SOME EXPERIMENTS ON FLOW-INDUCED VIBRATION OF A CIRCULAR CYLINDER WITH SURFACE ROUGHNESS

Aeroelastic instability of a circular cylinder with surface roughness was experimentally studied by free-oscillation tests in a wind tunnel. Flows at high Reynolds numbers could be simulated at relatively low wind velocities, by introducing surface roughness, so as to reduce the value of the critical Reynolds number. The response amplitudes of a roughened cylinder oscillating in the transverse (cross-flow) direction in the flow were measured. The measured range of reduced velocity is about 1·5–8, which includes the critical velocity. The value of a reduced mass-damping parameter (the Scruton number) is constant at about 6. For the aeroelastic instability in the transverse direction, it was found that the oscillation of the roughened cylinder induced by a vortex-excitation is damped down in a small velocity range covering the critical Reynolds number. At Reynolds numbers higher than the critical value, a roughened cylinder vibrates with a large amplitude again, associated with a lock-in phenomenon due to the coincidence of the wake-frequency and the natural frequency of the oscillating cylinder.     

横观各向同性圆柱体自由振动的弹性力学解

本文由横观各向同性的弹性力学方程出发，研究有限长圆柱体的自由振动问题。利用文献「１」的通解，将位移分量和应力分量分别表达成傅里叶－塞尔级数和双曲－贝塞尔级数的形式。通过边界条件和级数的正交关系，得到关于有限长圆柱自由振动频率的特征方程。利用数值方法求解特征根，从而得到圆柱体三维振动的自振频率。     

带有新型涡激振动抑制罩的圆柱体的水动力特性

通过模型实验和数值模拟计算,研究了带有涡激振动抑制罩的圆截面柱体的水动力特性.模型实验主要测试了柱体上附加谐波型和类圆锥型涡激振动抑制罩的单摆结构在不同流速下发生涡激振动的性质;数值模拟则针对谐波型和圆锥型扰动,在雷诺数Re为102到105范围内,研究其水动力参数,如阻力、升力和涡脱落频率等,随扰动波长和波动强度的变化.模型实验结果表明,在直圆柱开始发生共振的流速下,带抑制罩的柱体的振幅显著降低,而在更高流速下则显著增大.数值模拟结果表明,谐波型和圆锥型扰动具有相似的水动力特性;且在不同Re时,阻力、升力和涡脱落频率具有相似的变化规律;随波动强度的增大,阻力一般逐渐增大,升力则在多数情况下先减小而后增大,而涡脱落频率一般逐渐减小.      

高亚临界雷诺数圆柱绕流的尺度自适应模拟

借助γ-Re_θ转捩模型,实现了高亚临界雷诺数（Re=1.4×105）下圆柱层流分离流动的尺度自适应模拟.统计平均结果表明数值计算和实验测量较为接近,尤其在圆柱后半段的分离区中,压力系数和实验符合得很好,误差主要源于分离点预测的不准确. 瞬态流动则显示,层流分离的剪切层中出现了展向不稳定,且在向下游的输运过程中不断增强,最后转捩为完全湍流. 在湍流分离模拟中,由于缺乏剪切层失稳的非定常性,SST-SAS 模型的尺度分辨能力变弱,因此在分离区以及下游尾迹中求解出的湍流尺度要明显较层流分离时大.      

SCALE ADAPTIVE SIMULATION OF FLOWS AROUND A CIRCULAR CYLINDER AT HIGH SUB-CRITICAL REYNOLDS NUMBER

借助γ-Re_θ转捩模型,实现了高亚临界雷诺数（Re=1.4×10⁵）下圆柱层流分离流动的尺度自适应模拟.统计平均结果表明数值计算和实验测量较为接近,尤其在圆柱后半段的分离区中,压力系数和实验符合得很好,误差主要源于分离点预测的不准确. 瞬态流动则显示,层流分离的剪切层中出现了展向不稳定,且在向下游的输运过程中不断增强,最后转捩为完全湍流. 在湍流分离模拟中,由于缺乏剪切层失稳的非定常性,SST-SAS 模型的尺度分辨能力变弱,因此在分离区以及下游尾迹中求解出的湍流尺度要明显较层流分离时大.     

FLOW-INDUCED VIBRATION OF FREE EDGES OF THIN FILMS

During manufacturing processes of thin materials such as paper, photographic film, and magnetic film, which are handled as continuous sheets and subjected to drying air-flows, the interaction of the air with the web can cause the free edges to vibrate violently. This phenomenon is related to the waving motion of a flag in the wind, except that the thin films under consideration are under tension in the direction of the air-flow or at right angles to it. A travelling-wave analysis was done based on incompressible potential-flow theory; the critical flow speed, wave speed, wavelength, and flutter frequency were predicted. A closed-form solution of the critical flow speed is suggested. Experiments were carried out with stationary thin films mounted in a wind tunnel where the direction of tension was perpendicular to the flow direction. It was shown that the analysis, which assumes that the film is infinitely long in the flow direction, could successfully predict the critical flow speed above which violent edge vibrations occur.     

亚临界雷诺数下圆柱绕流的大涡模拟

本文应用Smagorinsky涡粘性模式和二阶精度的有限体积法对圆柱绕流的流场进行大涡模拟．求解了非正交曲线坐标系下的N-S方程,对雷诺数为100和20000的工况进行了计算．计算结果与实验及动力涡粘性模式的结果进行了比较,表明计算对于层流及高亚临界雷诺数的湍流流动是合理的     

小Kc数振荡流中圆柱绕流流场结构的数值分析

本文综合Chorin的涡方法、网格涡方法和泊松方程快速解法的优点,采用固定离散涡的数目和位置,随时间变化调整离散涡强度的计算方法,对Kc数在1到5之间,β数在10到1665之间圆柱在振荡流动中的流场结构进行了数值模拟。并与U型振荡水槽中的流场涡结构显示照片比较,计算结果表明,数值解在稳定性和收敛性方面都是比较好的。与显示照片相比,其符合程度也是令人满意的。     

圆柱绕流的非线性动力学

本文利用非线性动力学的概念和方法来研究粘性不可压缩流体的二维圆柱绕流问题．计算了定常流失稳以及出现混沌的临界雷诺数，并估计了混沌解的一些统计特征．     

非定常圆柱绕流的实验研究

本文采用流场显示方法研究单圆柱在非定常流动中的涡旋脱落规律。实验结果表明：在不同的ＫＣ数（ＫＣ＝Ｕ∞Ｔ／Ｄ）下，非常圆柱绕流的涡旋脱落特性是不相同的，一般随ＫＣ数值的增加，其涡旋脱落对数也增加，但有明显的阶梯性，同时圆柱分离点的周期变化后于流场的变化。     

幂律流体圆柱绕流的格子波尔兹曼模拟

基于插值补充格子波尔兹曼方法和幂律流体的本构方程,建立了贴体坐标系下适用于幂律流体的格子波尔兹曼模型,模拟了幂律流体的圆柱绕流问题,采用非平衡外推格式处理圆柱表面的速度无滑移边界,利用应力积分法确定曳力系数和升力系数,并与基于标准的格子波尔兹曼方法和有限容积法获得的数值数据进行对比,吻合良好. 进行了网格无关性验证之后,分析了稳态流动时,不同雷诺数下幂律指数对于尾迹长度、分离角、圆柱表面黏度分布、表面压力系数及曳力系数的影响,以及非定常流动中,幂律指数对于流场、曳力系数、升力系数和斯特劳哈尔数的影响. 获得的变化规律与基于其他数值模拟方法得到的结果相一致,充分验证了模型的有效性和正确性. 结果表明:插值补充格子波尔兹曼方法可以用来模拟幂律流体在具有复杂边界流场内的流动问题,通过引入不同的非牛顿流体本构方程,该方法还可以进一步应用于其他类型的非牛顿流体研究中.      

VORTEX-INDUCED VIBRATIONS OF AN ELASTIC CIRCULAR CYLINDER

A numerical study of a uniform flow past an elastic circular cylinder using the discrete vortex method incorporating the vortex-in-cell (VIC) technique has been undertaken. The Reynolds number is kept at 200 for all calculations and the cylinder motion is modelled by a spring–damper–mass system. The fluid motion and the structural responses are solved in an iterative way so that the interactions between the fluid and the structure can be accounted for properly. Analyses of the cylinder responses, the damping, the induced forces, the vortex shedding frequency and the vortex structure in the wake have been carried out. The results show that fluid damping is responsible for a limit-cycle oscillation behaviour even when the system natural frequency is close to the vortex-shedding frequency. Reasonable agreement with previous experimental data and computational results is obtained in the comparison of the amplitude of the limit-cycle oscillations. The results further show that the cylinder oscillations could be as large as 0·57 diameter under certain flow conditions and structural properties. Finally, it is shown that a one-degree-of-freedom structural model yields results that are only in qualitative agreement with a two-degree-of-freedom model. In other words, the streamwise oscillations also have a substantial effect on the transverse vibrations and their characteristics.     

幂律流体圆柱绕流的格子波尔兹曼模拟简

基于插值补充格子波尔兹曼方法和幂律流体的本构方程,建立了贴体坐标系下适用于幂律流体的格子波尔兹曼模型,模拟了幂律流体的圆柱绕流问题,采用非平衡外推格式处理圆柱表面的速度无滑移边界,利用应力积分法确定曳力系数和升力系数,并与基于标准的格子波尔兹曼方法和有限容积法获得的数值数据进行对比,吻合良好. 进行了网格无关性验证之后,分析了稳态流动时,不同雷诺数下幂律指数对于尾迹长度、分离角、圆柱表面黏度分布、表面压力系数及曳力系数的影响,以及非定常流动中,幂律指数对于流场、曳力系数、升力系数和斯特劳哈尔数的影响. 获得的变化规律与基于其他数值模拟方法得到的结果相一致,充分验证了模型的有效性和正确性. 结果表明：插值补充格子波尔兹曼方法可以用来模拟幂律流体在具有复杂边界流场内的流动问题,通过引入不同的非牛顿流体本构方程,该方法还可以进一步应用于其他类型的非牛顿流体研究中.     

A NUMERICAL STUDY OF FLOW PAST A ROTATIONALLY OSCILLATING CIRCULAR CYLINDER

Vortex shedding from a rotationally oscillating circular cylinder in a uniform flow is studied by numerical solutions of the two-dimensional incompressible Navier-Stokes equations using primitive variables. To demonstrate the viability and accuracy of the method, we calculate the approach flow past a rotating cylinder with constant angular velocity. For approach flow past a rotationally oscillating cylinder, the object of the study is to examine the effect of oscillating rotation on the flow structure. In the present study, some basic patterns of vortex shedding can be identified according to our calculated results and are in good agreement with available experiments. In addition, the influence of the oscillating frequency and amplitude on the forces acting on the cylinder is also investigated.     

并列双圆柱流致振动的不对称振动和对称性迟滞研究

对雷诺数Re = 100 间距比s/D = 2.5 和5.0 的并列双圆柱流致振动进行了数值模拟研究, 其中圆柱质量比m = 2.0, 折合流速U_r 在2.0~10.0 之间, 两圆柱仅能做横流向振动. 研究发现, 当间距比s/D = 2.5 时, 在折合流速4.4 < U_r< 4.8区间内, 两圆柱流致振动响应出现不对称振动现象, 在折合流速4.4 < U_r< 4.8 区间内, 两圆柱流致振动响应出现对称性迟滞现象; 而当间距比s/D = 2.5时, 圆柱流致振动响应与单圆柱涡激振动响应相似, 没有出现不对称振动和对称性迟滞现象. 在不对称振动区间内, 两圆柱的升、阻力参数也出现了不相等的情况. 此外, 当两圆柱不对称振动时, 圆柱间隙流稳定地偏斜向其中的一个圆柱; 相应地, 尾涡也出现了宽窄不等的模式. 窄尾流圆柱的振幅和升、阻力均较宽尾流圆柱的大. 通过对比不对称振动现象发生前后的尾涡模式, 对新现象的产生机制进行了阐述.      

低雷诺数串列双方柱流致振动质量比效应的数值研究

为澄清串列双方柱流致振动的质量比效应, 采用数值模拟方法, 在雷诺数为150时, 研究了质量比($m^{\ast }=3$, 10, 20)对下游方柱振动响应特性的影响规律, 分析了下游方柱尾流模态的演变过程, 探讨了导致下游方柱振动的流固耦合机制. 结果表明: 质量比对下游方柱的流致振动有重要影响, 低质量比($m^{\ast }=3$)时下游方柱的振动响应更为复杂, 随着折减速度的增大, 下游方柱并未出现传统“锁定”现象(即振动频率比$f_{y}$/$f_{\rm n} \approx1$的锁定), 而发生了“弱锁定”现象(即$f_{y}/f_{\rm n}<1$的锁定); 随着质量比的增加($m^{\ast }=10$和20), “弱锁定”现象消失, 而出现传统“锁定”现象, 且下游方柱横流向最大振幅减小. 质量比对串列双方柱的柱心间距有明显影响, 低质量比($m^{\ast }=3$)时的柱间距在振动锁定区内会急剧减小, 而较高质量比($m^{\ast }=10$和20)下的柱间距则变化不大. 此外, 质量比对串列双方柱的尾流模态和流固耦合机制也有显著影响, 其中低质量比($m^{\ast }=3$)下的情况更为多样.      

A NUMERICAL SIMULATION OF VORTEX SHEDDING FROM AN OSCILLATING CIRCULAR CYLINDER

Vortex shedding from an oscillating circular cylinder is studied by numerical solutions of the two-dimensional unsteady Navier–Stokes equations. A physically consistent method is used for the reconstruction of velocity fluxes which arise from discrete equations for the mass and momentum balances. This method ensures a second-order accuracy. Two phenomena are investigated and, in both cases, the cylinder oscillation is forced. The first is the flow induced by the harmonic in-line oscillation of cylinder in water at rest. The Reynolds number is equal to 100 and the Keulegan–Carpenter number is equal to 5. A comparison of phase-averaged velocity vectors between measurements and predictions is presented. Applying the widely used model of Morison to the computed in-line force history, the drag and the inertia coefficients are calculated and compared for different grid levels. Using these to reproduce the force functions, deviations from those originally computed are revealed. The second problem is an investigation of a transversely oscillating cylinder in a uniform flow at fixed Reynolds number equal to 185. The cylinder oscillation frequency ranges between 0·80 and 1·20 of the natural vortex-shedding frequency, and the oscillation amplitude is 20% of the cylinder diameter. As the frequency of excitation of the cylinder increases relative to the inherent vortex formation frequency, the initially formed concentration of vorticity moves closer to the cylinder until a limiting position is reached. At this point, the vorticity concentration abruptly switches to the opposite side of the cylinder. This process induces distinct changes of the topology of the corresponding streamline patterns.     

横向振荡圆柱绕流的格子Boltzmann方法模拟

基于格子Boltzmann方法(LBM)对不可压横向振荡圆柱绕流问题进行了数值研究. 与传统的求解宏观的N-S方程的数值方法不同, LBM求解此类问题不需要采用动网格, 而且不需要对网格进行特殊处理, 从而节约了计算成本. 结果显示, 当振荡频率增加到相应的静止圆柱绕流的自然涡脱落频率附近时, 圆柱后最新形成的集中涡距离柱体越来越近, 直到达到一个极限位置. 随后, 集中涡突然转向圆柱体另一侧脱落. 当振荡频率接近于静止圆柱的自然涡脱落频率时, 发生频率同步的现象. 随着振荡频率远离自然涡脱落频率, 同步现象消失. 在几种次谐振荡和超谐振荡下, 尾流区的涡脱落频率仍为相应的静止圆柱绕流的自然涡脱落频率.      

THERMOELASTIC FREE VIBRATION OF CLAMPED CIRCULAR PLATE

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圆柱尾迹涡的三维演化及结构特征

应用无粘涡丝运动学理论和局部诱导近似(LIA)方法,以Lagrange观点跟踪涡丝在背景流场中运动,用数值方法研究了中等Re数(≈10~3)下圆柱分离尾迹中Kármán涡和涡辫区涡丝的三维演化的机制和动力学过程,及其结构特征。背景流场考虑为尾迹时间平均速度流场和Kármán涡街流场。初始展向小扰动为指数形式和谐波形式。结果指出:Kármán涡和涡辫区中的涡丝具有展向不稳定性,形成流向涡量。在尾迹的初期输运过程中,表现出有序的大、小尺度涡结构。并进一步分析了其产生的机理。