水下航行体通气超空泡形态实验研究

在水洞中对航行体模型的通气超空泡形态及其影响因素进行了系列实验研究。在低速的情况下通过向空化器下游通入空气生成了超空泡。通过改变水洞速度、压力，通气参数，模型外形和状态产生了多种超空泡外形并研究了超空泡外形与空化器、空化数和通气参数之间的关系以及影响超空泡形状的因素，得出了有益的结论。对于下一步的研究工作具有指导意义，对于航行体超空泡外形控制技术的研究具有重要参考价值。     

超空泡形态及其流动特性的数值模拟

对圆盘空化器分别采用CFD、"1/3法则"和空泡截面独立扩展原理三种不同方法数值模拟了水下航行体定常自然超空泡外形及其流动特性。应用CFD方法基于粘性多相流的空泡捕捉法,采用六面体网格,选择Singhal空化模型和SST湍流模式,数值求解均质超空泡流场RANS方程。研究表明:泡形态时变特性是一种行之有效的工程估算方法,应用空泡截面独立扩展原理其计算结果与CFD方法吻合较好,说明了CFD方法用于超空泡流动仿真计算的可行性和独立性原理快速估算超空泡形态的准确性;同时超空泡外形主要与头部空化器有关,空泡长度会由于航行体本身存在变长;可优先选择空泡截面独立扩展原理对水下航行体超空泡外形进行快速估算。     

水下航行体通气超空泡非对称性研究

超空化水下高速航行体为了满足运动平衡性和稳定性要求,需要采用非对称超空泡流动模式。为了探索满足要求的非对称超空泡流型,在水洞中开展了水下航行体通气超空泡非对称性实验研究。采用通气的方法在较低水速(V=7～12m/s)下生成人工超空泡,通过分析和测量不同弗劳德数、模型攻角、空化器攻角与舵角等实验条件下的超空泡图像,获得了航行体超空泡非对称性与影响因素之间的定量和定性关系。研究表明,当组合参数F_r~2(1 σ)>50时,重力引起的超空泡变形量小于5‰;由模型攻角、空化器攻角和舵角产生的非对称超空泡流型可以满足超空化水下航行体的平衡性要求。     

水下亚声速细长锥型射弹超空泡流的数值计算方法

采用理想可压缩流体无旋定常流动及超空泡尾部Riabushinsky闭合方式假定,基于水动力学势流理论及细长体理论,建立了描述水下亚声速条件下细长锥型射弹超空泡流动的积分微分方程。发展了求解该方程的数值离散方法,提出多种超空泡外形初始解,分析了它们对超空泡形态计算结果的影响,优化了计算过程,简化了初始迭代条件。分析了流体压缩性对超空泡流动参数的影响,当马赫数大于0.3时,超空泡外形、射弹表面压力系数及射弹运动压差阻力系数均明显增大。计算得到的超空泡流动参数与相关文献的理论和实验结果吻合良好。     

小攻角下轴对称细长体的充气肩空泡试验研究

在减压水洞中进行的空泡试验研究受到各种条件因素的制约,充气空泡的研究是对减压水洞试验的一个很好补充。本文在小型重力式水洞中对轴对称细长体的充气空泡的特征、形态、底压力和水动力等作了尝试性研究。部分结果和自然空泡的测量结果及有关计算结果作了比较,得到了有益的结论。     

钝体倾斜和垂直冲击入水时引起的超空泡流动特性实验研究

对4种不同头型的钝体、以不同初始速度在小倾斜角度和垂直状态下入水,所产生的空泡流进行了的实验观察,分析了不同工况下空泡产生和发展的特性。实验结果表明:对于倾斜入水及垂直入水,圆台头和平头(即空化器均为圆盘)实验体均能形成较稳定的入水弹道;初始入水速度较低时,空泡的闭合方式为深闭合;初始入水速度较高时,空泡的闭合方式为表面闭合,且运动速度衰减得更快。测量得知,钝体倾斜入水产生的空泡的前部外形轮廓与Logvinovich的半经验公式给出的结果相吻合。在垂直入水的情况下,调查了物体头部对空泡的起始点位置及其形态的影响。     

通气法控制超空泡流动的实验研究

在高速水洞中运用人工通气方法进行了航行体模型超空泡形态特性的系列实验研究.获得了不同通气流量、不同空化数和不同弗鲁德数下的空泡形态,建立了以弗鲁德数为参数的空泡几何特征参数与空化数的对应关系、空化数与通气流量系数之间的对应关系.验证了空化数是决定空泡尺度的主要无因次参数,通过改变通气流量可以有效地调控空化数,进而达到控制空泡形态的目的.文中同时给出了通气流量系数的实验曲线拟合公式,并与国外的相关实验和公式进行了比较,两者基本一致.所得结论对进一步的水下超高速航行体空泡形态控制技术研究具有参考价值.     

水下亚声速细长锥型射弹超空泡形态的计算方法

采用理想可压缩流体无旋定常流动以及超空泡尾部Riabushinsky闭合方式假定,基于细长体理论和匹配渐近展开法,建立了描述水下亚声速条件下细长锥型射弹超空泡流动的积分微分方程。求解得到了考虑压缩性影响的超空泡形态1阶和2阶近似解,改进了超空泡形态的计算精度。分析了射弹高速冲击条件下流体压缩性对超空泡形态的影响,随着马赫数的增加,超空泡形态将发生更加显著的膨胀变化。计算得到的超空泡特征参数与相关文献的理论和实验结果吻合良好。     

阶梯式圆柱射弹小角度入水弹道特性研究

圆锥圆柱外形射弹小角度高速入水过程中,入水初期空泡呈不对称性发展.随着入水角度减小,入水空泡发展不对称性现象加剧,使得弹体受到阶跃性突变力矩作用,导致其姿态角发生大幅度变化,严重影响射弹入水弹道稳定性,甚至出现入水跳弹现象.为了改善高速射弹小入水角度入水过程弹道稳定性,基于“空化器空化效应”原理提出了一种阶梯式圆柱外形射弹设计方案.通过流体体积多相流模型和动网格技术,建立超空泡射弹小角度入水数值计算方法,并通过入水试验验证了数值方法的有效性.对阶梯圆柱外形射弹与圆锥圆柱外形射弹以5°入水角的入水过程进行了数值模拟研究,得到了不同射弹外形空泡演化特性对水动力特性及弹道稳定性的影响.结果表明:阶梯圆柱外形能够加快初生空泡的发展并伴随多空泡融合现象,在0°攻角条件下,当空泡充分发展后,空泡尺寸未发生改变,在小攻角(5°)工况下,空泡对弹体的包覆面积增大,改善了射弹的升力性能;在小角度入水过程中射弹锥段空泡发展形态对入水稳定性具有重要影响,阶梯圆柱外形能够有效加快入水空泡的发展,进而形成有效抑制攻角持续增大的恢复力矩,提升了高速射弹小角度入水初期弹道稳定性.     

小玫角下轴对称细长体的充气肩空泡试验研究

在减压水洞中进行控泡试验研究受到各种条件因素的制约,充气空泡的研究是对减压水洞试验的一个很好补偿,本文在小型重力式水洞中对轴我体的充气空泡的特征、形态、底压力和水动力等作了尝试性研究。部分结果和自然空泡的测量结果及有关计算结果作了比较,得到了有益的结论。     

Calculation of aerodynamic characteristics of a wing profile with discharge of a controllable jet into the external flow

A wing profile of infinite span, whose lower surface is replaced by a system of guide vanes, is placed in a flow of an ideal incompressible fluid. Fluid flows out through the system of guide vanes from the internal cavity of the wing into the external stream, forming a jet in the wake (Fig. 1). The total pressure in the wing cavity and in the jet differs from the total pressure in the outer free stream. The jet boundaries are streamlines extending to infinity, along which there is a discontinuity of the velocity value. The flow of fluid in the internal wing cavity is simulated by a flow caused by a system of suitably located sources, and the system of guide vanes is replaced by discrete vortices.The form of the profile arc is selected so that the fluid flow from the sources in the direction which is nearly opposite the direction of the freestream velocity is restrained by the segment of the contour with high curvature in the vicinity of the leading edge. We consider the flow regime about the profile with an exhausting jet for which the two ends of the arc the points of detachment of the stream and the velocity discontinuity line (profile arc, jet boundary) is a smooth curve, which imposes an additional condition on the magnitude of the circulation. As the model for the study of the flow about a profile with jet blowing we take the arc of a logarithmic spiral.Formulas are obtained for determining the over-all characteristics of the stream forces acting on the profile in the presence of the jet and the total pressure discontinuity. On the basis of the calculations made for a thin wing a qualitative analysis is made for the stream force acting on the profile.The authors wish to thank S. A. Khristianovich for formulating the problem and for his advice.     

Low-Dimensional Modeling of a Driven Cavity Flow with Two Free Parameters

By applying Proper Orthogonal Decomposition (POD) one is able to extract a limited amount of data which characterizes a flow of interest. The modes resulting from the decomposition form a basis in the phase space on which a Galerkin projection of the equations of motion can be performed. By carrying out such a procedure one obtains a low-dimensional model consisting of a reduced set of Ordinary Differential Equations (ODEs) which models the original equations. A technique called Sequential Proper Orthogonal Decomposition (SPOD) is developed to perform decompositions suitable for low-dimensional models. SPOD is capable of transforming data organized in different sets separately while still producing orthogonal modes. A low-dimensional model is constructed and used for analyzing bifurcations occurring in the flow in the lid-driven cavity with a rotating rod. The model allows one of the free parameters to appear in the inhomogeneous boundary conditions without the addition of any constraints. This is necessary because both the driving lid and the rotating rod are controlled simultaneously. Apparently, the results reported for this model are the first to be obtained for a low-dimensional model based on projections on POD modes for more than one free parameter. Received 29 October 2001 and accepted 27 August 2002 Published online 13 January 2003 Communicated by P. Hall     

Velocity profile measurement by ultrasonic doppler method

The ultrasonic velocity profile measuring method has been developed at PSI for application in fluid mechanics and fluid flow measurement. It uses pulsed ultrasonic echography together with the detection of the instantaneous Doppler shift frequency. This method has the following advantages over the conventional techniques: (1) an efficient flow mapping process, (2) applicability to opaque liquids, and (3) a record of the spatiotemporal velocity field. After a brief introduction of its principle, the characteristics and specifications of the present system are given. Then examples in fluid engineering for oscillating pipe flow, T-branching flow of mercury, and recirculating flow in a square cavity are described that confirm the method's advantages. Several other works under way by other investigators are introduced. A potential for in-depth study of fluid dynamics is demonstrated by several examples from an investigation of modulated wavy flows in a rotating Couette system. The position-averaged power spectrum and the time-averaged energy spectral density were used to study the dynamic characteristics of the flow, and subsequently the velocity field was decomposed into its intrinsic wave structure based on two-dimensional Fourier analysis.     

Identification of Material Damage Model Parameters: an Inverse Approach Using Digital Image Processing

A reliable prediction of ductile failure in metals is still a wide-open matter of research. Several models are available in the literature, ranging from empirical criteria, porosity-based models and continuum damage mechanics (CDM). One major issue is the accurate identification of parameters which describe material behavior. For some damage models, parameter identification is more or less straightforward, being possible to perform experiments for their evaluation. For the others, direct calibration from laboratory tests is not possible, so that the approach of inverse methods is required for a proper identification. In material model calibration, the inverse approach consists in a non-linear iterative fitting of a parameter-dependent load–displacement curve (coming from a FEM simulation) on the experimental specimen response. The test is usually a tensile test on a round-notched cylindrical bar. The present paper shows a novel inverse procedure aimed to estimate the material parameters of the Gurson–Tvergaard–Needleman (GTN) porosity-based plastic damage model by means of experimental data collected using image analysis. The use of digital image processing allows to substitute the load–displacement curve with other global quantities resulting from the measuring of specimen profile during loading. The advantage of this analysis is that more data are available for calibration thus allowing a greater level of confidence and accuracy in model parameter evaluation.     

Forced convective flow and heat transfer over a?porous plate in a?Darcy-Forchheimer porous medium in presence of?radiation

A mathematical model is presented for analyzing the boundary layer forced convective flow and heat transfer of an incompressible fluid past a plate embedded in a Darcy-Forchheimer porous medium. Thermal radiation term is considered in the energy equation. The similarity solutions for the problem are obtained and the reduced nonlinear ordinary differential equations are solved numerically. It is noticed that the boundary layer decreases with an increase in the value of inertial parameter and in this case the temperature profile is found to decrease smoothly within the boundary layer. In case of porous plate, fluid velocity increases whereas non-dimensional temperature decreases for increasing values of suction parameter. The rate of heat transfer increases with the increasing values of Prandtl number. The effect of thermal radiation on temperature field is also analyzed.     

An improved three-dimensional coupled fluid–structure model for Coriolis flowmeters

The paper presents a coupled numerical model built to simulate the operation of Coriolis flowmeters, which exploit the alteration of the vibration mode shape of the measuring tube for the mass flow rate measurement. The explained measuring effect is a consequence of the interaction between the motion of the tube, vibrating at its natural frequency, and the fluid flow in it. The numerical model is realized by coupling of a finite volume (FV) code for fluid flow analysis with a finite element (FE) code for structural analysis using the conventional staggered solution procedure, with added inner iterations to achieve strong coupling. The simulation algorithm is divided into two steps. A free vibration of the measuring tube considered in the first step is complemented in the second step, after the numerical free vibration response is properly stabilized, with the harmonic excitation force actuating the measuring tube at its resonant frequency of several hundreds of Hertz to resemble the operation of actual Coriolis flowmeters. Different scenarios using zero-order or three-point fluid load predictor and soft application of the fluid load in the initial stages of the simulation are compared to yield a simulation strategy, which will minimize the time needed to obtain the stabilized steady-state response of the vibrating measuring tube. The proposed simulation procedure was applied on a straight-tube Coriolis flowmeter and used for the estimation of the velocity profile effect. The results exhibit sufficient stability (low scatter) to be used for the estimation of sensitivity variations of order of magnitude around tenths of a percent.     

Dynamics of pipes conveying fluid with non-uniform turbulent and laminar velocity profiles

Previous analytical work on stability of fluid-conveying pipes assumed a uniform velocity profile for the conveyed fluid. In real fluid flows, the presence of viscosity leads to a sheared region near the wall. Earlier studies correctly note that viscous forces do not affect the dynamics of the system since these forces are balanced by pressure drop in the conveyed fluid. Although viscous shear has not been ignored in these studies, a uniform velocity profile assumes that the sheared region is infinitely thin. Prior analysis was extended to account for a fully developed non-uniform profile such as would be encountered in real fluid flows. A modified, highly tractable equation of motion was derived, which includes a single additional parameter to account for the true momentum of the fluid. This empirical parameter was determined by numerical analysis over the Reynolds number range of interest. The stability of cantilever pipes conveying fluid with two types of non-uniform velocity profile was assessed. In the first case, the profile was a function of Reynolds number and transition to turbulence occurred before the onset of flutter instability. This case had stability properties similar to the uniform velocity case except in specific narrow regions of the parameter space. The second case required that the Reynolds number be such that the flow was always laminar. For this case, lower fluid velocity was required to achieve instability, and the oscillation frequency at instability was considerably lower over much of the parameter space, compared to the uniform case.     

主导曲线法测定J阻力曲线的研究

本文利用主导曲线法测定了国产核容器用钢材料Ａ５０８－３的Ｊ阻力曲线，并将实验结果与卸载柔度法作了对比，进而说明这种新方法有其广阔的应用前景；同时，本实验引入了计算机数据实时采集及自动处理系统．这些系统的引入大大提高了实验精度和效率．