The Effect of Internal Fluid on the Response of Vortex-Induced Vibration of Marine Risers

Based on Iwan′s wake oscillator model developed with the classical van der Pol equation, the differential equation for the response of the vortex-induced vibration (VIV) of the riser considering the effect of the internal flowing fluid and the external marine environmental condition is derived. The effect of the internal flowing fluid on the response of VIV of the riser is studied by means of the Finite Element Method. The results show that the effect of the internal fluid velocity on the VIV of the riser is strong when the natural frequency of the riser is close to the vortex shedding frequency. In addition, the increase of the top tension can decrease the sensitivity of the riser to the internal fluid velocity.

Vortex-induced vibration of pipes conveying fluid using the method of multiple scales

The nonlinear dynamics of supported pipes conveying fluid subjected to vortex-induced vibration is evaluated using the method of multiple scales. Frequency response portraits for different internal fluid velocities under lock-in conditions are obtained and the stability of steady-state responses is discussed. Results show that the internal fluid velocity has a prominent effect on the oscillation amplitude and that the steady-state responses incorporating unstable solutions in the lock-in region are also obtained. In addition, the effects of two kinds of fluctuating lift coefficients on the steady-state responses are compared with each other.

Effect of perforation on flow past a conic cylinder at Re=100:vortex-shedding pattern and force history

The flow past a circular-section cylinder with a conic shroud perforated with four holes at the peak was simulated numerically at Re = 100, considering two factors, viz.the angle of attack and the diameter of the holes. The effects of the perforated conic shroud on the vortex shedding pattern in the near wake was mainly investigated, as well as the time history of the drag and lift forces. In the investigated parameter space, three flow regimes were generally identified, corresponding to weak, moderate, and strong disturbance effects.In regime I, the wake can mainly be described by alternately shedding Kármán or Kármán-like vortices. In regime II, the spanwise vortices are obviously disturbed along the span due to the appearance of additional vorticity components and their interactions with the spanwise vortices, but still shed in synchronization along the spanwise direction. In regime III,the typical Kármán vortices partially or totally disappear,and some new vortex shedding patterns appear, such as-type, obliquely shedding, and crossed spanwise vortices with opposite sign. Corresponding to these complex vortex shedding patterns in the near wake, the fluid forces no longer oscillate regularly at a single vortex shedding frequency, but rather with a lower modulation frequency and multiple amplitudes. An overview of these flow regimes is presented.

Numerical simulation of a short flexible pipe subject to forced motion and vortex-induced vibration

A series of numerical sinmlations about a small scale （aspect ratio： 63.2） flexible pipe undergoing forced harmonious oscillation and vortex-induced vibration （VIV） have been taken into account. The wake hydrodynamics and pipe deformation were accomplished by ANSYS MFX solution strat- egy designed for fluid-structure interaction （FSI） problem with well-performed LES model. The configuration of structured mesh, multi-domain design, different mesh stiffness admeasured by User Fortran ensured that the numerical task was competent to deal with large deformation related to this case. The introduction of instantaneous amplitude definition and modeless component decom- position method （Chen and Kim, 2008） was helpful to reveal much more information from modal analysis. Most results from numerical simulation are generally consistent with those from model test （Choi and Hong, 2000） via the comparison between them. As supplementary to model test, visualization of the vortex wake was also provided. It has been proved that the forced oscillation doesn＇t only excite a complicated dumbbell-like wake pattern around the outer thimble, but also results in inner flow inside the PVC pipe. The velocity of the inner flow increases with the frequency of forced oscillation.

Anomalous Viscosity of Vortex Hall States in Graphene

We study temperature effect on anomalous viscosity of Graphene Hall fluid within quantum many-vortex hydrodynamics. The commonly observed filling fraction, in the range is considered. An expression for anomalous viscosity dependent on a geometric parameter-Hall expansion coefficient is obtained at finite temperatures. It arises from strained induced pseudo-magnetic field in addition to an anomalous term in vortex velocity, which is responsible for renormalization of vortex-vortex interactions. We observed that both terms greatly modify the anomalous viscosity as well as an enhancement of weakly observed v fractions. Finite values of the expansion coefficient produce constant and infinite viscosities at varying temperatures. The infinities are identified as energy gaps and suggest temperatures at which new stable quantum hall filling fractions could be seen. This phenomenon is used to estimate energy gaps of already measured fractional Quantum Hall States in Graphene.

Heat Transfer Enhancement by Vortex Generators for Compact Heat Exchanger of Automobiles

The paper describes the effects of heat transfer enhancement and gas-flow characteristics by wing-type-vortex-generators inside a rectangular gas-flow duct of a plate-fin structure exhaust gas recirculation (EGR) cooler used in a cooled-EGR system. The analyses are conducted using computational fluid dynamics (CFD). The numerical modelling is designed as a gas-flow rectangular duct of an EGR cooler using two fluids with high temperature gas and coolant water whose flow directions are opposite. The gas-flow duct used to separate two fluids is assembled with a stainless steel material. The inlet temperature and velocity of gas flowed inside gas-flow duct are 400°C and 30 m/s, respectively. Coolant water is flowed into two ducts on both a top and a bottom surface of the gas-flow duct, and the inlet temperature and velocity is 80°C and 0.6 m/s, respectively. Wing-type-vortex-generators are designed to achieve good cooling performance and low pressure drop and positioned at the center of the gas-flow duct with angle of inclination from 30 to 150 degrees at every 15 degrees. The temperature distributions and velocity vectors gained from numerical results were compared, and discussed. As a result, it is found that the vortices guided in the proximity of heat transfer surfaces play an important role in the heat transfer enhancement and low pressure drop. The collapse of the vortices is caused by complicated flow induced in the corner constituted by two surfaces inside gas-flow duct.

Simulation of Vortex-Induced Vibrations of A Cylinder Using ANSYS CFX

In this paper, vortex-induced vibrations of a cylinder are simulated by use of ANSYS CFX simulation code. The cylinder is treated as a rigid body and transverse displacements are obtained by use of a one degree of freedom spring damper system. 2-D as well as 3-D analysis is performed using air as the fluid. Reynolds number is varied from 40 to 16000 approx., covering the laminar and turbulent regimes of flow. The experimental results of （Khalak and Williamson, 1997） and other researchers are used for validation purposes. The results obtained are comparable.

Numerical Simulation of the Vortex-Induced Vibration of A Curved Flexible Riser in Shear Flow

A series of fully three-dimensional(3 D) numerical simulations of flow past a free-to-oscillate curved flexible riser in shear flow were conducted at Reynolds number of 185–1015. The numerical results obtained by the two-way fluid–structure interaction(FSI) simulations are in good agreement with the experimental results reported in the earlier study. It is further found that the frequency transition is out of phase not only in the inline(IL) and crossflow(CF) directions but also along the span direction. The mode competition leads to the non-zero nodes of the rootmean-square(RMS) amplitude and the relatively chaotic trajectories. The fluid–structure interaction is to some extent reflected by the transverse velocity of the ambient fluid, which reaches the maximum value when the riser reaches the equilibrium position. Moreover, the local maximum transverse velocities occur at the peak CF amplitudes, and the values are relatively large when the vibration is in the resonance regions. The 3 D vortex columns are shed nearly parallel to the axis of the curved flexible riser. As the local Reynolds number increases from 0 at the bottom of the riser to the maximum value at the top, the wake undergoes a transition from a two-dimensional structure to a 3 D one. More irregular small-scale vortices appeared at the wake region of the riser, undergoing large amplitude responses.

不同生理条件下主动脉双叶机械心脏瓣膜血流动力学分析

采用计算流体动力学方法对不同生理条件下主动脉位置双叶机械心脏瓣膜下游的速度分布、涡演化、粘性剪切应力、雷诺剪切应力进行研究。收缩期峰值时,运动状态下血液射流较其他两种生理状态下更为强烈,最大血液流速为2.1 m/s。对于涡演化的分析表明3种生理状态下剪切层是比较明显的流动特征,在收缩期峰值时,血液与主动脉窦作用较为强烈。对于粘性剪切应力,当瓣叶完全打开时,瓣叶在瓣膜区阻碍了血液流过瓣膜,血流与瓣叶之间的相互作用导致了较高的剪切应力,3种生理状态下,观察到的最大粘性剪应力小于8 N/m^(2)。3种生理状态下观察到最大的雷诺剪切应力为700 N/m^(2)。本研究有助于为临床手术瓣膜选择以及术后康复提供理论依据。

大型岔管内螺旋流涡带数值分析研究

岔管部件广泛应用于常规水电站、抽水蓄能电站等重要水力发电工程。大型岔管(D主≥6 m)由于岔管外形,肋板结构及内部流态三因素耦合可能导致螺旋流涡带,其对岔管的水力损失具有不可忽略的消极后果。本文基于经典k-ε湍流模型联合考虑实际岔管材料壁面粗糙率,构建了考虑壁面真实粗糙度的钢岔管内部流态k-ε湍流模型。计算结果与实验数据对比验证了本文构建数值模型的有效性及准确性。随后采用导流叶片策略有效消除了大型岔管内的螺旋流涡带现象。研究结果表明:多导流叶片优化方案可有效减少和消除大型岔管中的螺旋流态,并减少岔管水力损失。具体地,对岔管支管路中不同流量工况,优化后的主管路部分流线分布均匀,最大流速位于岔管支管路弯管(内弯)附近;优化后岔管肋板两端涡旋流态基本消除。对岔管支管流量不均匀工况,支管路内螺旋流涡带流态明显减缓,只存在少量螺旋流态。本文研究结果可对岔管水力优化设计及相近研究领域提供一定参考。

Numerical Study on the Effect of Current Profiles on Vortex-Induced Vibrations in a Top-Tension Riser

In this paper, numerical simulations of vortex-induced vibrations in a vertical top-tension riser with a length-to-diameter ratio of 500 using our in-house code viv-FOAM-SJTU are presented. The time-dependent hydrodynamic forces on two-dimensional strips are obtained by solving the Navier-Stokes equations, which are, in turn, integrated into a finite-element structural model to obtain the riser deflections. The riser is discretized into 80 elements with its two ends set as pinned and 20 strips are located equidistant along the risers. Flow and structure are coupled by hydrodynamic forces and structural displacements. In order to study the effects of the shear rate, of the current profiles on the vortex-induced vibrations in the riser, vibrations, with varying shear rates, in both the in-line and cross-flow directions, are simulated. In addition to the time domain analysis, spectral analysis was conducted in both the temporal and spatial domains. Multi-mode vibration characteristics were observed in the riser. The relationship between dominant vibration mode number and the shear rate of current profiles is discussed. In general, the overall vibrations in the riser pipe include contributions from several modes and each mode persists over a range of shear rates. Moreover, the results suggest that with a larger shear rate the position of the maximum in-line time-averaged displacement will move closer to the end where the largest velocity is located.

控制杆-强制旋转对圆柱涡激振动的抑制作用研究

为实现圆柱涡激振动的高效抑制,本文基于控制杆-强制旋转组合结构讨论圆柱涡激振动的抑制问题。对低雷诺数下的圆柱涡激振动进行数值模拟研究,对比分析不同控制杆数量和圆柱转速比下的流场特性、振动响应和流体力响应等。结果表明:与裸圆柱相比,控制杆具有一定的涡激振动抑制效果;对于三、五控制杆,通过调节转速比,可以实现最高98%的圆柱振幅抑制(对三控制杆,转速比为0.4~0.6;对五控制杆,转速比为0~0.2);圆柱振幅抑制与圆柱减阻区间相同;对于四、六控制杆,旋转对振幅抑制的影响较小;主圆柱表面的漩涡脱落和漩涡的合并会引起流体力系数的高幅波动,导致圆柱高振幅响应;没有漩涡合并和圆柱表面脱涡不明显时,流体力系数则无明显波动,圆柱振幅也显著降低。

圆端形墩台尾迹水动力特性开缝控制三维数值分析

水流绕流水闸闸墩、导流墩等水工建筑物后产生旋涡分离流,其三维尾迹结构水动力特性复杂,对旋涡分离流进行流动控制,不仅是闸墩优化设计的需要,而且关系到工程运行的效率和工程安全。在验证三维水动力数学模型的基础上,对圆端形墩台开缝后的旋涡分离流和流动控制的三维水动力特性进行了研究,对比了无缝与开缝墩台方案下纵向、横向和竖向方向的时均流速分布,紊流动能以及紊流强度分布等水力学指标,探究了开缝墩台水动力特性。分析表明,墩台开缝后可以明显改善墩台后方的水流结构,开缝位置越靠近水深的中部位置,对墩后尾流旋涡的抑制越大,对绕流墩台绕流的控制越来越好,水深中部位置开缝最大程度的影响了开缝墩后的上部水流和下部水流,使得开缝对时均流速的影响最大,开缝对旋涡的抑制也最大最佳。为抑制墩后的旋涡对工程的不利影响,工程的设计中应首先确定墩台后的水深,开缝位置应尽量设置在墩台后水深的中部位置。

Interactive Visualization System of Taylor Vortex Flow Using Stokes' Stream Function

Taylor vortex flow between two concentric rotating cylinders with finite axial length includes various patterns of laminar and turbulent flows, and its behavior has attracted great interests. When mode bifurcation occurs, quantitative parameters such as the volume-averaged energy change rapidly. It is important to visualize the behaviors of vortices. In this study, a three-dimensional visualization system with respect to time is devised. This system can change the viewpoint of flow visualization, and we can observe the track of a vortex from any point. The volume-averaged energy is projected to the track of the center of a vortex. The proposed system can help to investigate the relationship between the mode bifurcation process and the volume-averaged energy.

停机时风力机塔筒与叶片的涡激机理

大型化风力机存在塔筒与叶片的脱落涡相互诱导的现象,极大影响机组的安全运行。为探究两者涡激互扰特性,该文以NREL 5 MW风力机为研究对象,分别选取距离地面65、70、75和80 m高度的塔筒与叶片共存的二维截面,开展6、8、10、12与14 m/s风况下数值模拟,并与单塔筒工况计算结果进行对比分析。研究结果表明,叶片与塔筒的尾涡存在严重相互诱导干涉;与单塔筒绕流工况相比,塔筒特定点压力值增长了1.5倍,升力系数出现多个响应幅值且其最大约为单塔筒的6倍;塔筒与叶片尾涡出现相互合并且脱落频率一致,涡激作用增强;塔筒-叶片工况下的涡脱频率较单塔筒降低,更易与风力机低阶固有频率出现交叉,风力机极可能出现共振。

高雷诺数下质量比对圆柱涡致振动影响分析

涡致振动分析可为柔性结构抗风研究与工程应用提供重要理论依据。采用基于非平稳雷诺均值Navier-Stokes方程(Unsteady Reynolds-Averaged Navier-Stokes equations,URANS)的数值方法模拟了高雷诺数、高质量比条件下单向自由度圆柱的涡激振动。在改进的剪切应力输运k-ω(Shear Stress Transport,SST k-ω)模型基础上,运用动网格技术实现流固耦合,并结合Newmark-β方法求解结构运动方程。在雷诺数为1.39×104~1.04×105、折算速度为2~12的条件下,模拟分析了圆柱在不同折算速度时的涡激振动响应,得到了不同质量比时圆柱涡激振动的振幅比、振动频率比、升力系数等的变化特点,讨论了圆柱振动响应和旋涡脱落特性,分析总结了高雷诺数下不同质量比的圆柱涡激振动特性规律。结果表明:数值结果与已有试验的吻合程度较好;雷诺数和质量比对圆柱的涡激振动影响效果明显,对进一步分析圆柱涡致振动具有参考意义。

“Click”运动高升力机制

研究了具有“Click”运动的模型翼在三维流场中的气动特性.采用计算流体力学方法求解三维非定常不可压缩Navier-Stokes方程,对比了简谐和“Click”运动所产生的升力与阻力,研究了升力最大时的前缘涡特性和展向流动.结果表明,在下拍过程的升力最大处,“Click”和简谐运动的前缘涡能够分别维持至翼展的80%和60%处.两种运动都会在翼展方向存在维持展向流动的压力梯度,而“Click”运动具有更显著的展向压力梯度,对展向涡度的能量输运效率更高,因此其前缘涡能够保持更完整的形态.在上拍过程中,两种运动的前缘涡形态差异不明显,“Click”运动快速上拍的运动特点使其能够快速地摆脱尾涡的干扰,从而维持升力.

脉动顶张力作用下输流立管涡激振动主共振研究

为定量描述海洋工程中输流立管受脉动顶张力作用时的涡激振动响应,本文研究了海洋输流立管受到内部流体、脉动顶张力和涡流激励作用时的参数主共振问题。首先建立了该问题的数学模型,运用Galerkin法和多重尺度法对其进行求解,引入调谐参数描述输流立管第一阶固有频率同顶张力脉动频率和涡流频率的接近程度,通过改变调谐参数,研究输流立管的动力学响应。结果表明:定常顶张力作用下,立管的振幅为单值,随着调谐参数增大,立管振幅先增大后减小,且出现明显的跳跃现象,内部流体速度和张力幅值的竞争机制也会影响立管振幅;脉动顶张力作用时,在一阶锁频的条件下,随着调谐参数变化,立管的振幅呈现“多值—单值—多值”的变化规律,顶张力均值或脉动幅值越大,立管振幅的变化范围越宽。

考虑质量比影响的双圆柱涡激振动发电装置数值模拟研究

双圆柱涡激振动发电装置是一种利用海流能发电的新型能源装置。该研究使用Fluent软件模拟了双圆柱涡激振动发电装置的水动力特性。利用用户自定义函数和动网格技术实现了圆柱振子的流固耦合模拟,分析圆柱振子的涡激振动特性。首先,进行网格和时间步长的无关性验证,证明了该数值模拟的准确性;其次,从位移时程曲线、幅值、频率三个方面分析圆柱振子的涡激振动特性;最后,通过对等质量比下串列双圆柱涡激振动发电装置圆柱振子的获能功率及能量转换效率的分析,研究质量比对双圆柱涡激振动发电装置的俘获能特性的影响。研究表明:当质量比m^(*)=3、4、5时,下游圆柱的横流向振幅随约化速度的增加而逐渐大于上游圆柱的横流向振幅;不同质量比下双圆柱振子的横流向无量纲振幅均呈现先随约化速度的增加而增加,然后又减小的趋势;当质量比m^(*)=4时,上、下游圆柱的能量转换效率最高,分别为32.6%、22.8%;当质量比m^(*)=5时,下游圆柱的能量转换效率最高,为30.5%。

考虑双向流固耦合的洪水作用下穿越河流管道力学行为分析

为了解决洪水作用下穿越河流管道单向流固耦合未考虑管道对洪水影响而导致结果不准确的问题,提出洪水-管道双向流固耦合有限元计算模型,基于该模型开展不同因素下管道应力、位移和振动频率的力学行为分析。研究结果表明:管道应力、位移与洪水流速、悬空长度呈强正相关,管道存在屈服破坏和变形破坏的风险,且输油管道尤其明显;管道涡激振动频率与洪水流速、悬空长度呈强相关,这2个因素增大导致管道共振风险增大,且输气管道尤其明显;涡激振动引起的交变荷载导致管道累计损伤,管道存在疲劳破坏的风险。因此,管道设计时应采取预防性措施以应对洪水灾害,特别关注洪水流速、悬空长度和输送介质的影响,以避免管道发生破坏。研究结果对穿越河流管道的安全评价及可靠性研究具有参考价值。