均匀来流条件下并行排列旗帜耦合运动模式的实验

利用高速摄影技术在低速风洞中记录了不同间距并行排列的两个旗帜在不同来流速度中的耦合运动。利用自编的时间-空间演化图像处理软件分析总结了旗帜的耦合运动模式以及旗帜摆动振幅、频率和St数的变化规律。实验结果显示,随着排列间距和来流速度的变化,两旗帜可能以静止、同向摆动、反向摆动和过渡状态这四种不同的模态耦合运动。两旗帜同向摆动时摆动频率明显低于单个旗帜在相同来流中的值,反向摆动时情况相反。在过渡状态中两旗帜摆动的振幅交替增减并且运动中同时包含有两个频率,而同向和反向摆动都是单频率的运动。     

高升阻比乘波构型优化设计

在M∞ =6, 30km高空条件下，以升阻比为目标函数，进行了锥形流乘波体的黏性优化设计，讨论 了影响乘波体升阻比的因素，并对优化结果进行了数值验证. 结果表明：对于升阻比最大的 黏性优化乘波体，存在最优圆锥角使得源自该基本流场的乘波体升阻比最大；摩阻和波阻处 于同一量级；体积率、细长比和展长比都随着基本流场圆锥角的增大而增大.     

不同交通流状态下桥梁气动导数的风洞试验研究

为了考虑实际运营车辆对桥梁气动导数的影响,根据车辆密度模拟了三种交通流状态,基于强迫振动装置,分别对每个交通流和无车状态下的桥梁气动导数进行风洞试验研究,讨论了不同攻角下不同车流的车辆对桥梁气动导数的影响,探究了车辆对气动导数影响的百分比以及气动导数变化量的变化规律。研究结果表明:不同攻角下不同车流的车辆均对直接导数A*2、H*4和交叉导数A*4、H*2影响显著,A*2、A*3变化量随着折减风速有一定的变化规律。虽然不同攻角下不同车流的车辆对气动导数的影响程度及影响规律不同,并且车流的繁忙程度对大多数气动导数的影响规律不明显,但是车辆对桥梁气动导数的影响不容忽略。     

EXPERIMENTS ON EVOLUTION CHARACTERISTICS FOR THE MUSHROOM-LIKE VORTEX STRUCTURE GENERATED BY A SUBMERGED LAMINAR ROUND JET

利用溢流恒压装置产生具有稳定出流速度的层流圆管潜射流,结合染色液流态显示方法,在多种射流时间和雷诺数组合下,实验研究了该射流动量在密度均匀黏性流体中的演化机理及其表现特征,定量分析了蘑菇型涡结构的无量纲射流长度L^*、螺旋型涡环半径R^*及其包络外形长度d^*等几何特征参数随无量纲时间t^*的变化规律。系列实验结果表明,蘑菇型涡结构的形成与演化过程可分为3个不同的阶段,分别为启动阶段、发展阶段和衰退阶段。在启动阶段,L^*和d^*随t^*线性变化,而R^*则近似为一个常数。在发展阶段,蘑菇型涡结构的演化具有自相似性,在各种射流时间和雷诺数组合下,L^*,R^*和d^*与t^*1/2均为同一正比关系,而且实验结果与基于斯托克斯（Stokes）近似的理论解结果一致。在衰退阶段,蘑菇型涡结构会发生两类形式的演化,第1类衰退出现在射流结束之后,其间L^*和R^*与t^*1/5相关,而d^*近似为一个常数;第2类衰退出现在射流结束之前,当射流动量达到某个临界值后,蘑菇型涡结构就会发生破碎等现象。     

双模态发动机的模态鉴别方法

双模态冲压发动机的不同燃烧模态具有不同的稳焰机制和流态特征,并且在模态转换时伴随着显著的推力变化. 因此,准确判断燃烧模态,对于捕捉发动机的燃烧区位置/范围、释热分布特征,以及为进一步优化燃烧室的设计(流道结构和供油布局) 具有重要意义. 目前尚无鉴别模态的有效试验方法,本文提出了一种模态鉴别的试验方法,并在超燃直连台上开展验证试验. 试验中使用的测量技术包括:壁面静压、高速阴影/纹影、多通道可调谐二极管吸收光谱和高能态碳氢自由基CH* 自发光成像. 利用多种测量方法的组合,可以同时获得燃烧室中气流静温、速度、马赫数分布,释热分布以及燃烧区位置/范围. 这些试验数据能够用于判别模态,并获得不同模态的流动和火焰特征.      

超声速边界层/混合层组合流动的稳定性分析

利用可压缩线性稳定性理论研究了超声速混合层考虑壁面影响流动时的失稳特性. 基本流场选取了具有不同速度特征的2 股均匀来流,进入存在上下壁面的流道中. 混合层与边界层的距离为1~3 个边界层厚度,其中壁面取为绝热壁. 分析了该流动在超声速情况下的稳定性特征,同时还讨论了不同波角下的三维扰动波的演化特点,并与二维扰动波进行了比较和分析. 研究结果表明,在此流动情况下,边界层流动和混合层流动的稳定性特征同时存在,并互有影响,其流动稳定性特征既有别于单纯的平板边界层,也有别于单纯的平面混合层,呈现出了新的稳定性特征.      

点载荷作用下密集颗粒物质的传力特性分析

利用颗粒离散元商业软件PFC3D, 模拟了在2m*1m*0.01m容器中直径分别为0.01m, 0.008m和0.006m的颗粒各1*10$^4$个, 受重力作用下的静态密集堆积; 以此为初始条件, 在表层随机选择7个颗粒分别施加5.2*10$^{ - 2}$N(100倍最大颗粒重量)的点载荷, 进行应力传播特点研究. 结果表明: 力的传递在局部范围内呈现很强的各向异性; 应力涨落随着距离的增加呈指数下降; 在大于5倍最大颗粒粒径时, 其分布可以使用弹性力学理论来计算. 探讨了摩擦系数$\mu =0$, 0.2, 1对应力传递的影响, 随着摩擦系数的增加, 各向异性范围减小.     

超声速边界层/混合层组合流动的稳定性分析简

利用可压缩线性稳定性理论研究了超声速混合层考虑壁面影响流动时的失稳特性. 基本流场选取了具有不同速度特征的2 股均匀来流,进入存在上下壁面的流道中. 混合层与边界层的距离为1~3 个边界层厚度,其中壁面取为绝热壁. 分析了该流动在超声速情况下的稳定性特征,同时还讨论了不同波角下的三维扰动波的演化特点,并与二维扰动波进行了比较和分析. 研究结果表明,在此流动情况下,边界层流动和混合层流动的稳定性特征同时存在,并互有影响,其流动稳定性特征既有别于单纯的平板边界层,也有别于单纯的平面混合层,呈现出了新的稳定性特征.     

含氢原子缺陷晶界的剪切行为

针对4个α-Fe对称倾斜晶界,采用分子静力学考察了4个晶界中H原子偏析能的分布特征,并采用分子动力学方法研究了晶界内植入不同数量H原子对其在室温条件下剪切行为的影响.H原子通过随机方式植入界面内,利用植入H原子数量与晶界面积的比值来定义H原子面密度ρ.在含H原子晶界剪切行为分析过程中,重点考察了在不同H原子密度ρ下,4个晶界的初始塑性临界应力和晶界迁移位移的变化趋势以及4个晶界在加载过程中的微观变形机理.研究表明:晶界内的H原子偏析能明显偏低,4个晶界附近的H原子会自发向晶界内偏析;随着植入H原子数量的逐渐增多,晶界的初始塑性临界应力和后续变形阶段应力均会降低.晶界内植入H原子会从本质上改变晶界的微观变形机理,进而影响晶界在外载荷条件下的迁移属性.与不含H原子晶界的变形机理对比发现,加载过程中晶界的微结构会发生剧烈的演化,H原子的扩散和团簇化效应会导致晶界内出现纳米孔缺陷.     

宏观交通流模型的余维2分岔分析

交通流特性是混合交通流建模的一个重要因素. 交通流模型中的分岔现象是导致复杂交通现象的因素之一. 交通流的分岔, 涉及复杂的动力学特征且研究较少. 因此, 提出了一个最优速度模型来研究驾驶员记忆对驾驶行为的影响. 基于带有记忆的最优速度连续交通流模型, 利用非线性动力学, 分析和预测了复杂交通现象. 推导了鞍结 (LP) 分岔存在条件, 并通过数值计算得到了余维1 Hopf (H) 分岔、LP分岔和同宿轨 (HC) 分岔以及余维2广义Hopf (GH) 分岔、尖点 (CP) 分岔和Bogdanov-Takens (BT) 分岔等多种分岔结构. 根据双参数分岔区域的特点, 研究了记忆参数对单参数分岔结构的影响, 分析了不同分岔结构对交通流的影响, 并用相平面描述了平衡点附近轨迹的变化特征. 选择Hopf分岔和鞍结分岔作为密度演化的起点, 描述了均匀流、稳定和不稳定的拥挤流以及走走停停现象. 结果表明, 驾驶员记忆对交通流的稳定性有重要影响; 动力学行为很好地解释了交通拥堵现象; 考虑余维2分岔的影响, 能更好地理解交通拥堵产生的根源, 并为制定有效抑制拥堵的方法提供一定的理论依据.      

Vortex shedding in cylinder flow of shear-thinning fluids: I. Identification and demarcation of flow regimes

An experimental study on the flow of non-Newtonian fluids around a cylinder was undertaken to identify and delimit the various shedding flow regimes as a function of adequate non-dimensional numbers. The measurements of vortex shedding frequency and formation length (l_f) were carried out by laser-Doppler anemometry in Newtonian fluids and in aqueous polymer solutions of CMC and tylose. These were shear thinning and elastic at weight concentrations ranging from 0.1 to 0.6%. The 10 and 20 mm diameter cylinders (D) used in the experiments had aspect ratios of 12 and 6 and blockage ratios of 5 and 10%, respectively. The Reynolds number (Re^*) was based on a characteristic shear rate of U_∞/(2D) and ranged from 50 to 9×10³ thus encompassing the laminar shedding, the transition and shear-layer transition regimes. Increasing fluid elasticity reduced the various critical Reynolds numbers (Re_etr^*, Re_lf^*, Re_bbp^*) and narrowed the extent of the transition regime. For the 0.6% tylose solution the transition regime was even suppressed. On the other end, pseudoplasticity was found to be indirectly responsible for the observed reduction in Re_otr^*: it increases the Strouhal number which in turn increases the vortex filaments, precursors of the transition regime. Elasticity was better quantified by the elasticity number Re′/We than by the Weissenberg number. This elasticity number involves the calculation of the viscosity at a high characteristic shear rate, typical of the boundary layer, rather than at the average value (U_∞/(2D)) used for the Reynolds number, Re^*.     

Bifurcation and chaos of a flag in an inviscid flow

A two-dimensional model is developed to study the flutter instability of a flag immersed in an inviscid flow. Two dimensionless parameters governing the system are the structure-to-fluid mass ratio M^⁎ and the dimensionless incoming flow velocity U^⁎. A transition from a static steady state to a chaotic state is investigated at a fixed M^⁎=1 with increasing U^⁎. Five single-frequency periodic flapping states are identified along the route, including four symmetrical oscillation states and one asymmetrical oscillation state. For the symmetrical states, the oscillation frequency increases with the increase of U^⁎, and the drag force on the flag changes linearly with the Strouhal number. Chaotic states are observed when U^⁎ is relatively large. Three chaotic windows are observed along the route. In addition, the system transitions from one periodic state to another through either period-doubling bifurcations or quasi-periodic bifurcations, and it transitions from a periodic state to a chaotic state through quasi-periodic bifurcations.     

Effect of the melting system sizes on the temperature and concentration combined melting of a vertical ice plate in a rectangular cavity

This paper is concerned with the melting of a vertical ice plate into a calcium chloride aqueous solution inside a rectangular cavity. The initial temperature of the ice plate and the mixture are both −5°C and the initial concentration of the mixture is 20 wt%. The effect of the liquid height H, the width W, the aspect ratio of the liquid region A (=H/W) and the initial ice plate thickness δ _i on the transient melting mass per unit melting front area, M, is mainly considered numerically. M keeps a similar value in spite of H varied for A = 1 at early melting stage, however, becomes considerably influenced by H as melting progresses. The ice plate melts influenced by A for H = 20 mm fixed at early melting stage due to the fast development of the stagnant region and M decreases with increasing A (=1∼ 10). A dimensionless correlation of the transient melting mass, the aspect ratio and the melting time was presented under the restricted condition of H = 20 mm.     

Effects of aspect ratio on the drag of a wall-mounted finite-length cylinder in subcritical and critical regimes

The effect of cylinder aspect ratio (??H/d, where H is the cylinder height or length, and d is the cylinder diameter) on the drag of a wall-mounted finite-length circular cylinder in both subcritical and critical regimes is experimentally investigated. Two cases are considered: a smooth cylinder submerged in a turbulent boundary layer and a roughened cylinder immersed in a laminar uniform flow. In the former case, the Reynolds number Re _d (??U _?? d/??, with U _?? being the free-stream velocity and ?? the fluid viscosity) was varied from 2.61?×?10⁴ to 2.87?×?10⁵, and two values of H/d (2.65 and 5) were examined; in the latter case, Re _d ?=?1.24?×?10⁴?C1.73?×?10⁵ and H/d?=?3, 5 and 7. In the subcritical regime, both the drag coefficient C _D and the Strouhal number St are smaller than their counterparts for a two-dimensional cylinder and reduce monotonously with decreasing H/d. The presence of a turbulent boundary layer causes an early transition from the subcritical to critical regime and considerably enlarges the Re _d range of the critical regime. No laminar separation bubble occurs on the finite-length cylinder immersed in the turbulent boundary layer, and consequently, the discontinuity is not observed in the C _D?CRe _d and St?CRe _d curves. In the roughened cylinder case, the Re _d range of the critical regime grows gradually with decreasing H/d, while the C _D crisis becomes less obvious. In both cases, H/d has a negligible effect on the critical value of Re _d at which transition occurs from the subcritical to critical regime.     

Stability and coupled dynamics of three-dimensional dual inverted flags

Fluid–structure interaction of an inverted flag, which has a free leading edge and a clamped trailing edge, has drawn attention recently because of its novel properties such as divergence stability, a low stability threshold, and large-amplitude flapping motion. In this study, the stability and flapping behaviors of dual inverted flags with finite height are investigated for a side-by-side arrangement, and their noticeable characteristics are compared to those of dual conventional flags. The critical velocity at which the inverted flags break the equilibrium of a straight configuration reduces monotonically when a gap distance between the two flags becomes smaller and an aspect ratio becomes larger, which is also predicted by our linear stability analysis using simple theoretical models of two-dimensional flags and slender flags. After bifurcation, in addition to the synchronized in-phase and out-of-phase modes commonly observed in dual conventional flags, a novel attached mode appears which is mainly observed for small gap distance and small aspect ratio. In this non-linear mode, the leading edges of the two inverted flags touch each other on a midline, and the deformed inverted flags maintain static equilibrium. In a non-linear flapping regime, a new mechanism of a mode transition from an out-of-phase mode to an in-phase mode is identified, which is allowed by the collision of the two flags flapping with large amplitude.     

Analysis of fluid–structure interaction by an arbitrary Lagrangian–Eulerian finite element formulation

In this paper, the interaction fluid–rigid body is analysed by a finite element procedure that incorporates the arbitrary Lagrangian–Eulerian (ALE) method into a well‐known two‐step projection scheme. The flow is assumed to be two‐dimensional, incompressible and viscous, with no turbulence models being included. The flow past a circular cylinder at ℛℯ=200 is first analysed, for fixed and oscillating conditions. The dependence of lock‐in upon the shift between the mechanical and the Strouhal frequencies, for a given amplitude of forced vibration, is illustrated. The aerodynamic forces and the wake geometry are compared for locked‐in conditions with different driving frequencies. The behaviour of a rectangular cylinder (B/D=4) at ℛℯ=500 (based on height D) is also analysed. The flutter derivatives associated with aerodynamic damping (H₁* and A₂* in Scanlan's notation) are evaluated by the free oscillation method for several values of reduced flow speed above the Strouhal one (namely for 3≤U*≤8). Torsional flutter was attained at U*≥5, with all the other situations showing stable characteristics. Copyright © 1999 John Wiley & Sons, Ltd.     

Blade manipulators in turbulent channel flow

We report here the results of a series of careful experiments in turbulent channel flow, using various configurations of blade manipulators suggested as optimal in earlier boundary layer studies. The mass flow in the channel could be held constant to better than 0.1%, and the uncertainties in pressure loss measurements were less than 0.1 mm of water; it was therefore possible to make accurate estimates of the global effects of blade manipulation of a kind that are difficult in boundary layer flows. The flow was fully developed at the station where the blades were mounted, and always relaxed to the same state sufficiently far downstream. It is found that, for a given mass flow, the pressure drop to any station downstream is always higher in the manipulated than in the unmanipulated flow, demonstrating that none of the blade manipulators tried reduces net duct losses. However the net increase in duct losses is less than the drag of the blade even in laminar flow, showing that there is a net reduction in the total skin friction drag experienced by the duct, but this relief is only about 20% of the manipulator drag at most.List of symbols A, A log law constants - c chord length of manipulator - D drag of the manipulator - dp/dx pressure gradient in the channel - h half height of the channel - H height of the channel (2h) - K log law constant - L length of the channel - L.E. leading edge of the manipulator - P static pressure - P _x static pressure at a location x on the channel - P _xm static pressure at the location x in the presence of manipulator - p _ref static pressure at any reference location x upstream of the manipulator - Re Reynolds number - t thickness of the manipulator - T.E. trailing edge of the manipulator - u velocity in the channel - U friction velocity - U ^* average velocity in the channel - u _c centre-line velocity in the channel - U ₊ U/U _* - u _m velocities in the channel downstream of the manipulators - u _ref velocities in the channel at reference location upstream of the manipulators - w Coles's wake function - W width of channel Also National Aeronautical Laboratory, Bangalore 560 017, India     

Effects of riblet bends on pipe flows

Four riblet bends were tested to investigate the effects of riblets on pipe flows including the secondary flow on the Reynolds numbers; Re _D =6×10³–4×10⁴. The pressure gradients on the smooth pipe downstream from the riblet bends were measured, and also the pressure losses of the bends only were measured. All riblet bends reduced the pressure gradient on the smooth pipe downstream from them, which means a drag reduction. Two of the riblet bends showed the maximum drag reduction of about 4 percent at Re _D = 6500; this reduction rate was significant considering the uncertainty of the present experiments. Since the pressure losses of these two riblet bends were almost identical to that of the smooth bend at Re _D = 6500, they could cause a net drag reduction of about 4 percent on the piping system including these bends at that Reynolds number. Furthermore, the velocity profiles measured by LDV indicated that the secondary flow becomes weaker downstream from the riblet bends when a drag reduction is recognized there.Nomenclature D pipe diameter - D ₀ the distance from the valley to the valley passing through the pipe center - H height of groove - P nondimensional static pressure (p/it/(U ₀ ² ):p is gauge pressure) - dP/dX nondimensional pressure gradient - Rc curvature of bend - Re _D Reynolds number based on bulk velocity and pipe diameter - s spacing of groove - U mean streamwise velocity along the horizontal diameter - U ₀ bulk velocity - V mean vertical velocity along the horizontal diameter - x streamwise direction along the pipe axis (see Fig. 1) - X nondimensionalx (=x/D) - y radial direction in the horizontal plane which is perpendicular to the plane including the bend (see Fig. 1) - yUV swirl intensity (nondimensional swirl intensity:yUV/(DU ₀ ² ))     

Unsteady near wake of a flat disk normal to a wall

The unsteady wake of a flat disk (diameter D) located at a distance of H from a flat plate has been experimentally investigated at a Reynolds number Re _D  = 1.3 × 10⁵. Tests have been performed for a range of gap ratio (H/D), spanning from 0.3 to 1.75. The leading edge of the flat plate is either streamlined (elliptical) or blunt (square). These configurations have been studied with PIV, high speed PIV and multi-arrayed off-set fluctuating pressure measurements. The results show a progressive increase of the complexity of the flow and of the interaction as the gap ratio decreases. For large values of H/D (1.75), the interaction is weak and the power spectral densities (PSD) exhibit a strong peak associated with the vortex shedding events (St = 0.131) – St = fD/U _∞ is the Strouhal number. For lower values of H/D (0.75), the magnitude of the wall fluctuating pressure increases significantly. A large band contribution is associated with the unsteady wake structure and turbulence. A slight increase of the shedding frequency (St = 0.145) is observed. A critical value of the gap ratio (about 0.35) has been determined. Below this critical value, a three-dimensional separated region is observed and the natural vortex shedding process is very strongly altered. These changes induce a great modification of the fluctuating pressure at the wall. Each interaction reacts in a different way to perturbed upstream conditions. In particular, the disk is an overwhelming perturbation for the lowest H/D value studied here and the relative influence of the upstream turbulence on the wall fluctuating pressure below the near wake region is moderate.     

Mixing characteristics of a film-exciting flapping jet

We have recently discovered a new type of self-excited flapping jets due to a flexible film whose leading edge is fixed at the nozzle exit [Exp Ther Fluid Sci, 106, 226-233]. This paper is to report the experimental investigation on mixing characteristics of the jet induced by a rectangular FEP film. Hot wire anemometry and flow visualization are used to examine the flapping jet flow versus the non-flapping counterpart. Experiments are conducted under the following conditions: i.e., L/D = 1.0 (fixed), W/D = 0.03 ~ 1.0 (varying) and Re = 10000 ~ 45000 (varying); where W and L are the film's width and length, D is the nozzle-exit diameter, and Re is the Reynolds number defined by Re U_oD/ν with U_o and ν being the jet-exit velocity and fluid viscosity.It is found that the jet-flapping frequency f_F varies with W in a complex fashion while it grows roughly linearly with increasing U_o for W/D ≥ 0.5. The flapping Strouhal number St_F f_FD/U_o ranges in 0.13 ≤ St_F ≤ 0.23 for Re = 15,000 ~ 45,000. These Strouhal numbers are substantially lower than that (≈ 0.45 ~ 0.7) for the primary vortex generation in the free jet, but one to two orders of magnitude higher than those from the conventional self-exciting fluidic devices. In general, the flapping jet decays and spreads more rapidly than does the free jet. As W increases, the decaying and spreading rates both grow. Of significance, the centerline evolutions of Taylor and Kolmogorov scales versus the integral scale are examined to characterize the small scales of turbulence against the large-scale motion.