串列双方柱干扰效应流动机理研究

为了研究建筑群体周围的流场结构,减小工程设计中由于干扰效应造成的损失,利用粒子图像测速(PIV)结合数值模拟,研究在较大雷诺数及不同间隙工况下,双方柱流场受干扰时的流动特性及流场空间结构。分析升阻力系数、涡脱频率、斯特劳哈尔数等流场特征参数,探究不同间隙对串列双方柱的影响。当Re=3.42×104时,存在临界间隙比G=4使串列双方柱流场结构发生突变,试验观察到流场中出现双稳态现象;当G<4时,下风向方柱平均阻力系数为负值,小于单方柱情况下的阻力,屏蔽效应明显,上风向方柱后方涡脱落被抑制,平均阻力系数出现了明显的降幅,最大降幅约达10%;当G>4时,上下方柱均有涡旋脱落。该结果对于工程应用具有参考意义。     

基于表面涡方法的单列圆柱流体诱导振动及流弹性不稳定研究

基于表面涡方法和流固耦合模型研究了Re=2.67×104时的单列圆柱流体诱导振动问题,计算了流体力、振动响应、涡脱落频率等,并给出了涡云图。计算模拟结果很好地重现了刚性单列圆柱在T/D=1.5(小间隙比)下以宽窄尾涡交替和多频为特征的非均匀流态,以及T/D=2.0的涡脱落现象。此外,该文还研究了单列弹性圆柱在T/D=1.5时的流体诱导振动以及流体弹性不稳定问题,计算了SG=1.29时圆柱列的无量纲临界速度。     

基于IBM法的低雷诺数下涡激振动高质量比效应的研究

由深圳市赛格广场大厦在低速风场作用下的强烈有感振动事件可知,高质量比系统的振动问题仍较突出。为澄清涡激振动中的高质量比效应,该文采用一种锐利界面浸入边界法,通过C++编程计算了低雷诺数(Re=80~150)流场中,不同高质量比(m*=14.8~280)、阻尼比(ζ=0.0012~0.036)和质量-阻尼比组合m*ζ对涡激振动的影响。结果表明:通过与文献和实验结果的对比,验证了该方法的准确性和有效性;在高质量比情况下,Re<100时,结构发生"弱锁定"现象,Re=100~130时,发生传统的"锁定"现象,且发生共振时Re=110,位于锁定区间靠近Re数较小的一侧,当Re=130时,开始摆脱锁定,且升力与振动响应出现"相位突变"现象;m*、ζ对锁定区间的影响并不大,但是质量-阻尼比组合m*ζ相同时,质量比对涡激振动的影响更加显著,即质量比低的结构系统发生涡激振动时的锁定区间更广(Re=90~140),m*=14.8的高质量比系统比m*=148的较高质量比系统提高了1.67倍,而且共振时对应的雷诺数也减小;发生共振时,尾涡脱落均为"2S"模态,最大振幅均为0.5D左右,无太大变化,即高质量比和较高质量比对振幅和锁定区间的影响并不大,但是随着ζ的增加,振幅比Y逐渐减小,振动受到了抑制。     

翼梢涡风洞研究中的摆动问题

采用激光粒子成像速度仪(PIV)对一矩形机翼(NACA0012)模型所产生的翼梢涡进行了风洞测试研究.测量位置为机翼近场尾迹,即x/c=3垂直于流动方向的截面,这里x为机翼后缘和测量截面之间的距离,c是机翼弦长.实验中基于弦长的雷诺数范围在3.4×104～26.6×104,通过分析所测得的涡量、切向速度和环量等,发现翼梢涡的摆动幅度与流过机翼上流体是否发生边界层分离有直接关系.     

考虑大变形的大柔性立管涡激振动模型

在总结深海大柔性立管涡激振动研究基础上,提出考虑大变形的大柔性立管涡激振动数学模型,开发出相应的计算程序。在验证新模型计算程序可靠的基础上,分别在雷诺数Re=6300、Re= 情况下研究深水顶张式立管涡激振动特征。结果表明,低雷诺数时立管涡激振动以低阶单模态振动为主;随雷诺数的增大,立管涡激振动出现多模态参与振动及涡激振动非完全对称大变形现象。考虑大变形的大柔性立管涡激振动数学模型可用于深水、极深水立管系统涡激振动响应分析预测。     

不同间距串列双方柱流致振动运动特性分析

多钝体流致振动是涉及流体工程安全性的重要问题,为了探究多钝体流致振动的相互影响,针对不同间隙距离串列弹性支撑双方柱流致振动进行研究,方柱间距变化范围为2-6倍方柱边长D,分析折减速度在4.04-13.46时双方柱振幅及频率响应特性,并与串列被动控制双圆柱流致振动和相关方柱实验进行比较。研究结果表明,当方柱间距大于4 D时,上游方柱振动响应与单方柱实验结果相似,受下游方柱的影响较弱;双方柱流致振动在间距为4 D时得到加强,上下游方柱达到最大振幅,分别为1.06 D和1.10 D;当间距小于4 D时,双方柱流致振动减弱,与单方柱实验结果相比振幅减小;间距对双方柱的振动频率没有表现出明显影响。在折减速度为8.00时,随间距增大,上游方柱能在双方柱间隙内形成完整脱体旋涡,下游方柱的尾涡模态经历了2S(双单涡模态)、P+2S(涡对+双单涡组合模态)、2S的转变过程。     

小控制杆对矩形柱尾流旋涡脱落的抑制

采用小控制杆方法对主柱宽厚比B/H为2.0和3.0的两种矩形柱尾流的旋涡脱落进行了抑制实验.实验在风洞中进行,雷诺数(Re)范围为1.5×103至5.5×103,控制杆与矩形柱长度相同,两者相互平行放置.对三种不同宽度的控制杆进行了实验,小杆宽厚比分别为b/H=0.2,0.32和0.4.流动显示和热线测量结果表明:在柱体附近存在一个有效区,控制杆位于此区内时,柱体两侧的旋涡脱落被抑制.另外还存在一个单侧有效区,当控制杆位于该区内时,柱体一侧的旋涡脱落被抑制,而另一侧未被抑制,形成"单侧旋涡脱落现象".文中还研究了小杆宽厚比、矩形柱宽厚比和雷诺数对有效区和单侧有效区的影响,发现b/H增大时,有效区和单侧有效区不是扩大,而是有所缩小;当B/H增大时,有效区和单侧有效区扩大,而且有效区由一个变为两个;Re增大时,有效区和单侧有效区略有缩小.     

方柱绕流的二维和三维数值模拟

为了深入研究方柱绕流中流动情况,详细分析方柱受力,通过采用标准k-ε模型和DES模型,运用有限体积法对单方柱绕流进行了数值模拟,着重研究了流场形态、升阻力系数变化曲线,给出了流场中方柱在不同雷诺数下受到的升力和阻力的变化情况.通过与前人的实验和数值结果比较,证明了DES计算模型对于三维钝体绕流计算是可行的.表明三维数值模拟优于二维数值模拟,且随着雷诺数的增加,CD的平均值增加而St数先增大再减小.     

基于拓扑网格方法的多钝体流致振动分析

多钝体流致振动是一个较为复杂的流固耦合过程,普遍存在于自然界和工程领域。为了减小高幅振动时网格变形引起的计算误差,基于非定常Navier-Stokes方程对二维双圆柱和三圆柱、三维双圆柱流致振动进行数值求解,采用耦合界面结合拓扑网格变形技术,实现流体与多个运动钝体之间的耦合计算。将数值结果与实验进行比较分析,验证了该数值方法是处理高振幅多钝体流致振动的有效方法。研究结果表明上游圆柱的存在对下游圆柱流致振动和旋涡形成产生明显影响。串列双圆柱流致振动振幅和频率响应与实验测试趋势一致,清晰观察到了涡致振动初始分支和上部分支;并且当Re8×10~4时,圆柱流致振动由涡致振动向驰振过渡。圆柱尾涡形态随流致振动分支切换发生变化,当驰振发生时,下游圆柱的尾涡形态受上游圆柱影响难以捕捉。随着双圆柱间距增大,低Re时下游圆柱受到上游圆柱的抑制作用减弱。三维多柱体流致振动计算结果更接近实验值,如何提高三维数值计算速度将是下一步研究工作的重点。     

不同迎流攻角下正三角柱流致振动数值模拟研究

采用嵌入式迭代浸入边界法对不同迎流攻角下正三角柱的流致振动(VIV)进行了二维数值模拟研究,其中迎流攻角为α=0°～60°(当α=60°时三角柱顶点迎流),雷诺数为Re=100,质量比为m *=5和折合流速为Ur=2～20.详细分析了不同迎流攻角下横流向振幅、振动频率、水动力系数、升力与位移的相位差以及尾涡模式随折合流...     

Numerical simulation of the separated fluid flows at large Reynolds numbers

The variety of flow regimes (steady separated, periodically separated-‘Karman vortex street’, unsteady turbulent) and their characteristic peculiarities (separation and reattachment points, secondary separation, boundary layer, instability of the shear mixing layer, etc.) require the construction of effective numerical methods, which will be able to simulate adequately the considered flows. MERANGE ? SMIF–a splitting method for physical factors of incompressible fluids¹-is used for calculations of the steady and unsteady fluid flows past a circular cylinder in a wide range of Reynolds numbers (10° < Re < lo6). The finite-difference scheme for this method is of second order accuracy in the space variables, has minimal numerical viscosity and is also monotonic. Use of the Navier-Stokes equations with the corresponding transformation of Cartesian co-ordinates allows the calculations to be made by one algorithm both in a boundary layer and out of it. The method allows calculations at Re = ∞ cc and simulation of d‘Alembert’s paradox. Some results on the classical problem of the flow around a circular cylinder for a wide range of Reynolds numbers are discussed. The crisis of the total drag coefficient and the sharp rise of the Strouhal number are simulated numerically (without any turbulence models) for the critical Reynolds numbers (Re ≈ 4 × 10⁵), and are in a good agreement with experimental data.     

高雷诺数下错列双圆柱气动干扰的机理研究

为了进一步澄清小间距错列双圆柱的气动干扰机理,该文采用大涡模拟方法,在高雷诺数下（Re=1.4×10⁵）,研究了间距为2倍圆柱直径的错列双圆柱的气动性能和流场特性随风攻角的变化规律,分析了两个圆柱气动力系数相关性,探讨了下游圆柱气动力与流场结构的内在联系,对下游圆柱平均升力的流场机理提出了新的解释。研究表明,大涡模拟得到的结果与风洞试验值吻合良好;下游圆柱的气动性能、流场结构和两个圆柱气动力相关性均会随风攻角发生剧烈变化;风攻角在0°~10°时,下游圆柱受平均负阻力作用,其原因分别为两圆柱间的回流区和间隙流;风攻角在10°附近时,下游圆柱受很大平均升力作用,风压停滞点偏移、两圆柱间高速间隙流、下游圆柱间隙侧剪切层的提前分离和再附是平均升力出现的三个因素。     

Computational study of unsteady viscous flow around a transversely and longitudinally oscillating circular cylinder in a uniform flow at high reynolls numbers

A finite difference simulation method for the time dependent viscous incompressible flow around a transversely and longitudinally oscillating circular cylinder at Reynolds numbers of Re=4×10³ and 4×10⁴ is presented. The Navier-Stokes equations in finite difference form are solved on a moving grid system, based on a time dependent coordinate transformation. Solution of the vortex street development behind the cylinder is obtained when the cylinder remains stationary and also when it is oscillating. Time eholution of the flow configuration is studied by means of stream lines, pressure contours and vorticity contours. The computer results predict the lock-in phenomenon which occurs when the oscillation frequency is close to the vortex shedding frequency in the transverse mode or around double the vortex shedding frequency in the longitudinal mode. The time dependent lift and drag coefficients are obtained by the integration of the pressure and shear forces around the body. The drag, lift and the displacement relations are also discussed.     

圆柱涡激振动数值模拟研究

基于k-ωSST湍流模型研究典型亚临界状态下(Re=3 900)二维圆柱的涡激振动。通过将圆柱简化为不同刚度的质量弹簧系统,并运用CFX的动网格与CEL功能,研究刚度系数对圆柱涡激振动的影响。研究观察到圆柱涡激振动的自限定现象以及旋涡脱落模态的转变过程,并得到圆柱涡激振动特性随刚度变化的规律。     

Heat transfer and icing of a rough cylinder

Heat transfer from a rough ice surface to air is a fundamental factor determining the icing rate and ice shape on an object under conditions where the impinging water flux exceeds the icing rate. In order to examine the effect of the roughness characteristics on icing, a boundary-layer model of heat transfer from a front half of a rough circular cylinder is proposed. The model is based on the integral equations of the boundary layer, and predicts the local heat transfer coefficient along the cylinder surface and, subsequently, the overall heat transfer rate. Comparison between the results of the model and the available experimental data for three different roughnesses in the cylinder Reynolds number range 5 × 10⁴ < Re < 4 × 10⁶ show that the model simulates the heat transfer more precisely than the previous formulations used in icing models.     

Channel flow past bluff-body: outlet boundary condition,vortex shedding and effects of buoyancy

 Structure of laminar flow and heat transfer, in a two-dimensional horizontal plane channel differentially heated, with a built-in triangular prism is investigated from the numerical solutions of complete Navier–Stokes and energy equations. Results are obtained for Reynolds and Grashof numbers ranging respectively from 30 to 200 and from 0 to 1.5 × 10⁴ at Pr=0.71. In forced convection, results are specially presented to show how the vortex shedding at downstream affects the upstream. Also, two correlations giving the Strouhal and the averaged Nusselt numbers as functions of the Reynolds number are proposed. In mixed convection, the superposition of Von Karman street and convective cells is discussed. Received 15 May 2001     

Thermal analysis of airflow inside a refrigerated container

A numerical study of conjugated heat transfer in ceiling-slot refrigerated containers is carried out to analyze the temperature distribution effectiveness and to determine the ventilation characteristics. The effect of slot size on thermal characteristics is studied by considering half-span and full-span injection. The container walls are defined as conductive opaque and are interacting with outside environment. The outer surface heat transfer coefficients of conductive walls are computed by studying the flow around the refrigerated truck. The Reynolds number at the slot exit varied between 2 × 10⁴ ≤ Re ≤ 2 × 10⁵. The gravity effect is taken into account, and the coupled mass, momentum, and energy equations are discretized in finite volumes. The heat transfer coefficients of inner flow are presented as plots of the mean Nusselt number versus the modified Reynolds number. The maximum dispersion in the numerical data being at 14.54-percent, the mean Nusselt number, the modified Reynolds number, and the aspect ratio of the container are correlated.     

Numerical study of bluff body flow structures

Our recent progress in numerical studies of bluff body flow structures and a new method for the numerical analysis of near wake flow field for high Reynolds number flow are introduced. The paper consists of three parts. In part one, the evolution of wake vortex structure and variation of forces on a flat plate in harmonic oscillatory flows and in in-line steady-harmonic combined flows are presented by an improved discrete vortex method, as the Keulegan-Carpenter number (KC) varies from 2 to 40 and ratios ofU _m toU ₀ are ofO(10⁻¹),O(1) andO(10), respectively. In part 2, a domain decomposition hybrid method, combining the finite-difference and vortex methods for numerical simulation of unsteady viscous separated flow around a bluff body, is introduced. By the new method, some high resolution numerical visualization on near wake evolution behind a circular cylinder at Re=10², 10³ and 3×10³ are shown. In part 3, the mechanism and the dynamic process for the three-dimensional evolution of the Kármán vortex and vortex filaments in braid regions as well as the early features of turbulent structure in the wake behind a circular cylinder are presented numerically by the vortex dynamics method. This study was supported by the National Natural Science Foundation of China and the Laboratory for Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, as well as by the National Basic Research project “Nonlinear Science”.     

Numerical simulation of unsteady viscous flow around a circular cylinder

A finite difference solution is obtained for the time-dependent viscous incompressible 2-dimensional flow past a circular cylinder by direct integration of the Navier-Stokes equations expressed in a general curvilinear coordinate system. The solution describes the development of the vortex street developed behind the cylinder. Evolution of flow configuration is studied by means of streamlines, pressure contours, and vorticity contours for different Reynolds numbers. The time-dependent lift and drag coefficients are also obtained.