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.      

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-ε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.     

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.      

单液滴内空化气泡的生长及溃灭研究

超空化燃油射流使得喷雾中部分燃油分裂液滴内含有空化气泡;空化气泡的生长及溃灭对液滴的分裂与雾化具有重要影响. 基于VOF 方法首次对超空化条件下燃油液滴内空化气泡的生长及溃灭过程进行了数值模拟. 通过研究发现,单液滴内空化气泡的生长过程可以按控制机理划分为表面张力控制阶段、综合竞争阶段和惯性力控制阶段;在第I 阶段,空泡的生长主要受表面张力的控制作用,惯性力对空泡生长的促进作用及黏性力对空泡生长的抑制作用可以忽略;在第II 阶段,空泡的生长受表面张力、惯性力及黏性力三者的综合作用,空泡的生长速率是促进空泡生长的惯性力和抑制空泡生长的表面张力及黏性力相互竞争、共同作用的结果;在第III 阶段,空泡的生长主要受惯性力的控制作用,抑制空泡生长的表面张力及黏性力的作用基本可以忽略. 单液滴内空化气泡的溃灭过程由多个溃灭阶段和反弹阶段构成,类似于有阻尼弹簧振子的振动过程;根据每个溃灭周期结束时空泡半径随时间的变化历程,可以将空泡的溃灭分为快速溃灭期、缓慢溃灭期以及稳定期;溃灭初期空泡溃灭压力的变化非常剧烈,但空泡溃灭体积的变化则要相对平缓得多;空泡反弹压力随时间的变化与空泡反弹体积随时间的变化基本对应.      

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.     

泡内气体热力学性质对空泡溃灭的影响

数值研究固壁附近轴对称空泡溃灭问题. 忽略泡内气体与周围流体之间的质量和热交换, 假设气体瞬时处于热平衡状态, 通过引入不同的热力学模型, 考察泡内气体在空泡溃灭过程中的作用. 采用原始变量的Navier-Stokes方程作为流场的控制方程, 用流体体积方法跟踪运动空泡壁. 数值结果显示空泡溃灭过程中, 伴随空泡变形, 空泡发出多个高压脉冲和高速射流. 对于不同的热力学模型, 等温, 绝热和准绝热过程, 绝热过程能够最大程度抑制空泡溃灭, 从而减弱空泡溃灭对固壁造成的空蚀破坏. 在绝热及其类似过程中, 出现空泡回弹现象.     

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.      

淹没磨料射流的空泡运动分析

通过对淹没条件下磨料射流的空泡运动分析研究,建立了淹没磨料射流的空泡运动方程,揭示了淹没磨料射流中空泡的溃灭特性,数值模拟了淹没磨料射流的磨料体积浓度以及空泡所处流场压力对空泡运动及溃灭的影响规律.分析表明:淹没磨料射流中磨料的存在增大流体的粘性系数,增大空泡溃灭历时,减弱射流的空蚀破坏能力;流场压力的改变对空泡溃灭过程影响显著,压力越高,空泡溃灭历时越短.     

N表面张力对近固壁二空化泡影响的数值研究

在忽略浮力下，用边界积分方法数值模拟了表面张力对固壁之上且靠近固壁的二轴对称空化泡生长和溃灭的影响，发现在下空泡最大等效半径为上空泡一半情形，若固壁对下空泡的Bjerknes力大于上空泡对下空泡的Bjerknes力，则表面张力的作用将使下空泡溃灭加速，使其向下的液体射流变强变宽；若固壁对下空泡的Bjerknes力小于上空泡对下空泡的Bjerknes力，则表面张力的作用将使下空泡溃灭变慢，使其向上射流变弱变细长；若这两个Bjerknes力近于相等，则表面张力将会对下空泡溃灭有重大作用，如改变下空泡射流的方向甚至形式(如由环状变向下或由向上变环状)，当上空泡等于或小于下空泡时，表面张力将不会对这两个空泡的行为产生显著影响，定性地分析了表面张力作用的机理。     

Combined experimental observation and numerical simulation of the cloud cavitation with U-type flow structures on hydrofoils

Sheet/cloud cavitation is an important topic that is a very common type of cavitation in turbo-machinery and marine propeller. Up to now we still have limited understanding of the cavitation shedding dynamics and cloud cavity formation and development. The present study used experimental and numerical studies to gain a better understanding of the complex physics involved in this problem. A series of experimental observations around hydrofoils are carried out in the cavitation tunnel of the China Ship Scientific Research Center (CSSRC) to illustrate the spatial–temporal evolution of the cloud cavity in detail. The results demonstrate that U-type flow structures are common in cloud cavities and can be divided into three stages and the closure line in a sheet cavity often has a convex–concave profile. Reentrant flows occur in the convex region with the jet direction normal to the contour edge so the shedding is mainly caused by the converging reentrant flows. Further analysis demonstrated that there was a striking difference with the cavity growth suppressed substantially in the twisted hydrofoil case if compared with straight hydrofoil and the effect of side entrant jets might make the cavity more uniform across the span. Numerical simulations were used to simulate the formation and development of the cloud cavity. The results show that the strong adverse pressure gradient in the stagnation region at the downstream end of the attached cavity forces the re-entrant flows into the vapor structure with a radially-diverging re-entrant jet and a pair of side-entrant jets, which causes the cavity shedding. Further analyzes of the local flow fields show that the interactions between the circulating flow and the shedding vapor cloud may be the main reason for the formation of the U-type cloud cavity structures.     

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.      

Experimental study of ventilated cavity flow over a 3-D wall-mounted fence

Ventilated cavity flow over a fixed height 3-D wall-mounted fence is experimentally investigated in a cavitation tunnel for a range of free-stream conditions. The impact of 3-D effects on cavity topology is examined, along with the dependence of the cavitation number and drag on the volumetric flow rate coefficient, fence height based Froude number and vapour pressure based cavitation number. Three different flow regimes are identified throughout the range of cavitation numbers for a particular free-stream condition. Generally, the cavity has a typical re-entrant jet closure the intensity of which is found to increase linearly with increasing Froude number. This increase in re-entrant jet intensity causes an increase in drag with Froude number for constant volumetric flow rate coefficient. At low Froude numbers the closure mechanism transitions from a single to a split re-entrant jet. The parameters used to characterize the cavity topology show a linear dependence on Froude number irrespective of the closure mode. The cavity topology and drag are found to be independent of vapour pressure based cavitation number.     

Multiscale tow-phase flow modeling of sheet and cloud cavitation

A multiscale two-phase flow model based on a coupled Eulerian/Lagrangian approach is applied to capture the sheet cavitation formation, development, unsteady breakup, and bubble cloud shedding on a hydrofoil. No assumptions are needed on mass transfer. Instead natural free field nuclei and solid boundary nucleation are modelled and enable capture of the sheet and cloud dynamics. The multiscale model includes a micro-scale model for tracking the bubbles, a macro-scale model for describing large cavity dynamics, and a transition scheme to bridge the micro and macro scales. With this multiscale model small nuclei are seen to grow into large bubbles, which eventually merge to form a large scale sheet cavity. A reentrant jet forms under the sheet cavity, travels upstream, and breaks the cavity, resulting in the emission of high pressure peaks as the broken pockets shrink and collapse while travelling downstream. The method is validated on a 2D NACA0015 foil and is shown to be in good agreement with published experimental measurements in terms of sheet cavity lengths and shedding frequencies. Sensitivity assessment of the model parameters and 3D effects on the predicted major cavity dynamics are also discussed.     

The influence of surface roughness on cloud cavitation flow around hydrofoils

The aim of this study is to investigate experimentally the effect of surface roughness on cloud cavitation around Clark-Y hydrofoils. High-speed video and particle image velocimetry(PIV) were used to obtain cavitation patterns images(Prog. Aerosp. Sci. 37: 551–581, 2001), as well as velocity and vorticity fields. Results are presented for cloud cavitating conditions around a Clark-Y hydrofoil fixed at angle of attack of α = 8?for moderate Reynolds number of Re = 5.6 × 10~5. The results show that roughness had a great influence on the pattern, velocity and vorticity distribution of cloud cavitation. For cavitating flow around a smooth hydrofoil(A) and a rough hydrofoil(B), cloud cavitation occurred in the form of finger-like cavities and attached subulate cavities, respectively. The period of cloud cavitation around hydrofoil A was shorter than for hydrofoil B.Surface roughness had a great influence on the process of cloud cavitation. The development of cloud cavitation around hydrofoil A consisted of two stages:(1) Attached cavities developed along the surface to the trailing edge;(2) A reentrant jet developed, resulting in shedding and collapse of cluster bubbles or vortex structure. Meanwhile, its development for hydrofoil B included three stages:(1) Attached cavities developed along the surface to the trailing edge, with accumulation and rotation of bubbles at the trailing edge of the hydrofoil affecting the flow field;(2) Development of a reentrant jet resulted in the first shedding of cavities. Interaction and movement of flows from the pressure side and suction side brought liquid water from the pressure side to the suction side of the hydrofoil, finally forming a reentrant jet. The jet kept moving along the surface to the leading edge of the hydrofoil, resulting in large-scale shedding of cloud bubbles. Several vortices appeared and dissipated during the process;(3) Cavities grew and shed again.     

High-speed visualization and PIV measurements of cavitating flows around a semi-circular leading-edge flat plate and NACA0015 hydrofoil

Cavitating flows around a flat plate with semi-circular leading edge and a NACA0015 hydrofoil at attack angles ranging from 0° to 9° and with varying cavitation number are investigated using high-speed-imaging visualization (HIV) and particle-imaging velocimetry (PIV). Several known types of cavitation common to both foils, but also some different patterns, were observed. At small angles of incidence (less than 3°), cavitation on the plate begins in the form of a streak array (bubble-band) whereas on the hydrofoil as traveling bubbles. For the regimes with developed cavitation on the NACA0015 hydrofoil, the scattered and discontinuous bubble streaks branch and grow but subsequently merge into bubble clouds forming a remarkably regular lattice pattern. Once the incidence angle increased to 9°, the cavitation on the hydrofoil changed to a streaky pattern like that on the plate at small attack angles, whereas the regime on the plate showed no significant changes. The PIV method proved to be usable for measuring the instantaneous velocity also in the gas–vapor phase, albeit with reduced accuracy that was evaluated and accounted for on the basis of the effective (validation-surviving) number of imaging samples. The time-averaged velocity and turbulence moments show that the incipience of cavitation is governed by the development of the carrier-fluid flow around the foil leading edges, but the subsequent flow pattern depends strongly on the cavitation regime displaying markedly different distributions compared to the non-cavitating case. The main cavitation parameters: the maximum cavity length, the cloud cavity streamwise dimensions and the cloud shedding Strouhal number are analyzed and presented in function of the cavitation number and the attack angle in different scaling. The measurements confirm qualitatively the trends reported in the literature, but show also some quantitative differences, notably between the two foils considered.     

Investigation of a re-entrant jet reflection at an inclined cavity closure line

Cavitation on two-dimensional hydrofoils with swept leading edges always displays some 3-dimensional effects. It is well known that the cavity closure on such hydrofoil is not perpendicular to the channel walls, but is curved in a distinctive pattern. The cavitation pocket is longer in the region where the hydrofoil is the shortest. Also the dynamics of cavitation is very distinctive. In the region where the hydrofoil is the longest attached and steady cavitation with no cloud separation exists. On the other side, where the hydrofoil is the shortest, cavitation cloud separations occur.Different explanations for this pattern were proposed in the past but they have not jet been clearly confirmed neither experimentally nor by numerical simulation.In the present paper a clear explanation supported by the numerical simulation and also by experimental measurements, is given.     

Numerical investigation of periodic cavitation shedding in a Venturi

Unsteady partial cavitation is mainly formed by an attached cavity which presents periodic oscillations. Under certain conditions, instabilities are characterized by the formation of vapour clouds convected downstream the cavity and collapsing in higher pressure region. Two main mechanisms have been identified for the break-off cycles. The development of a liquid re-entrant jet is the most common type of instabilities, but more recently, the role of pressure waves created by the cloud collapses has been highlighted. This paper presents one-fluid compressible simulations of a self-sustained oscillating cavitation pocket developing along a Venturi geometry. The mass transfer between phases is driven by a void ratio transport equation model. The importance of traveling pressure waves in the physical mechanism is put in evidence. Moreover, the importance of considering a non-equilibrium state for the vapour phase is exhibited.