带整流罩隔水管流场特性的大涡模拟研究

为了深入揭示整流罩抑制涡激振动的机理,采用CFD软件FLUENT,结合大涡模拟方法,分别对附加整流罩前后隔水管模型的绕流流场进行模拟,将附加整流罩前后隔水管周围的流场特性、升力系数、阻力系数及其频谱特性等进行对比分析.结果表明,整流罩能有效抑制隔水管尾流区的漩涡脱落,隔水管背部的压力升高,升力系数和阻力系数显著降低,从而抑制了隔水管的涡激振动.     

不同控制角下附加圆柱对圆柱涡激振动影响

在弹性支撑的圆柱周围布置直径更小圆柱会影响剪切层发展以及旋涡脱落,进而改变其涡激振动状态.通过不同的布置形式和附加小圆柱个数可以实现对圆柱涡激振动的促进或抑制.激励更大幅值的振动可以更好地将水流动能转化为可利用的机械能或电能,抑制其振动则可以实现对海洋平台等结构物的保护.采用基于迭代的嵌入式浸入边界法对前侧对称布置两个小圆柱的圆柱涡激振动进行数值模拟研究,系统仅做横向振动,其中基于主圆柱直径的雷诺数为100,质量比为2.0,折合流速为3~11.小圆柱与主圆柱的直径比为0.125,间隙比为0.125.结果表明,在研究的控制角范围内(30°~90°),附加小圆柱可以很大程度上改变圆柱涡激振动的状态.当控制角较小(30°)时,附加小圆柱对主圆柱的振动起抑制作用;当控制角为45°~60°时,圆柱的振动分为涡振和弛振两个阶段,在弛振阶段,圆柱振幅随折合流速增加而持续增加;当控制角较大(75°~90°)时,附加小圆柱的促进作用随着控制角增加而减小.进一步地,结合一个周期内不同时刻旋涡脱落以及圆周压强分布,解释了附加小圆柱对主圆柱涡激振动的作用机制.应用能量系数对圆柱系统的进一步分析发现,弛振阶段由流体传递到主圆柱的能量系数随折合流速的增加逐渐下降,旋涡结构的改变是产生这种变化的直接原因.      

不同控制角下附加圆柱对圆柱涡激振动影响

在弹性支撑的圆柱周围布置直径更小圆柱会影响剪切层发展以及旋涡脱落,进而改变其涡激振动状态.通过不同的布置形式和附加小圆柱个数可以实现对圆柱涡激振动的促进或抑制.激励更大幅值的振动可以更好地将水流动能转化为可利用的机械能或电能,抑制其振动则可以实现对海洋平台等结构物的保护.采用基于迭代的嵌入式浸入边界法对前侧对称布置两个小圆柱的圆柱涡激振动进行数值模拟研究,系统仅做横向振动,其中基于主圆柱直径的雷诺数为100,质量比为2.0,折合流速为3～11.小圆柱与主圆柱的直径比为0.125,间隙比为0.125.结果表明,在研究的控制角范围内(30°～90°),附加小圆柱可以很大程度上改变圆柱涡激振动的状态.当控制角较小(30°)时,附加小圆柱对主圆柱的振动起抑制作用;当控制角为45°～60°时,圆柱的振动分为涡振和弛振两个阶段,在弛振阶段,圆柱振幅随折合流速增加而持续增加;当控制角较大(75°～90°)时,附加小圆柱的促进作用随着控制角增加而减小.进一步地,结合一个周期内不同时刻旋涡脱落以及圆周压强分布,解释了附加小圆柱对主圆柱涡激振动的作用机制.应用能量系数对圆柱系统的进一步分析发现,弛振阶段由流体传递到主圆柱的能量系数随折合流速的增加逐渐下降,旋涡结构的改变是产生这种变化的直接原因.     

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.      

错列角度对双圆柱涡激振动影响的数值模拟研究

为研究错列角度α对双圆柱涡激振动问题的影响,采用自主研发的基于CIP(constrained interpolation profile)方法的数值模型,对雷诺数Re=100、错列角度α=0?～90?(间隔15?)的等直径双圆柱涡激振动问题进行数值模拟.模型在笛卡尔网格系统下建立,采用具有三阶精度的CIP方法求解N-S(Navier--Stokes)方程,采用浸入边界法处理流-固耦合问题,避免了任意拉欧方法下的网格畸变和重叠动网格技术中的大量信息交换问题,保证了模型的计算效率.重点分析不同错列角度α上下游圆柱的升阻力系数、位移响应、涡脱频率和尾涡模态等.结果表明:折合速度Ur=2.0～3.0时,上下游圆柱升阻力随错列角度的增大基本呈单调增大的趋势;Ur=5.0～8.0时,随错列角度的增大,上下游圆柱阻力变化较小,升力呈"上凸"趋势,在α=15?～30?取得最大值;Ur=10.0～13.0时,随错列角度的增大,上下游圆柱阻力变化较小,升力呈"下凹"趋势,在α=30?～45?取得最小值,且柱体横流向振幅和升力没有明显的对应关系.最后,结合尾涡模态对以上规律的成因进行分析.研究结果可为相关海洋工程设计提供参考.     

低雷诺数下串列三圆柱涡激振动中的弛振现象及其影响因素

对间距比为1.2和雷诺数为100的串列三圆柱涡激振动进行数值模拟, 发现在某个折合流速之后, 三圆柱的响应均呈现为随着折合流速增大而增大的弛振现象, 平衡位置偏移、低频振动以及旋涡脱落与圆柱运动之间的时机三个因素共同决定了弛振现象的出现. 进一步的研究发现, 串列三圆柱的弛振现象仅出现在质量比不大于2.0和雷诺数不大于100的工况下. 当质量比较大时, 串列三圆柱的平衡位置固定不变, 且圆柱的振动不规律, 使得旋涡脱落与圆柱运动的时机处于变化之中. 当雷诺数较高时, 最上游圆柱的平衡位置在折合流速较大时回到初始位置, 不再参与对圆柱振动的调节, 使得圆柱的振动响应不再规律, 旋涡脱落与圆柱运动的时机也一直处于变化之中.      

低雷诺数下串列三圆柱涡激振动中的弛振现象及其影响因素

对间距比为1.2和雷诺数为100的串列三圆柱涡激振动进行数值模拟,发现在某个折合流速之后,三圆柱的响应均呈现为随着折合流速增大而增大的弛振现象,平衡位置偏移、低频振动以及旋涡脱落与圆柱运动之间的时机三个因素共同决定了弛振现象的出现.进一步的研究发现,串列三圆柱的弛振现象仅出现在质量比不大于2.0和雷诺数不大于100的工况下.当质量比较大时,串列三圆柱的平衡位置固定不变,且圆柱的振动不规律,使得旋涡脱落与圆柱运动的时机处于变化之中.当雷诺数较高时,最上游圆柱的平衡位置在折合流速较大时回到初始位置,不再参与对圆柱振动的调节,使得圆柱的振动响应不再规律,旋涡脱落与圆柱运动的时机也一直处于变化之中.     

柔性圆柱涡激振动流体力系数识别及其特性

涡激振动是诱发海洋立管、浮式平台系泊缆和海底悬跨管道等柔性圆柱结构疲劳损伤的重要因素.目前,海洋工程中用于柔性圆柱涡激振动预报的流体力系数主要来源刚性圆柱横流向受迫振动的实验数据,存在一定缺陷和误差.本文综合考虑横流向与顺流向振动耦合作用,建立了柔性圆柱涡激振动流体力模型,运用有限元法和最小二乘法确定升力系数、脉动阻力系数和附加质量系数.为了准确识别柔性圆柱涡激振动流体力系数,设计并开展了拖曳水池模型实验,实验用柔性圆柱模型的质量比为1.82,长径比为195.5.通过与刚性圆柱流体力系数对比,深入分析了柔性圆柱流体力系数的特性.结果表明:柔性圆柱在一阶模态控制区,流体力系数随约化速度变化趋势与刚性圆柱大致相似;二阶模态控制区,升力系数和脉动阻力系数显著增大;附加质量系数在响应频率较低时与振动位移的相关性增强;当响应频率较低时,振动位移较大区域为能量耗散区,当响应频率较高时,振动位移较大区域为能量输入区.     

利用O型环抑制圆柱绕法流涡脱落的数值研究

圆柱绕流涡脱落诱发较大的振动和声,如何有效地抑制值得关注.利用大涡模拟技术求解了Navier-Stokes方程,得到了涡脱落频率,升力脉动幅值及平均阻力系数.计算表明二维模拟不能体现流动基本特征,三维计算与实验吻合较好.为了抑制涡脱落,在直径为D的圆柱表面装入间距为1D,直径为0.0167D的O型环.通过升力、速度谱分析以及柱向横截面流场分析可知,在光滑圆柱外表面加入O型环能诱发流体边界层分离,有效地抑制涡脱落现象,升力脉动和观测点速度脉动幅值几乎完全消失,阻力系数也略微降低,适合在实际工程中采用.     

利用O型环抑制圆柱绕流涡脱落的数值研究

圆柱绕流涡脱落诱发较大的振动和声,如何有效地抑制值得关注。利用大涡模拟技术求解了Navier—Stokes方程,得到了涡脱落频率,升力脉动幅值及平均阻力系数。计算表明二维模拟不能体现流动基本特征．三维计算与实验吻合较好。为了抑制涡脱落,在直径为D的圆柱表面装入间距为1D．直径为0．0167D的O型环。通过升力、速度谱分析以及柱向横截面流场分析可知．在光滑圆柱外表面加入O型环能诱发流体边界层分离,有效地抑制涡脱落现象．升力脉动和观测点速度脉动幅值几乎完全消失,阻力系数也略微降低,适合在实际工程中采用。     

Numerical study of the suppression mechanism of vortex-induced vibration by symmetric Lorentz forces

In this paper, the electro-magnetic control of vortex-induced vibration (VIV) of a circular cylinder is investigated numerically based on the stream function–vorticity equations in the exponential–polar coordinates attached on the moving cylinder for Re=150. The effects of the instantaneous wake geometries and the corresponding cylinder motion on the hydrodynamic forces for one entire period of vortex shedding are discussed using a drag–lift phase diagram. The drag–lift diagram is composed of the upper and lower closed curves due to the contributions of the vortex shedding but is magnified, translated and turned under the action of the cylinder motion. The Lorentz force for controlling the vibration cylinder is classified into the field Lorentz force and the wall Lorentz force. The symmetric field Lorentz force will symmetrize the flow passing over the cylinder and decreases the lift oscillation, which, in turn, suppresses the VIV, whereas the wall Lorentz force has no effect on the lift. The cylinder vibration increases as the work performed by the lift dominates the energy transfer. Otherwise, the cylinder vibration decreases. If the net transferred energy per motion is equal to zero, the cylinder will vibrate steadily or be fixed.     

On the development of lift and drag in a rotating and translating cylinder

The two-dimensional flow around a rotating cylinder is investigated numerically using a vorticity forces formulation with the aim of analyzing quantitatively the flow structures, and their evolutions, that contribute to the lift and drag forces on the cylinder. The Reynolds number considered, based on the cylinder diameter and steady free stream speed, is Re=200, while the non-dimensional rotation rate (ratio of the surface speed and free stream speed) selected was α=1 and 3. For α=1 the wake behind the cylinder for the fully developed flow is oscillatory due to vortex shedding, and so are the lift and drag forces. For α=3 the fully developed flow is steady with constant (high) lift and (low) drag. Each of these cases is considered in two different transient problems, one with angular acceleration of the cylinder and constant speed, and the other one with translating acceleration of the cylinder and constant rotation. We characterize quantitatively the contributions of individual fluid elements (vortices) to aerodynamic forces, explaining and quantifying the mechanisms by which the lift is generated in each case. In particular, for high rotation (when α=3), we explain the relation between the mechanisms of vortex shedding suppression and those by which the lift is enhanced and the drag is almost suppressed when the fully developed flow is reached.     

圆柱绕流的电磁控制

流体绕过非流线物体时，在物体尾部形成涡街，使其表面周期性变化的阻力和升力增加，从而导致物体振荡，产生噪音。本文通过实验和计算，研究圆柱绕流的电磁控制，阐述浸于弱电介质溶液中，表面包覆电磁激活板的圆柱，在电磁力作用下的流体控制原理，讨论电磁力的消涡、减阻和减振过程。     

Suppression of lift fluctuations on a circular cylinder by inducing the symmetric vortex shedding mode

The flow around a stationary circular cylinder modified by two synthetic jets positioned at the mean separation points is numerically studied. The Reynolds number based on the free-stream velocity and the circular cylinder diameter is Re=500. The focus is to present a novel way to suppress the lift fluctuations by changing the vortex shedding mode, and thus particular attention is paid to the interactions between the synthetic jets and wake shear layers and the resulting vortex dynamics. The overall influences of both momentum coefficient and excitation frequency are discussed. In some simulated cases, the vortex lock-on phenomenon is discovered, which causes the typical Kàrmàn type vortex shedding to be converted into the symmetric shedding modes, leading to the complete suppression of lift fluctuations. In other cases, the asymmetric shedding mode still dominates the wake evolution. Detailed vortical evolution for each typical wake pattern is analyzed to reveal the control mechanism. Additionally, the control effectiveness is evaluated, indicating that the present control strategy contributes an effective way to suppress the lift fluctuations and reduce the mean drag.     

Frequency effects on lift and drag for flow past an oscillating cylinder

A transversely oscillating cylinder in a uniform flow is modeled to investigate frequency effects of flow-induced wake on lift and drag of the cylinder. Specifically, verified unsteady fluid dynamic simulations using an immersed-boundary method in a fixed Cartesian grid predict the flow structure around the cylinder and reveal how the integration of surface pressure and shear distributions provides lift and drag on the oscillating cylinder. In this study, frequency ranges to be considered are both near and away from the natural frequency of wake vortex shedding. Subsequently, the effects of frequency lock-in, superposition and demultiplication on lift and drag are discussed based on the spectral analysis of time histories of lift and drag.     

Numerical simulation with a TVD–FVM method for circular cylinder wake control by a fairing

A water drop-shaped fairing is applied to control the wake behind a circular cylinder and to suppress the formation of Karman vortex street in this paper. The results are evaluated using high resolution CFD technique. A finite-volume total variation diminishing (TVD) approach based upon the recently proposed elemental velocity vector transformation (EVVT) method, which aims at solving the incompressible turbulent flow for irregular boundary conditions with renormalization group (RNG) turbulence model, is used to simulate the flow field around circular cylinder systems. The calculations are carried out with cylinder systems with and without fairings, while the fairings have different top shape angles within the range of $30°~90°$. The Reynolds number ranges from 1000 to 50 000. It is shown that the simulation results of present numerical method reaches good agreement with the available experimental and numerical simulation data of typical circular cylinder flow and a fixed fairing cylinder system flow. Compared with bare cylinder, the faired bluff structures can obviously reduce the lift and drag forces and alter the vortex shedding frequency. Overall, the mean drag coefficient can be reduced up to about (10–31)% and the RMS lift coefficient can be reduced up to (30–99)% for all faired systems at given Reynolds numbers. The influence of Reynolds number and attack angles on the flow field characters of bare cylinder and faired cylinders is also discussed. The faired structures with shape angles within $30°~45°$under zero-attack-angle-inflow case are considered as the optimal structures, with which the mean drag coefficient and the RMS lift coefficient can be reduced up to (26–31)% and (98–99)%, respectively. Considering the influence of attack angles on lift and drag coefficients reduction, 75° shaped faired structure may be taken as a proper option.     

Passive control of wake flow by two small control cylinders at Reynolds number 80

Passive control of the wake behind a circular cylinder in uniform flow is studied by numerical simulation at Re_D=80. Two small control cylinders are placed symmetrically along the separating shear layers at various stream locations. In the present study, the detailed flow mechanisms that lead to a significant reduction in the fluctuating lift but maintain the shedding vortex street are clearly revealed. When the stream locations lie within 0.8≤X_C/D≤3.0, the alternate shedding vortex street remains behind the control cylinders. In this case, the symmetric standing eddies immediately behind the main cylinder and the downstream delay of the shedding vortex street are the two primary mechanisms that lead to a 70–80% reduction of the fluctuating lift on the main cylinder. Furthermore, the total drag of all the cylinders still has a maximum 5% reduction. This benefit is primarily attributed to the significant reduction of the pressure drag on the main cylinder. Within X_C/D>3.0, the symmetry of the standing eddy breaks down and the staggered vortex street is similar to that behind a single cylinder at the same Reynolds number. In the latter case, the mean pressure drag and the fluctuating lift coefficients on the main cylinder will recover to the values of a single cylinder.     

The PELskin project: part IV—control of bluff body wakes using hairy filaments

The passive control of bluff body wakes using a sparse layer of elastic hairy filaments has been investigated via a series of numerical simulations and compared to selected experiments under well-controlled boundary conditions. It has been found that a distribution of filaments spaced half of the dominant three dimensional instability and resonating with the main shedding frequency can drastically delay the three dimensional transition of the wake behind a circular cylinder. It will also be shown that when using a pair of rows of filaments symmetrically spaced by an azimuthal angle, the wake topology can be deeply affected as well as the value of the integral force coefficients of the cylinder. In the most favourable case, a coupled three dimensional transition delay and strongly reduced values of the drag and of the lift fluctuation can be simultaneously achieved. These results hold also for higher Reynolds-number flows as shown in experiments on a cylinder with hairy flaps attached to the aft part. The lock-in effect of structural vibration of the flaps with the vortex shedding is assumed to be the reason for a sudden change in the shedding cycle as soon as the motion amplitude is high enough to modify the wake. In line with this hypothesis, it has been demonstrated that a long elastic filament pinned on the centerline of a forced spatially developing mixing layer can interact with the vortex dynamics delaying the pairing process-leading to a reduced thickness of the layer. These findings show that a properly designed fluid structure interaction can indeed lead to technological benefits in terms of wake control: drag reduction, vibration control and possibly palliation of aeroacoustic emissions.