FLOW PAST A 2-D BACKWARD-FACING STEP WITH AN OSCILLATING WALL

The spatial-temporal movements of the reattachment (x_r) and separation (x_s) points in a 2-D backward-facing step flow with a bottom wall oscillated sinusoidally at selected frequencies and amplitudes were characterized using multiple hot-film sensor arrays and smoke flow visualization methods. The results also show that both x_rand x_smoved toward (away from) the step during upward (downward) motion of the wall, and that their rate of variation or the covered distance is a strong function of the wall oscillation. The present measurements would provide a practical means for the study of unsteady separated flows, as well as for the validation of CFD modelling.     

UNSTEADY DETACHED SEPARATION FROM A CIRCULAR CYLINDER PERFORMING ROTATIONAL OSCILLATIONS IN A UNIFORM VISCOUS INCOMPRESSIBLE FLOW

The origination of detached separation is studied on the basis of a numerical solution of the full Navier–Stokes equations. Fluxes of vorticity with different signs generated with twice the frequency of cylinder oscillation move from the cylinder to the outer surface of a detached liquid layer in the form of concentric rings. Near the critical layer between the attached layer and the main flow these rings are torn and crimped to the regions of separated vortices of the corresponding sign. The form of detached separated vortices is similar to that of vortices originating from a stationary circular cylinder in a uniform flow. Transition of the flow to a non-symmetric form with Karman vortex street generation at a Reynolds number (based on the radius) greater than 17 is revealed. This critical Reynolds number is smaller than that for a stationary circular cylinder in a viscous stream (where Re=20 has been determined to be a critical value) and corresponds to the Reynolds number extrapolated from the critical value for the stationary cylinder by increasing the cylinder radius by the attached layer thickness. The vorticity flux from the cylinder surface immediately into the separation region decreases as the frequency of cylinder oscillation increases. Violation of the flow potentiality in the detached separation region is the main cause of the vorticity generation on the outer surface of the attached liquid layer. © 1997 John Wiley & Sons, Ltd.     

振动翼型和襟翼的绕流特性研究

用数值模拟手段详细地研究了振动翼型和襟翼的绕流问题,数值模拟的出发方程为Euler和N-S方程,格式为Bcam-Warming格式的改进型。数值实验主要针对流场的二大特性进行的,即振动对激波的影响和振动对分离的抑制作用,结果表明:(1)随翼型或襟翼的振动激波强度和位置也相应地变化但这一变化滞后于攻角的变化;(2)振幅加大激波强度的变化和激波运动范围也加大;(3)振动频率越高对激波的影响反而较低频时要小;(4)流动条件的不同可使升力回线的走向发生变化;(5)振动对分离有明显的抑制作用。     

数值模拟孤立波通过水下孤立方柱的粘性流动

本文用完整二维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程和ＶＯＦ方法，研究孤立波通过淹没水下孤立直立方柱水域时的波形变化和粘性流场运动。本文对孤立波通过水下 孤立直方柱的情形进行了实例计算。给出了波形随时间的演化图，可以看到反射波、前传波和跟随的振荡型小波列的生成及涡流场的运动演化，并与势流计算结果进行了比较。     

CALCULATION OF STEADY AND OSCILLATING FLOWS IN TUBES USING A VORTICITY TRANSPORT ALGORITHM

Steady and oscillating axisymmetric tube flows are modelled using a vorticity transport algorithm. The axisymmetric convective –diffusive Navier–Stokes equations are solved using a splitting technique. Axisymmetric ring vortex filaments are introduced on the walls and subsequently convected and diffused throughout the flow field. An axisymmetric equation similar to the Oseen diffusion equation is used to diffuse the ring vortex filaments. Vorticity is reflected from the tube walls using two techniques. Results are presented for the developing Poiseuille flow and for the developed flow in the form of the entrance length and the axial velocity and vorticity profiles. Good agreement is achieved with a finite difference method in the developing region of Poiseuille flow. The developed flow results are compared with the analytical solutions. The developed profiles of velocity and vorticity have errors of less than 0ċ3 per cent for both methods of dealing with reflection of diffusion at the bounding surfaces and similar accuracy is obtained for the velocity profiles in oscillating flow except at the wall. Oscillating flow is produced with a discretized sinusoidal piston motion. Velocity profiles, boundary layer thickness and entrance length are presented for oscillating flow. Good agreement is achieved for low-Womersley-number non-dimensional frequency. At higher values of this parameter, flows are inaccurately simulated, because the number of piston positions used to discretize the piston motion is inversely proportional to the non-dimensional frequency.     

A NEW OPTIMAL SHAPE DESIGN PROCEDURE FOR INVISCID AND VISCOUS TURBULENT FLOWS

A new approach for optimal shape design is introduced. The main ingredients are an unstructured CAD-free framework for geometry deformation and automatic differentiation (AD) in reverse mode. Transonic inviscid and viscous turbulent flows are investigated. Both two- and three- dimensional configurations are considered. These cases involve up to several thousand control parameters. © 1997 John Wiley & Sons, Ltd.     

COMPUTATION OF COBBLESTONE EFFECT WITH UNSTEADY VISCOUS FLOW UNDER A STERN SEAL BAG OF A SES

The concept of a surface effect ship (SES) is to lift the hull partly by the air cushion enclosed within two side hulls, a bow skirt and a stern seal. Consequently, it results in lower draft, resistance and motions than equivalent length catamarans in most sea states. In very low sea states, however, there is a significant design problem, which is high vertical accelerations, referred to as the cobblestone effect. The oscillations are based on resonance phenomena and are caused by the change of the cushion volume due to the incident waves. The resonance oscillations have an important damping mechanism which is derived from the air leakage flow under the stern seal bag of a SES. Hence, the accurate prediction of the leakage flow is required for the estimation of the cobblestone effect. In order to solve the unsteady flow field under the stern seal bag, a viscous flow code for numerically simulating two-dimensional incompressible flows has been developed. The governing equations to be solved are the time-dependent Navier–Stokes equations, using the artificial compressibility approach. The spatial discretization is based on a cell-centred finite volume formulation. The inviscid fluxes are evaluated by Roe's scheme with the third-order-accurate MUSCL approach. Time integration is conducted by the second-order accurate backward Euler formula and the linear equation system is solved by an approximate Newton relaxation scheme with the symmetric Gauss–Seidel iteration approach. For the resulting time integration to be conservative on a moving grid system, a geometric conservation law is introduced. A numerical procedure is presented and contributions of the viscous effects to the cobblestone effect problem are discussed.     

旋转振动圆柱绕流周期解和Floquet稳定性

对低雷诺数旋转振动圆柱绕流问题运用低维Galerkin方法将N－S方程约化为一组非线性常微分方程组。运用打靶法数值求解了这组方程的周期解,并用Tloquet理论对周期解的稳定性进行了分析,确定了流动失稳的机制。     

分层流体中内孤立波在台阶上的反射和透射

基于匹配渐近展开和格林函数的方法,研究了两层流体系统中内孤立波在台阶地形上透射、 反射及其分裂的演化特征. 通过保角变换和求解奇异Fredholm积分方程,获得了反映地形 效应对Boussinesq方程影响的约化边界条件,藉此建立了KdV演化方程的``初值'问题, 根据散射反演理论获得了反射波和透射波的解析表达式. 分析结果表明：上下流体层的厚度 比、密度比以及台阶高度对于反射和透射波振幅及其分裂具有显著的影响. 尤其当上层流体 厚度小于下层厚度时,由于存在临界点,在其附近反射波的幅值随台阶高度的演化由单调增 变为单调减,透射波的幅值由单调减变为单调增;上台阶的反射波与入射波反相,其最大幅 值可达到入射波的数倍;此外,下台阶反射波也可发展为单支孤立波,它区别于单层流体中 反射波仅为衰减的振荡波列.     

非牛顿幂律流体绕可渗透平板非定常流动的相似解

文（１）研究了平板表面吸吮速度按照某一规律变化时，非牛顿幂律流体绕可渗透平板非定常流动的运动方程的相似解。本文补充研究吸吮速度按照其它一些规律变化时该问题的准确的或近似的相似解。     

AN ARTIFICIAL DISSIPATION FORMULATION FOR A SEMI-IMPLICIT,PRESSURE BASED SOLUTION SCHEME FOR VISCOUS AND INVISCID FLOWS

The formulation of artificial dissipation terms for a semi-implicit, pressure based flow solver, similar to SIMPLE type formulations, is presented and is applied to both the Euler and the Navier-Stokes equations. The formulation uses generalized coordinates and a non-staggered grid. This formulation is compared to some SIMPLE and time marching formulations. The relationship between SIMPLE and time marching formulations is discussed briefly. The artificial dissipation inherent in some commonly used semi-implicit formulations, e.g. upwind differencing, powerlaw, QUICK and pressure weighting, is investigated. The scheme used here includes these dissipation terms directly, but retains the ability to mimic previous schemes. The potential for errors introduced by the simultaneous use of artificial dissipation in the continuity equation and central differencing of convective terms, is revealed. The effect of the amount of dissipation on the accuracy of the solution and the convergence rate is quantitatively demonstrated for two-dimensional inviscid flow in a mildly curved duct, three-dimensional laminar flow in a square cross section elbow with strong secondary flows, and two-dimensional turbulent flow through a turbine nozzle. The spurious effects of artificial dissipation, particularly second order dissipation, inherent in some commonly used algorithms, is clearly shown. The effect of artificial dissipation on the convergence rate is also demonstrated. The main conclusion drawn from the results is that the minimum amount of artificial dissipation that gives the required accuracy, but also an adequate convergence rate for a particular case, has to be used. This amount of dissipation is case dependent. The direct inclusion of artificial dissipation terms provides control over the amount of dissipation used.     

DYNAMICS OF A SUBMERGED AND INCLINED CONCENTRIC PIPE SYSTEM WITH INTERNAL AND EXTERNAL FLOWS

In this paper, we formulate a mathematical model to study the dynamics of submerged and inclined concentric pipes with different lengths. The governing equations of motion for the inner pipe are derived under small deformation assumptions and with the consideration of gravitational forces, turbulent boundary layer thickness of external flow, fluid frictional forces, and inertia effects. We obtain discretized dynamical equations using spatial finite-difference schemes and calculate the resonant frequencies of a particular pipe system design. In addition, by varying the operating conditions, we identify a few critical parameters pertaining to the proper design of such pipe systems.     

NUMERICAL SIMULATION OF LINEAR AND NONLINEAR DISTURBANCE EVOLUTION IN A BOUNDARY LAYER WITH COMPLIANT WALLS

A computational method capable of simulating the spatial evolution of disturbances in a boundary-layer flow with compliant coatings has been developed. The flow geometry being an unknown of the problem, this difficulty is overcome by use of a mapping, the domain being fixed in the computational coordinates. The model takes into account the nonlinear fluid-structure interaction all over the flow field, as well as nonparallel effects due to the wall displacement and to the boundary-layer growth. First, the numerical solution procedure is tested by focusing on the linear and nonlinear spatial disturbance evolution for a spring-backed elastic plate which is only unstable with respect to Tollmien–Schlichting-type travelling waves. The numerical procedure is then used to study the influence of the initial disturbance amplitude on the disturbance development for a tensioned membrane. Finally, to simulate a true physical experiment, a spring-backed elastic plate of finite length is considered. It is shown that the numerical model is capable of detecting the interaction between Tollmien–Schlichting instabilities and flow-induced surface instabilities at the interface.     

一类高效率高分辨率加映射的WENO格式及其在复杂流动问题数值模拟中的应用

由于映射操作会带来额外的计算时间消耗,传统加映射的WENO格式存在计算效率低的缺陷.为了提高传统加映射WENO格式的计算效率,通过利用标准符号函数的一种近似逼近函数构造出一族近似常值映射函数,本文提出了一种新的加映射WENO格式,称为WENO-ACM.新映射函数满足文献中已有WENOPM6格式映射函数的全部设计要求,其中WENO-PM6是一种为了克服经典WENO-M格式潜在的精度丢失缺陷而提出的格式.新格式保留了WENO-PM6在低耗散和高分辨率方面的优势,同时,显著的减少了每个时间步映射过程中的数学运算操作数,进而在计算效率方面获得了明显的提升.理论分析表明,新格式在即使包含临界点的光滑区域也能够获得最佳收敛精度.对近似色散关系的研究表明,新格式的频谱特性也得到了显著的提升.对大量标准测试算例进行了模拟计算,包括精度测试、激波管问题、激波-熵波相互作用、爆炸波相互碰撞、二维黎曼问题、双马赫反射、前台阶流动、瑞利-泰勒不稳定性和开尔文-亥姆霍兹不稳定性问题等.与广泛认可的WENO-JS, WENO-M, WENO-PM6格式综合比较发现,新提出的WENO-ACM格式在高效率、低数值耗散...     

任意多孔多裂纹有限大板的应力强度因子分析

采用各向异性体平面弹性理论中的复势方法,以Faber级数为工具,应用保角映射技术和最小二乘边界配点法,导出内边界条件精确满足,外边界条件近似满足的含多椭圆孔及裂纹群有限大板在任意载荷作用下的应力场、位移场的级数解,建立了任意多椭圆孔及裂纹群有限大板应力强度因子的有效分析方法,讨论了各参数对裂尖应力强度因子及孔边应力集中的影响．数值结果表明,该方法具有计算精度高、收敛速度快、方便快捷等优点,有利于全面系统地研究各参数对结构断裂性能的影响．     

ELAFINT: A MIXED EULERIAN–LAGRANGIAN METHOD FOR FLUID FLOWS WITH COMPLEX AND MOVING BOUNDARIES

In this work a mixed Eulerian–Lagrangian technique is devised, hereinafter abbreviated as ELAFINT (Eulerian–Lagrangian Algorithm For INterface Tracking). The method is capable of handling fluid flows in the presence of both irregularly shaped solid boundaries and moving/free phase boundaries. The position and shape of the boundary are tracked explicitly by the Lagrangian translation of marker particles. The field equations are solved on an underlying fixed grid as in Eulerian methods. The interface passes through the grid lay-out and details regarding the treatment of the cut cells so formed are provided. The issues involved in treating the internal boundaries are dealt with, with particular attention to conservation and consistency in the vicinity of the interface. The method is tested by comparing with solutions from well-tested body-fitted co-ordinate methods. Test cases pertaining to forced and natural convection in irregular geometries and moving phase boundaries with melt convection are presented. The capability developed here can be beneficial in solving difficult flow problems involving moving and geometrically complex boundaries.     

非稳态非线性油膜力作用下柔性轴裂纹转子的动力特性分析

分析了在动载轴承非稳态非线性油膜力作用下,具有横向裂纹柔性轴Jeffcott转子在非线性涡动影响下的动力特性.通过数值计算表明,在油膜失稳转速前,随着裂纹轴刚度变化比的增大,系统在低转速区域内具有丰富的非线性动力行为,出现倍周期分叉及混沌现象,涡动振幅随转速升高而减小,直到非稳态非线性油膜失稳.在无裂纹转子油膜临界失稳点处发现了类Hopf分叉现象,系统运动由平衡变为拟周期运动;裂纹转子在油膜临界失稳时的系统运动亦为拟周期运动.裂纹转子轴刚度变化对油膜失稳点及油膜失稳之后转子的运动影响不大,转子系统作拟周期运动.     

多工况下连续体结构的多刚度拓扑优化设计和二重敏度过滤技术

针对多工况载荷条件下连续体结构拓扑优化设计的研究现状,以各载荷下对应结构的最小柔度作为目标函数,基于带权重的折衷规划法和SIMP密度函数插值模型,提出并建立了多载荷条件下线弹性连续体结构拓扑优化设计的数学模型.对优化过程中出现的棋盘格式和网格依赖性等数值计算问题进行了研究,在此基础上提出一种二重敏度过滤技术,该方法能有效消除优化结构中的棋盘格式,并使优化结构体现出较好的网格无关性.通过几个典型的算例证明了该文方法的有效性.     

STABILITY AND TRANSITION TO TURBULENCE IN BOUNDARY LAYERS WITH A PRESSURE GRADIENT OVER A COMPLIANT MONOLITHIC COATING

Journal of Applied Mechanics and Technical Physics - This paper describes the study of a linear formulation of a problem of stability of boundary layers with a pressure gradient, which are...     

A NECESSARY AND SUFFICIENT CONDITION FOR THEOSCILLATION OF SOLUTIONS OF LIENARD TYPE SYSTEM WITH MULTIPLE SINGULAR POINTS

I.IntroductionJ.R.Graer']andG.VillariI2]gavenecessar\JandsufficientconditionsfortheoscillationofsolutionofLienardsystemf~])~F(x),y~~glXJ(l.l)HanMaoanI3]gaveoscillatirytheoremsforLienardt}'pesystemt'.ithasingularpointi~h(.t/)~F(x),#~~g(x)(1.2)Whereas,aloto…