横向喷流对低速大攻角细长旋成体非对称气动特性影响研究

采用风洞试验和数值模拟相结合的方法,对雷诺数Re=55 000条件下细长旋成体有、无横向喷流时大攻角非对称特性进行了分析.通过风洞试验发现了旋成体在法向和侧向进行喷流时其大攻角非对称气动特性与无喷流时的区别,通过数值模拟方法对几个典型工况下旋成体有、无横向喷流时的非对称气动特性进行了分析,揭示了喷流对旋成体非对称流动分离的影响.通过风洞试验发现当细长旋成体进行法向控制时无喷流、喷流位于迎风区和喷流位于背风区的旋成体表现出了不同的非对称流动特性:首先喷流位于迎风区时攻角范围在20°～40°之间有喷流和无喷流旋成体所产生的侧向力方向相反,攻角大于40°之后侧向力系数的方向发生了改变,与无喷流时的侧向力系数方向相同,但是其绝对值要比无喷流时的侧向力系数小.其次喷流位于背风区时攻角在15°～35°之间有喷流时的侧向力系数绝对值要明显比无喷流时大,在随后的40°～70°之间旋成体侧向力系数变化规律与无喷流的趋势相似.当细长旋成体进行侧向控制时由于沿侧向的喷流所产生的直接力使得攻角范围在0°～20°之间和大于45°时有喷流的旋成体侧向力系数绝对值要比无喷流时大,但是攻角在25°～40°之间时旋成体...     

大迎角细长体绕流背涡结构与气动特性分析

结合实验成果，对大迎角细长体绕流结构及其气动特征进行了分析。利用势流理论，探讨了大迎角下非对称侧向力沿物体轴线的变化特征与分离背涡结构之间的关系，得出截面侧向力系数沿轴向呈现正弦型曲线的变化特征是由轴向旋涡不断脱落所形成的多涡系统诱导的结果。对于三涡以上的多涡系统中，对称的物面压强分布并非一定对应对称的涡结构。     

矢量喷流对细长体大迎角非对称流动影响研究

采用测压方法研究了矢量喷流对细长旋成体大迎角非对称流动的影响特性.实验结果表明:矢量喷流对细长旋成体大迎角非对称侧向力有明显的抑制作用,该抑制作用是通过喷流诱导作用,改变其空间绕流涡系结构的分布来实现的,但是矢量喷流的存在并不能改变大迎角机身空间绕流涡系的本质结构;随着迎角的增大,矢量喷流对细长旋成体大迎角非对称流动的影响区域不断前移,甚至影响到头部;随着喷流落压比的增加,矢量喷流对细长旋成体大迎角非对称侧向力的抑制作用加强,但当喷流落压比达到临界落压比后(即喷管出口处达到设计马赫数时),喷流影响作用将不会随喷流落压比的增加而改变.     

涡流发生器对细长体侧向力的控制研究

细长体在大攻角情况下会出现很大的侧向力. 在10°半顶角细长旋成体头部安置涡流发生器（微型三角翼）,通过调节涡流发生器相对模型轴线的滚转安装角,实现了对大迎角状态下旋成体侧向力的近似比例控制.研究了涡流发生器半展长、后掠角和攻角等参数对侧向力控制效果的影响. 研究发现半展长为6mm、后掠角为45°的涡流发生器具有较好的控制效果,在一系列攻角下均能够实现侧向力的近似比例控制. 由于该机构非常简单,在工程中具有一定的应用前景.      

90°弯管内流动的理论模型及流动特性的数值研究

从三维不可压缩雷诺时均Navier-Stokes方程出发,对90°弯曲管道内湍流流动进行数值模拟。网格划分采用六面体网格,湍流模型为RNGk-ε模型,在近壁区采用两层壁面模型进行修正,流场的计算结果与实验数据吻合较好。在此基础上,本文数值研究了来流方向对流场结构和流动特性的影响。得出在弯管流场中发生了分离现象,且随着来流侧滑角的增大,分离区范围增大。此外,随着来流从同一侧滑角变换至同一攻角时,横截面的二次流图像中也从具有两个对称主涡变成只具有一个主涡的现象。     

Reynolds数对大攻角细长旋成体绕流滚转角效应影响的数值研究

使用低耗散的Roe格式,数值模拟了Reynolds数(Re)对大攻角细长旋成体绕流滚转角效应的影响.模型头部加了几何小扰动块以引发流场的不对称.在较大的Re数(Re=10 5)下,本文的计算结果与实验是相符的,此时细长体的滚转会导致双稳态、双周期现象,即侧向力随滚转角呈现类似方波形式的双周期变化,方波中侧向力基本保持不变的状态对应于流场的正则态,且两个正则态的侧向力方向相反,方波中侧向力基本保持不变的状态对应于流场的正则态,且两个正则态的侧向力方向相反;而在较小的Re数(Re=4 000)下,如果扰动足够大,细长体的滚转将导致不同的双稳态现象,此时两个正则态的侧向力方向相同,而在较小扰动下双稳态现象不再出现;Re数更小时(Re=1 000),即使在较大的扰动下,双稳态现象也不再出现,侧向力随滚动角仍是连续变化的.本文的计算结果表明,Re数越小,流场对头部扰动的感受性越弱.     

细长体大迎角非对称涡流的数值研究

通过数值方法对大迎角细长体低速湍流流场的模拟,探讨头部顶端极小扰动对细长体非对称绕流形成与发展的影响.结果表明在细长体顶端附近施加极小扰动可以模拟出实验观测到的非对称流场,非对称的涡系结构沿轴向是逐步发展的,截面侧向力沿轴向的分布呈现正弦型曲线的变化特征,扰动能量经过指数增长后达到饱和,有效扰动的规模影响涡流非对称性的大小及分布,单侧扰动产生的流场非对称性随扰动周向位置的变化呈现单周期性规律.小扰动诱发非对称的数值算例表明非对称绕流的形成是源于流场的空间不稳定性机制.     

高机动飞行器非指令运动及其控制的研究进展

高机动飞行器往往都是通过大攻角飞行来实现高机动科目的, 在发展高机动飞行器的过程中, 其非指令运动是伴随着大攻角飞行而常常出现的运动形态. 为此, 应在飞行器设计的早期阶段, 充分研究所设计布局的大攻角流动性态及其相应的非指令运动的形态;揭示这类运动形态的主控流动;在此基础上形成和发展流动控制新技术, 以达到抑制非指令运动的目的. 由于大攻角前体非对称涡往往与非指令运动密切相关, 为此本文首先指出前体非对称涡流动对头部微扰动十分敏感, 以致长期以来让人们误认为这类流动具有不确定性. 研究表明, 通过设置人工微扰动可使前体非对称涡流动具有可重复性, 并揭示该流动随扰动周向角变化的响应、演化规律. 通过利用大、小后掠翼两类翼身组合体的典型布局形式, 研究它们所呈现的摇滚运动形态, 揭示其摇滚运动的不同主控流动机理, 在此基础上分别发展了抑制小、大后掠翼身组合体摇滚运动的流动控制技术: 快速旋转头部扰动和适当设置扰动位使翼、身的两对非对称涡处于反相. 在抑制非指令运动的研究中, 深入理解和揭示头部微扰动对非对称涡流动的响应、演化机理是至关重要的, 应予以特别关注.     

EARSM和DRSM在亚音速旋涡流动中的应用

基于非结构网格,采用带曲率修正的显式代数应力非缌出漠型（EARSM）和微分雷诺应力模型（DRSM）,建立了用于模拟大攻角旋涡流动的计算方法。分别以尖前缘和钝前缘的65°三角翼为例,验证了EARSM模型和DRSM模型在两种典型亚音速计算状态下对复杂涡系干扰的模拟能力和旋涡的产生、发展、破裂过程。分别利用SA、BSL两种线性湍流模型对相关问题进行了计算;通过对多种计算的流场与气动力详细结果的比较分析,就几种湍流模型对大攻角复杂旋涡流动的预测能力和敏感性等进行了评估。结果表明：EARSM和DRSM能较好地预测旋涡流动的发展破裂,强于经典线性湍流模型;对边界层发展状况预测较差,导致旋涡启动位置较早。基于算例计算,对两种先进湍流模型的改进提出了适当的建议。所得结论将可为进一步开展大攻角旋涡流动模拟方法的研究提供参考。     

飞机大迎角非对称涡组合扰动主动控制研究

采用测压、测力以及流动显示方法研究了头部微三角扰动块对飞机大迎角非对称背涡的主控作用和背风侧单孔位微吹气对背涡空间位置及相应侧向力的控制作用,以此为基础提出了基于微三角块扰动和单孔位微吹气扰动的组合扰动主动控制新技术,并在某飞机模型上进行了验证.实验是在北京航空航天大学D4风洞中进行的.研究结果表明:该组合扰动控制技术能够实现对飞机大迎角非对称侧向力的有效主动控制.     

Experimental investigation of side force control on cone-cylinder slender bodies with flexible micro balloon actuators

Side forces on slender bodies of revolution at medium to high angles of attack (AOA > 30°) has been known from a large number of investigations. Asymmetric vortex pairs over a slender body are believed to be the principle cause of the side forces. Under some flight conditions, this side force may be as large as the normal force acting on the slender body. In this paper, experimental results are presented for side force control on a cone-cylinder slender body by using microfabricated balloon actuators. The micro balloon actuators are made of polydimethylsiloxane (PDMS) elastomer by using micro molding techniques. They can be packaged on curve surfaces of a cone-cylinder slender body. As actuator is actuated, the micro balloon actuator inflates about 1.2 mm vertically, which is about 2.4% of the cylinder diameter D (=50 mm) of the cone-cylinder slender body. Micro balloon actuators are actuated at different roll angles of a cone-cylinder slender body. Aerodynamic force measurement results indicate the effects of micro balloon actuators vary at different actuation locations on the cone-cylinder slender body. The side forces can be significantly reduced if the actuators are actuated in the weak vortex side (the side corresponding to the asymmetric vortex which is far from the surface) and actuation angles are located at about 50–60° (the actuation angle here is measured from stagnation line of the incidence plane toward weak vortex side direction). Significant changes are noticed from the surface pressure, as well as leeside vortex flow field, measurement. Micro balloon actuators change nose shapes of the slender body which decide adverse-pressure-gradient values and directly influence the origin of the separation lines and characteristics of the separated vortices over the leeside surface.     

Structure and evolution of the airflow generated by a slender body entry near a tube

When a slender body moving forward in open air enters into a confined region, two important unsteady aerodynamic phenomena are generated. An exiting flow is created with a direction opposite to the body movement and inside the confined region, a compression wave is formed. Generation mechanism of compression wave have been extensively studied but so far, no detailed investigation of the exiting flow has ever been reported. The experimental study presented in this paper was undertaken to gain insight into the structure and the evolution of the exit-flow. Experiments were conducted with an axisymmetric apparatus and the explored range of the moving body speed was 5–50 m/s. The study focused on the influence of the body speed and the body nose geometry on the flow. It was shown that the air ejected from the tube entrance generates an annulus jet accompanied by a vortex ring. The vortex development was clarified using laser sheet visualizations associated with unsteady pressure and velocity measurements at the tube entrance. It is constituted by four phases, the pre-vortex phase, the vortex development phase, the vortex convection phase and the vortex breakdown phase. The duration of each of these steps was found to be independent of both the studied parameters in a non-dimensional time scale. Furthermore, neither the body speed nor the nose geometry induced significant changes on the vortex ring evolution, except for extreme conditions (low body speed, V_M.B.<15 m/s, and/or very long nose geometry, L_nose/D_M.B.>6). The evolution of the vortex ring was compared to that of ‘classical’ vortex ring generated at a tube exit by a piston motion with large non-dimensional stroke length. Main similarities and differences were discussed in the paper. In particular, the formation number of vortex ring observed in our experiments was found to be significantly smaller.     

Obstacle-induced spiral vortex breakdown

An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-stream velocity, and the adverse pressure gradient. They were controlled through the incidence angle of the elliptical hydrofoil, the free-stream velocity and the sphere diameter. A single helical breakdown of the vortex was systematically observed over a wide range of experimental parameters. The helical breakdown coiled around the sphere in the direction opposite to the vortex but rotated along the vortex direction. We have observed that the location of vortex breakdown moved upstream as the swirl number or the sphere diameter was increased. LDV measurements were corrected using a reconstruction procedure taking into account the so-called vortex wandering and the size of the LDV measurement volume. This allows us to investigate the spatio-temporal linear stability properties of the flow and demonstrate that the flow transition from columnar to single helical shape is due to a transition from convective to absolute instability.     

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.     

细长体水下运动空化流场及弹道特性实验

为了研究细长体水下高速运动时空泡的产生、闭合及脱落特性,及影响细长体空泡形态及弹道特性的复杂因素等,初步开展了细长体模型水下高速运动的实验研究,分析了不同初始空化数下细长体模型在水中高速运动的一系列流动现象,重点研究了空泡的发展、闭合、尾部回射流和尾部脱落特性,以及轴对称细长体模型弹道特性与空泡形态变化之间的关系和转动特性随时间的变化历程等。结果表明:细长体水下高速运动时形成超空泡,空泡头部光滑透明,尾部凝结有汽水混合物且有交替脱落的含气漩涡;初始空化数对细长体的速度衰减有所影响;受初始扰动影响,细长体水下运动伴随有绕头部的转动且初始扰动影响细长体俯仰角随时间的变化历程。     

Study of the flow around surface-mounted pyramids

The turbulent flow around square-based wall-mounted pyramids in thin and thick boundary layers was experimentally investigated as a function of the pyramid apex angle, ‘, and angle of attack, f, based on mean flow surface patterns, pressure and velocity measurements. For thin boundary layers, wake periodicity is observed. For slender pyramids (15°<‘<75°), the periodic formation and shedding of vortices is observed. The shedding frequency scales with the length scale L=w'(w'/h)^-1/4, where h is the pyramid height and w' is the frontal (projected) base width. For broad pyramids, wake periodicity exists but cannot be related to vortex shedding. Vortex shedding appears related to the existence of a double vortex structure along the side faces of the slender pyramids. The location of the separation point upstream and the attachment point downstream of the obstacle also scales with L. For thick boundary layers, no wake periodicity is observed and the mean flow structure in the wake differs from that seen for thin boundary layers. The location of the separation and attachment points scales only approximately with L.     

Vortex shedding from a square cylinder in presence of a moving wall

A numerical study on the flow past a square cylinder placed parallel to a wall, which is moving at the speed of the far field has been made. Flow has been investigated in the laminar Reynolds number (based on the cylinder length) range. We have studied the flow field for different values of the cylinder to wall separation length. The governing unsteady Navier–Stokes equations are discretized through the finite volume method on a staggered grid system. A SIMPLE type of algorithm has been used to compute the discretized equations iteratively. A shear layer of negative vortex generates along the surface of the wall, which influences the vortex shedding behind the cylinder. The flow‐field is distinct from the flow in presence of a stationary wall. An alternate vortex shedding occurs for all values of gap height in the unsteady regime of the flow. The strong positive vortex pushes the negative vortex upwards in the wake. The gap flow in the undersurface of the cylinder is strong and the velocity profile overshoots. The cylinder experiences a downward force for certain values of the Reynolds number and gap height. The drag and lift are higher at lower values of the Reynolds number. Copyright © 2005 John Wiley & Sons, Ltd.