure for its spanwise bending and torsional flexibilities. The fluid flow is modeled with the incompressible, Unsteady Reynolds Averaged Navier–Stokes equations using an eddy-viscosity turbulence closure model that is corrected for the presence of cavitation, and with a transport equation based cavitation model. The results show that, in general, massive cavitation tends to: (i) reduce the mean lift, (ii) increase the mean drag, (iii) lower the mean deformations, and (iv) delay static divergence, while unsteady sheet/cloud cavitation promotes flow induced vibrations. Such vibrations and load fluctuations could be as large as (and even greater than) the mean values for cases with unsteady cavitation, so dynamic and viscous fluid–structure models are needed to simulate flexible hydrofoils in cavitating flows. In general, the flow induced vibrations, and hence the drag force, are higher with decreasing stiffness. For small leading edge partial cavitation, increasing foil flexibility increases the maximum cavity length and reduces the cavity shedding frequency; however, the influence of foil flexibility is limited for cases where the maximum cavity length is near or beyond the foil trailing edge, because of the relocation of the center of pressure at the elastic axis, near the mid-chord. The results show that the mean deformations are generally limited by stall, and by the quasi-steady linear theory predictions at the fully-wetted and supercavitating limits. Furthermore, frequency focusing can occur when the cavity shedding frequency is near the fundamental system resonance frequencies, and broadening of the frequency spectrum can occur due to excitation of the sub-harmonics and/or modulation induced by the fluctuating cavities, if the cavity shedding frequency is away from the fundamental system resonance frequencies.     

射流对绕水翼云空化流动抑制机理研究

为理解绕水翼云空化流动的发展机理和探究水翼吸力面开孔射流的影响,采用密度 修正的RNG $k$-$\varepsilon $湍流模型和Schnerr-Sauer空化模型对原始NACA66(mod) 水翼和采用射流后的 水翼的云空化非定常过程进行模拟和对比分析;采用在水翼吸力面近壁区设立监测线的方法对近壁区的流场进行监测,得到 近壁区汽相体积分数、回射流速度、压力及压力梯度的时空分布云图;开展了云空化流场特性的涡动力学分析,进而分析水 翼云空化的发生机理和射流抑制空化的抑制机理. 结果表明:游离型空泡在下游溃灭时产生强烈的局部高压,其向上游传播 导致前缘空穴的一次回缩,而空穴的二次回缩受回射流的影响. 回射流的发展区域受限于较高的压力梯度,高的压力梯度一 直存在,但回射流在一个周期内的首次出现需要时间的积累. 在水翼吸力面射流使得射流孔附近压力升高,弥补了由于空化 和绕流造成的压降,压力梯度增大,抗逆压能力增强,对回射流起到阻挡作用;另一方面,射流使得回射流区域面积和回射 流的强度也有所减小,从而对云空化的发展起到抑制的效果. $Q$准则的涡结构云图相比于汽相体积分数云图能显示复杂的 流动结构,前缘附着型空穴和尾缘游离型空穴内存在旋涡,回射流对空穴存在剪切作用造成空穴脱落. 而射流对空穴和回射 流的剪切和阻挡使云空化发展得到抑制.      

Experimental investigation of a hydrofoil designed via hydrostructural optimization

In the last decade, there has been an increased interest in the use of multidisciplinary optimization techniques for the design of aerospace, maritime, and wind engineering systems. However, validation of numerically optimized results using experimental measurements has been scarce. In this paper, numerical predictions are compared with experimental measurements of the hydrodynamic forces, deformations, and cavitation performance for a baseline NACA 0009 hydrofoil and an optimized hydrofoil. Both hydrofoils are made of solid aluminum, and are cantilevered at the root. One of the hydrofoils is optimized using a high-fidelity hydrostructural solver combined with a gradient-based optimizer, as detailed by Garg et al. (2017). The numerical predictions agree well with experimental measurements for both the baseline NACA 0009 and the optimized hydrofoils. For the optimized hydrofoil, the mean differences between the predicted and measured values for mean lift, drag coefficient, and moment coefficients, are 2.9%, 5.1%, and 3.0%, respectively. For the non-dimensional tip bending deflection, the mean difference is 3.4%. Although the optimized hydrofoil is significantly thicker to withstand higher loads than the baseline, it yields an overall measured increase in the lift-to-drag ratio of 29% for lift coefficients ranging from $-0.15$ to 0.75 and exhibits significantly delayed cavitation inception compared to the baseline. The improvement in hydroelastic and cavitation performance is attributed to the changes in the distribution of camber, twist, thickness, and the leading edge radius of the optimized hydrofoil. The results validate the analysis and optimization of the high-fidelity hydrostructural design optimization approach, and opens up new possibilities for the design of high-performance hydrofoils, marine propellers, and turbines.     

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.     

Determination of the lift of a partially cavitated hydrofoil

New methods for the velocity circulation around partially cavitated hydrofoils are proposed for both short and long cavities. The calculated results are compared with available experimental data. The calculated lift and cavitation number dependences remain bounded as the cavity end approaches the trailing edge of the hydrofoil.St. Petersburg. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, pp. 71–74, November–December, 1994.     

Fluid-structure interaction simulation of three-dimensional flexible hydrofoil in water tunnel

The closely coupled approach combined with the finite volume method (FVM) solver and the finite element method (FEM) solver is used to investigate the fluid-structure interaction (FSI) of a three-dimensional cantilevered hydrofoil in the water tunnel. The FVM solver and the coupled approach are verified and validated by comparing the numerical predictions with the experimental measurements, and good agreement is obtained concerning both the lift on the foil and the tip displacement. In the noncavitating flow, the result indicates that the growth of the initial incidence angle and the Reynolds number improves the deformation of the foil, and the lift on the foil is increased by the twist deformation. The normalized twist angle and displacement along the span of the hydrofoil for different incidence angles and Reynolds numbers are almost uniform. For the cavitation flow, it is shown that the small amplitude vibration of the foil has limited influence on the developing process of the partial cavity, and the quasi two-dimensional cavity shedding does not change the deformation mode of the hydrofoil. However, the frequency spectrum of the lift on the foil contains the frequency which is associated with the first bend frequency of the hydrofoil.