基于面元法回转体定长局部空泡的绕流计算

基于面元法 ,通过在回转体和空泡壁面放置源汇 ,对回转体定长局部空泡的绕流问题进行计算和分析 ,并讨论了空泡尾部速度过渡闭合模型对绕流计算的影响。计算结果表明 :本文的方法具有快速收敛的特征 ,第 1次叠代和最终收敛时空泡壁面切向速度的误差不超过 5 % ;随着回转体面元总数N的增加 ,局部空泡的空泡数趋于稳定 ;当回转体线型一定时 ,空泡数将随着局部空泡长度的增大而减小     

空泡流非稳态现象的流动控制

处于跨临界阶段的空泡流必然导致强烈的周期性冲击和振动,空泡流的激振来源于空泡云的周期性大规模脱落,空泡云的形成和发展与流动的边界层效应有着强烈的相关性,且空泡末端的局部流动直接影响空泡流的整体稳定性,本试验在NACA16012水翼表面粘附一条展向1mm厚10mm宽的挡流条,尝试以干扰水翼上表面局部流动的方法来影响整个空泡流的形态及其流动稳定性,最终在一定的空泡数范围内抑制了空泡流激振现象,并从试验研究的角度探索了空泡云脱落的机理。     

空泡流粘性效应研究

我们用激光多普勒技术(LDV)对钝头圆柱体、带小槽1.5度卵形头回转体、带背阶半球头回转体的绕流进行了测量。试验在600×600mm~2水洞中进行,雷诺数从0.75×10~5到6×10~5。给出了平均速度和脉动速度均方根值的分布并做了脉动速度的功率谱。得出了与过去热线流速计测量结果略有不同的结论。并且通过小槽内流动的测量对空泡的产生进行了分析。此外,本文提出一种捡测流动转捩现象的方法,用此法的实验结果与前人一致。     

计算回转体粘性绕流的流线迭代法

本文提出了一种计算回转体定常.不可压缩粘性绕流问题的新数值方法。通过一组坐标变换把径向和轴向都延伸到无限的外流场变换到有限区域的内流场,从而把计算内流场的粘性流动的流线迭代法推广应用到计算回转体粘性绕流流动。用本方法理论预测的轴对称物体表面压力分布与在不同试验条件下(风洞和水池)、几何形状差别较大的两条回转体得到的试验数据相比较,一致性好。     

两空泡运动特性研究

本文应用边界方法研究了两个相邻空泡的运动特性,得到了空泡的演化规律,以及空泡溃灭时的射流速度与溃灭时间的变化趋势,对于两个空泡之间的距离和半径比的影响进行了讨论。计算结果表明：不同大小的空泡在一起时则小泡会先溃灭,且人泡的存在时间与两泡的半径比成正比;大泡对小泡来说其作用相当于－固壁面,小泡会形成－指向大泡的溃灭射流。相同大小的空泡在一起溃灭时,会同时形成指向中间的射 流,与单空泡在固壁面附近的溃     

回转体头部通气入水流场演化与载荷特性数值预报研究

为探究周向通气对回转体入水表面载荷的影响，基于VOF(volume of fluid)模型和Realizable k-ε两层湍流模型，开展了周向通气回转体低速入水流场演化数值预报和表面载荷特性分析。通过将数值预报的空泡形态与试验结果相对比，验证了所采用的数值方法的有效性，并分析了不同通气率对空泡形态、流场特性和表面载荷特性的影响。结果表明，通气会改变回转体入水空泡演化过程以及侧壁表面压力，在通气作用下空泡第一次脱落时间延缓，并且通气气体流向空化器后方负压区，改善了空化器后方的负压情况；其次，通气气体在通气口附近形成了明显的涡结构，之后与壁面处由空化器形成的涡融合，增强了空泡中部的涡流强度；最后，通气率越大，空泡闭合时间越晚，空泡体积越大，尾部空泡越不容易发生脱落，同时通气会减缓回转体表面的压力波动，通气率越大压力波动越小。综合分析可以认为，侧向通气对于回转体低速入水流场及表面载荷特性有一定的改善作用。     

翼型空泡周期性流动的数值模拟及机理分析

采用基于正压关系的均质平衡流空化模型和低雷诺数修正的k-ε湍流模式,自行开发了空泡流数值模拟方法和计算软件,对绕翼型空泡的周期性流动现象进行了数值模拟.计算结果与实验数据的对比表明,空泡的宏观特征、流动特性、周期性脱落的斯坦顿数St等与试验结果接近,验证了计算结果的可靠性.空泡在大约一个周期的2/3时间段内成长,并在大约1/3周期时刻发生断裂脱落.这两个特征时间与高速摄像实验结果一致.所取工况对应的组合参数σ/2α=2.865,以翼弦长计算可得St=0.217,与文献的最新试验结果吻合.空泡周期性运动过程中升阻系数也周期性振荡,时均值C<,1>=0.41,C<,d>=0.097,振荡频率与空泡脱落频率一致.对空泡运动过程中流场结构的变化进行了分析,结果表明在大攻角条件下,空泡闭合区后的逆压梯度导致涡的形成及回射流的发展,沿壁面逆向流动的混合介质射流是引起空泡断裂的原因,回射流发展、涡结构变化与空泡非稳态演化过程存在密切的联系,探讨了翼型空泡发生周期性脱落的一些机理.     

Fluent环境中近壁面微空泡溃灭的仿真计算

基于FLUENT软件环境,采用VOF模型和非稳态方法求解Navier-Stokes方程,模拟了近壁面的空泡溃灭过程,同时计算了空泡溃灭处与壁面的距离对射流强度的影响.结果表明：在近壁面,空泡将形成非对称溃灭,因水锤作用,引发高速水射流在壁面产生高压而形成空蚀破坏;基于FLUENT环境的计算结果与已有的实验和计算结果相符,为研究空泡溃灭和空蚀机制提供了类比的数值计算方法.     

回转体高速倾斜入水的流场特性及结构响应

回转体高速入水过程涉及液体和固体的耦合作用，是一个复杂的非线性、非定常过程。为研究回转体高速入水的结构动响应及流场演变规律，本文中基于STAR-CCM+和ABAQUS平台，建立了回转体高速入水的双向流固耦合数值模型，开展了不同入水速度的回转体高速倾斜入水流固耦合数值计算。结果表明:数值计算的入水速度、位移曲线和空泡形态与实验结果良好吻合，验证了流固耦合方法的有效性；回转体倾斜高速入水的载荷先集中在触水部分边缘处，后集中于回转体底部中心处；流固耦合方法的入水冲击载荷峰值小于刚体的，弹性回转体的载荷曲线产生明显波动；撞水阶段，回转体空泡呈现不对称形态，随着入水加深，空泡不对称性变弱；入水速度60 m/s下，空泡发生表面闭合，回转体入水初速度越快，空泡表面闭合越晚；冲击载荷与入水速度有关，入水速度越大，峰值出现越早，震荡越明显，速度超过100 m/s时，回转体产生塑性形变。     

空泡绕流

物体在水中高速运动,使物体表面甚至尾流区内的局部压力降低到饱和蒸汽压力时,液体便发生汽化,形成空穴,即所谓空泡。亦有通过人工充气的办法来形成空泡。如空泡区结束于物体表面的区域之内,称为局部空泡;空泡区延伸至物体以外的下游,即称为超空泡。空泡绕流问题研究的主要目标是,在给定条件下分析物体所受的流体动力,物体附近的流场及空泡形状。这三者又以分析流体动力为首要任务。当前研究的主要是具有实际意义的具体对象如超空泡或局部空泡水翼、超空泡螺旋桨、对称物体的阻力及低阻力剖面等。空泡绕流的理论研究,可以说在50年代初期到60年代中期曾经是个高潮,有了很大的发展。但至今仍然存在着困难和一些问题。主要困难在于:(1)由于空      

A numerical method for simulation of attached cavitation flows

A new numerical algorithm for attached cavitation flows is developed. A cavitation model is implemented in a viscous Navier–Stokes solver. The liquid–vapour interface is assumed as a free surface boundary of the computation domain. Its shape is determined with an iterative procedure to match the cavity surface to a constant pressure boundary. The pressure distribution, as well as its gradient along the wall, is taken into account in updating the cavity shape iteratively. A series of computations are performed for the cavitating flows across three kinds of headform/cylinder bodies: conic, ogival and hemispheric heads. A range of cavitation numbers is investigated for each headform/cylinder body. The obtained results are reasonable and the iterative procedure of cavity shape updating is quite stable. The superiority of the developed cavitation model and algorithm is demonstrated. Copyright © 2006 John Wiley & Sons, Ltd.     

ON THE PARTIALLY CAVITATING FLOW AROUND TWO-DIMENSIONAL HYDROFOILS

The steady partially cavitating flow around two-dimensional hydrofoils was simulated numerically by the low-order potential-based boundary integration method. The cavity shape and length are determined for given cavitating numbers in the course of iteration by satisfying the kinematic and dynamic boundary conditions. The re-entrant jet model and the pressure-recovery close model are adopted to replace the high turbulent and two-phase wake forming behind the cavity. The results are compared with the other published numerical ones.     

On the partially cavitating flow around two-dimensional hyprofoils

The steady partially cavitating flow around two-dimensional hydrofoils was simulated numerically by the low-order potential-based boundary integration method. The cavity shape and length are determined for given cavitating numbers in the course of iteration by satisfying the kinematic and dynamic boundary conditions. The re-entrant jet model and the pressure-recovery close model are adopted to replace the high turbulent and two-phase wake forming behind the cavity. The results are compared with the other published numerical ones. Paper from LU Chuang-jing, Member of Editorial Committee, AMM Biographies: CCHENG Xiao-jun (1993-) LU Chuang-jing (1946-)     

Axially symmetric cavitation flow at small cavitation numbers

A method for computing the drag coefficient of a body in an axially symmetric, steady-state cavitation flow is presented. A ‘vortex ring’ distribution along the wetted body surface and along the cavity interface is assumed. Since the location of the cavitation interface is unknown a priori, an iterative procedure is used, where, for the first stage, an arbitrary cavitation interface is assumed. The flow field is then solved, and by an iterative process the location of the cavitation interface is corrected. Even though the flow field is governed by the linear Laplace equation, strong non-linearity resulting from the kinematic boundary conditions appears along the cavitation interface. An improved numerical scheme for solving the dual Fredholm integral equations is obtained by formulating high-order approximations to the singular integrals in order to reduce the matrix dimensions. Good agreement is found between the numerical results of the present work, experimental results and other solutions.     

开放空腔壳体倾斜入水运动特性试验研究

基于高速摄像试验方法,研究了开放空腔壳体的倾斜入水运动特性,重点分析了开放空腔结构引起的空泡流动特征和壳体运动规律. 通过试验数据分析了开放空腔内气体运动将引起独特的空泡流动和阶段性的运动规律,探讨了初始入水速度、入水姿态对入水弹道和空泡形态等运动特征的影响. 结果表明:开放空腔壳体入水空泡出现阶段波动演化现象,并先后经历两次闭合;入水空泡演化改变流体动力分布,直接影响壳体运动方式,进而改变水下弹道特征;空腔内部形成相对独立流场环境和开放端周期性流动,在重力作用下液体对空腔内下侧壁面作用力较大,加剧壳体偏转,从而改变入水运动过程的稳定性;随着入水速度的增大,空泡波动特征逐渐明显,闭合时间延迟,非对称深闭合引起的横向位移减小,但偏转角度与入水速度无关;随着初始姿态倾角减小,空泡波动程度减弱、闭合时间延迟,偏转角速度增大,闭合引起的横向位移增大.      

环形槽对通气空泡融合的促进作用分析

航行体出水过程中肩部的自然空化将影响航行体的载荷分布及出水姿态,工程上常采用肩部开孔通气的方式改善航行体表面的力学环境, 进而解决此类问题.本文针对水下航行体通气空泡周向融合效果不理想问题, 基于有限体积法,采用VOF (volume of fluid)多相流模型和动态铺层的动网格技术,数值研究了排气孔下游增加小尺度环形凹槽促进空泡融合的作用机制,以及不同发展阶段、不同工况下环形凹槽对空泡融合的促进作用. 结果表明,凹槽可以有效改善通气空泡的融合效果.运动坐标系下来流经过环形凹槽时因为流动膨胀发生了边界层的流动分离,其诱导产生的卷吸作用一方面迟滞了空泡的轴向发展,促进空泡产生周向尺寸更大的剪切涡并沿周向膨胀发展,另一方面将部分通气气体吸入环形凹槽,吸入槽内的气体通过挤压破碎实现了周向融合,且槽内融合气泡溢出下泄促进融合边界上移. 此外,凹槽可以通过改善空泡内部的流动状态,使空泡的形态和内部压力在不同工况下都更加稳定.      

Analysis of the nonstationary spatial propagation of elastic waves from cavities subject to mechanical and thermal loads

A numerical analytic method is proposed to solve nonstationary coupled problems of thermoelasticity with regard to the finite velocity of thermal waves. The method is used to analyze the nonstationary spatial propagation of elastic waves from a cavity subjected on its surface to mechanical and thermal loads. The ray theory of propagation of wavefield discontinuities is used. To determine the time dependence of the field parameters behind the wavefront and to account for the relationship between the mechanical and thermal fields with prescribed accuracy, a numerical iterative procedure that employs the properties of characteristics is used. Plots are presented for the nonstationary stresses and temperature near a prolate spheroidal cavity subject to step mechanical loading and near an elliptical cylindrical cavity subject to thermal shock __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 8, pp. 79–88, August 2006.     

Perturbation solution to 3-D nonlinear supercavitating flow problems

A 3-D nonlinear problem of supercavitating flow past an axisymmetric body at a small angle of attack is investigated by means of the perturbation method and Fourier-cosine-expansion method. The first three order perturbation equations are derived in detail and solved numerically using the boundary integral equation method and iterative techniques. Computational results of the hydrodynamic characteristics and cavity shapes of each order are presented for nonaxisymmetric supercavitating flow past cones with various apex-angles at different cavitation numbers. The numerical results are found in good agreement with experimental data. The project supported by the National Natural Science Foundation of China     

Impact interaction of cylindrical body with a surface of cavity during supercavitation motion in compressible fluid

The paper deals with the early stage of impact of a solid cylindrical body on the surface of a cylindrical cavity for zero and non-zero gap between the cavity surface and the body surface. As a result, the stated mixed non-stationary boundary value problem with the unknown variable in the time boundary is formulated. Its solution is reduced to a joint solution of an infinite system of linear integral Volterra equations of the second kind and the differential equation of the body movement. In the case of simplified formulation, the solution is reduced to the infinite sequence of the linear integral Volterra equations. Hydrodynamic and kinematic characteristics are also obtained.