高速射弹流体动力特性及空泡形态数值仿真

准确获得流体动力特性和空泡形态是研究水下高速射弹运动稳定性优化控制问题,由于攻角的影响,对空泡形态产生扰动,并影响航行器的稳定性。为研究攻角引起的流体动力特性和空泡形态变化强耦合性对射弹的同时作用,建立了射弹自然空化的三维仿真计算模型,在射弹可变攻角下的流体动力特性及空泡形态进行了数值仿真,得到了二者随攻角的变化规律。结果表明:在小攻角条件下射弹流体动力参数处于线性变化工作域,射弹临界攻角大于自力超空泡航行器,说明数字仿真方法正确,模型合理,为研究射弹弹道稳定性提供了理论基础。     

二维水翼超空化状态下流体动力特性数值仿真

基于均匀多相流假设,建立了二维水翼自然超空化流动的多相流CFD模型。运用Navier—Stokes方程加k-ε两方程湍流模型对NACA0012水翼的超空化流动进行了数值仿真。分别研究了固定空化数和固定攻角时,超空化状态下水翼的流体动力参数的特性和空泡形态的变化。仿真结果表明：水翼表面流体动力系数随来流攻角和空化数的增大而增大,压力中心沿弦向和展向的位置随着攻角变化,空泡尺寸随空化数的增大而减小,整个流场的流动可分为两个区域,前部对应附着空泡区,后部对应汽液两相流动区,并改进了性能,对速度梯度有较大的改善。     

航行器垂直入水空泡特性与流体动力研究

研究超空泡航行器垂直入水空泡形态与流体动力特性,针对空泡出现流体动力特性改变,为了确定入水参数,根据标准k-E模型,采用三维动网格技术,对超空泡航行器入水过程重力场下非定常空泡特性以及流体动力参数进行了数值仿真.得出结果,攻角越大,空泡形态不对称性越明显,迎流面空泡越短越薄,背流面空泡越长越厚,迎流面与背流面的空泡闭合时差越大,且流体动力系数越大.证明对同-入水深度时,从曲线上分析,阻力系数的斜率基本一致,升力系数与俯仰力矩系数的斜率随攻角的增大而增大.研究结果可为入水弹道的研究提供了流体动力参数.     

轴对称物体空化绕流的数值研究

空化绕流是水中普遍存在的现象,会对物体周围流场造成很大阻力,进而改变物体的水动力特性,必须加以优化研究.为此提出用多相流混合模型并耦合自然空化模型,对圆盘空化器的超空泡流和典型回转体的部分空泡流进行了数值仿真,并与边界元方法、分析解和实验数据进行了对比分析,利用软件FLUENT6.2进行仿真,结果表明,所建立的空化流计算模型是可行的,可为相关水动力研究提供参考和依据.     

基于SST湍流模型的空化流场的仿真分析

空化器是安装在超空泡航行器前端的关键部件,它的基本功能是诱导和促进航行器空泡的生成.为了使水中航行器系统的稳定性和快速性增强,根据k-ω的SST(剪切应力输运)湍流模型,对圆盘空化器的三维空化流动进行数值仿真,对空化流仿真计算的结果,得到了超空泡外形、阻力特性以及空泡最大直径和长度随空化数的变化规律,并与经验公式的计算结果进行对比.研究结果表明比经验公式计算结果具有良好的一致性,说明所采用的湍流模型和数值方法能够准确预报空化流问题,为研究空化流问题提供了一定的参考依据.     

轴对称弹体非定常空泡流数值仿真

在水下弹体空泡优化建模的研究中,由于弹体水下运动过程中生成的空泡形态及特性,直接影响弹体出水时的表面冲击载荷及空中弹道的稳定性,最终决定射弹打击目标的准确性.为研究弹体水下变减速运动过程中的空化现象,利用FLU-ENT的自然空化模型与动网格技术,对水下轴对称弹体的非定常空泡流进行模拟计算.结果显示,弹体在变减速运动过程中所形成空泡的主体尺寸变化规律与SCAV软件一致,非定常空泡的长度和最大直径均随着自然空化数的增加呈现先增后减的变化趋势;在初始速度一定情况下,质量越大的弹体运动速度衰减越慢,同一空化数下所形成空泡的尺寸越大.     

回转体自然空化阻力特性数值仿真

针对水下航行器壁面出现自然空化现象影响航行速度问题,为了研究由自然空化引起的局部空泡对水下航行器阻力系数的影响.根据标准k-ε双方程,采用混合模型对不同空化数下回转体自然空泡特性以及阻力系数在Fluent6.3软件上进行了数值仿真.结果表明,局部空化的出现增大了总阻力系数.由于存在一临界空化数,使得航行器表面出现自然空化,存在另一临界空化数,使得压差阻力系数达到最大,并且此空化数与总阻力系数达到最大时的空化数相差不大.求解结果证明,方法的正确性和有效性.     

底部结构对飞行器阻力特性的影响北大核心CSCD

为优化底部形状减小底部阻力,研究底部流场结构对超声速飞行器的阻力特性影响,采用计算流体动力学方法,建立了雷诺平均NS方程数值求解方法。通过与实验数据比较,验证了方法的可靠性,对含圆柱尾部、船尾的典型飞行器在不同马赫数和攻角下进行数值仿真。仿真结果表明:船尾设计具有减阻效果,且升阻比优于圆柱尾部。攻角变化对底部流场结构影响显著。飞行器的升力系数、阻力系数、升阻比及俯仰力矩系数,随攻角增加而增大。随马赫数增加,飞行器头部压力升高,底部压力降低,导致总阻力、底部阻力增大,然而,由于动压增大,使得相应的总阻力系数及底阻系数随之减小。计算分析为飞行器底部设计和优化提供了参考。     

基于流固耦合的叶片颤振分析

研究航空发动机性能问题,叶片颤振过程属于流固耦合问题,为了对叶片颤振进行预测和分析,保证发动机稳定,在提出对流固耦合原理和求解过程进行分析的基础上,采用 ANSYS 进行结构计算,CFX 进行流场计算,并利用两者间的数据交互平台传递流场压力载荷和结构位移数据,实现了流固耦合数值仿真计算.在不同来流速度及攻角下进行叶片和流场的耦合计算,根据得到的叶片振动位移响应判断叶片是否会发生颤振.计算结果表明,颤振频率与叶片低阶固有频率一致,证明来流速度和攻角是影响叶片气动弹性稳定性的重要因素,并可做为叶片颤振的预测依据.     

求解最小费用最大流问题的信念传播算法

最小费用最大流问题是一种组合优化问题,在经济、工业等领域具有重要研究意义和应用价值.针对部分最小费用最大流问题求解算法效率较低的情况,依据最小费用最大流问题的线性规划方程,将问题模型映射为对应因子图模型,改进描述函数,给出迭代方程,设计了求解最小费用最大流问题的信念传播算法.利用迭代方程优先对最大可行流特征值进行收敛计算,得到最大流,设置最大流阈值,在此基础上进行最小费用计算,从而求得问题最优解.最后选取若干带权有向图模型进行数值实验,验证了算法的可行性及有效性,且算法在求解效率上优于部分算法.     

超空泡射弹弹道特性影响因素仿真研究

建立并简化了水下超空泡射弹动力学和运动学模型,基于空泡模型假设和空泡截面独立扩张原理,确定了空化数与速度及深度的耦合计算关系和水下射弹尾翼穿刺前的临界空化数,进而对水下超空泡射弹弹道特性进行了仿真研究。仿真结果表明,水下射弹有效射程受自身质量和发射深度影响很大,有效射程与自身质量成线性增长关系,与发射深度成近似线性递减关系;在实际作战中,需考虑射弹发射深度对有效射程的影响,可以通过适当增加自身质量的方法,增大射弹有效射程。     

Numerical simulation of cavitating flows with homogeneous models

The simulation of cavitating flows is a challenging problem both in terms of modelling the physics and developing robust numerical methodologies. Such flows are characterized by important variations of the local Mach number and involve thermodynamic phase transition. To simulate these flows by applying homogeneous models, an appropriate equation of state (EOS) is necessary to cover all possible fluid states (pure liquid, two-phase mixture and pure vapour). Moreover, the numerical method has to handle any Mach number accurately. This paper presents a one-fluid compressible Reynolds-Averaged Navier–Stokes (RANS) solver with a preconditioning scheme. The cavitation phenomenon is modelled by two different liquid–vapour mixture EOS. The mathematical and thermodynamic properties are studied. Steady and unsteady numerical results are given for a Venturi geometry and comparisons are made with experimental data.     

An implicit low-diffusive HLL scheme with complete time linearization: Application to cavitating barotropic flows

A numerical method for generic barotropic flows is presented, together with its application to the simulation of cavitating flows. A homogeneous-flow cavitation model is indeed considered, which leads to a barotropic state equation. The continuity and momentum equations for compressible flows are discretized through a mixed finite-element/finite-volume approach, applicable to unstructured grids. P1 finite elements are used for the viscous terms, while finite volumes for the convective ones. The numerical fluxes are computed by shock-capturing schemes and ad-hoc preconditioning is used to avoid accuracy problems in the low-Mach regime. A HLL flux function for barotropic flows is proposed, in which an anti-diffusive term is introduced to counteract accuracy problems for contact discontinuities and viscous flows typical of this class of schemes, while maintaining its simplicity. Second-order accuracy in space is obtained through MUSCL reconstruction. Time advancing is carried out by an implicit linearized scheme. For this HLL-like flux function two different time linearizations are considered; in the first one the upwind part of the flux function is frozen in time, while in the second one its time variation is taken into account. The proposed numerical ingredients are validated through the simulations of different flow configurations, viz. the Blasius boundary layer, a Riemann problem, the quasi-1D cavitating flow in a nozzle and the flow around a hydrofoil mounted in a tunnel, both in cavitating and non-cavitating conditions. The Roe flux function is also considered for comparison. It is shown that the anti-diffusive term introduced in the HLL scheme is actually effective to obtain good accuracy (similar to the one of the Roe scheme) for viscous flows and contact discontinuities. Moreover, the more complete time linearization is a key ingredient to largely improve numerical stability and efficiency in cavitating conditions.     

均匀流场中四边形锥柱的压力试验研究

通过水池试验研究了在雷诺数Re为3.19×104~1.15×106,攻角为0°~180°,以15°为间隔,表面倾斜且前后不对称的四边形锥柱状结构在均匀流场中所受的压力载荷。分析了模型所受的周向压力和斯特劳哈尔数Sr随Re和攻角的变化趋势。结果表明:与圆柱绕流相比,倾斜壁面和攻角使压力在背压区和边界层分离点发生变化,同时,Sr也由0.2降到0.13。     

On predicting abrupt contraction flows with differential and algebraic viscoelastic models

The numerical simulation of flows through a planar contraction at low Reynolds number is considered for Newtonian and for viscoelastic fluids. The recently proposed algebraic extra-stress model (AESM) derived from the differential constitutive equation for an Oldroyd-B fluid is extended to a Phan-Thien–Tanner fluid. The approach is based on the exact polynomial representation using a three-term tensor basis. It is also shown that the algebraic formulation reproduces exactly the extra-stress tensor components for pure shear and for pure elongation flow. A parameter based on the strain rate and the rotation rate tensors is presented to identify the regions of the flow where the AESM model produces exact results. A second-order numerical scheme accurate in time and space, based on the finite volume method using a staggered grid has been applied to solve the conservation and constitutive equations for the Newtonian and viscoelastic flows. The numerical simulations for the viscoelastic fluids have been done using the classical constitutive equations in a differential form and the algebraic extra-stress model. Excellent agreement between the extra-stress values is obtained with the two different approaches, showing the viability of AESM.     

基质中农药回收率与超声空化效应的关系

通过超声动力学分析与实验相结合的研究方法,阐述了超声波促进农药溶解和引发农药分解两种效应的内在机理和相互制约关系。首先,基于Rayleigh-Plesset方程,利用数值迭代法(龙格-库塔算法)计算不同能量和频率组合下超声波的空化效应类型及强度;然后,进行纯溶剂中农药的分解率实验和菠菜基质中农药的回收率实验。研究结果表明:超声波的瞬态空化效应一方面通过破坏植物组织,促进被吸附的农药的释放,另一方面产生羟自由基(-OH)等游离子,导致农药被氧化分解。瞬态空化效应的强弱可通过单位时间内空化泡崩溃释放的压力值(Pmax/t)直观表示,当Pmax/t≤2.1×107atm/s时,溶解效应强于分解效应,农药回收率随Pmax/t增大而增大;当Pmax/t>2.1×107atm/s时,分解效应大于溶解效应,回收率降低;9种农药的平均回收率在Pmax/t=2.1×107atm/s时达最高值(此时超声波的频率和能量分别为45kHz和400W),可实现菠菜中多残留农药高溶解、低分解的超声萃取。     

Coupling of free surface and groundwater flows

In this paper we present some preliminary results about the coupling of shallow water equations for free surface flows and Darcy equation for groundwater flows. A suitable set of interface conditions is discussed: the Beavers and Joseph formula for the bottom stress is used. An iterative algorithm to solve the coupled problem is proposed and some numerical results are presented.     

Numerical flow-field analysis and design optimization of a high-energy first-stage centrifugal pump impeller

Described is the numerical flow-field analysis and design optimization of the first-stage impellers of a so-called high-energy centrifugal pump having two single-suction first-stage impellers and one double-suction second stage impeller. This study has been carried out with the aid of three-dimensional computational-fluid-dynamics calculations, employing the potential-flow approximation of the governing equations. The study was conducted because the first-stage impellers of the pump considered appeared to suffer from severe premature wear due to cavitation attack on the vane leading edges, which situation had to be improved. The analysis carried out for the existing design produced suggestions for improvement, and based on these suggestions a new first-stage impeller design was developed. Subsequently, this new design was numerically analyzed to substantiate its potentially better (cavitation) performance. It appeared that the blade inlet angle of the original impeller design was too excessive at mid span, causing best cavitation performance to occur at 160 percent of the rated flow. The new design has its best cavitation point at the rated flow, and will not suffer from premature wear due to cavitation attack like the existing design. Received: 1 March 1999 / Accepted: 21 September 1999     

激光等离子体空泡动力学特性研究

从理论模型、实验、数值模拟三方面综合研究了液体粘度和激光能量对激光空泡产生、成长、溃灭特性的影响。理论方面,建立了一个有所改进的空泡动力学模型;实验方面,利用两套基于光纤耦合光偏转法的实验装置,研究了空泡脉动、冲击波以及射流的细节过程;数值模拟方面,采用有限差分法进行数值计算,分析了所提出模型的优越性。研究结果可为水下激光加工、激光医疗、空化空蚀相关流体力学的研究提供一定的理论和实验支持。