Computational and experimental study on flow around a rotationally oscillating circular cylinder in a uniform flow

The effect of rotational oscillation on flow field and fluid forces of a circular cylinder in a uniform flow is studied by three-dimensional computation by large eddy simulation and flow measurement by particle image velocimetry at Reynolds number 2000. The experimental characteristics of the flow indicate the enhancement of vortex shedding at natural frequency, the suppression of vortex shedding at high frequency and the recovery that follows. The above are accurately predicted by numerical simulations except for the slightly smaller estimate of wake suppression effect at high frequency. On the other hand, the fluid force characteristics on the cylinder are studied by numerical simulation, which indicates the drag enhancement and reduction corresponding to the variations of flow field at low and high frequencies, respectively. The drag reduction at high frequency is magnified with an increase in oscillation amplitude, but it can be saturated at higher amplitude. The mechanism of drag reduction is caused by the combined effect of forcing frequency and oscillation amplitude due to the modification of wake flow at high frequency and the delay in flow separation, while the fluctuating lift force slightly increases at a high frequency caused by the small-scale vortex shedding in synchronous with the rotational oscillation.     

An examination of the effect of boundary layer thickness on vortex shedding from a square cylinder near a wall

A computational study has been undertaken to examine the effect of boundary layer thickness δ/D on vortex shedding from a square cylinder in proximity to a solid wall. The computations were carried out in a second-moment turbulence modeling framework using a finite-volume technique. The computed results show that, in general, thickening of the wall boundary layer causes wake periodicity to persist for increasingly smaller cylinder-to-wall gap widths, S/D. The nature of the periodic motion changes as S/D approaches the critical value for complete suppression of vortex shedding. Similar to experimental observations, the location at which coupled shear layer motion is first observed shifts downstream of the base region. This shift is characterized by a rise in the shedding frequency and a drop in the time-averaged drag and lift on the cylinder. In addition, the pressure distribution along the lower wall is seen to change significantly due to the reduced size of the recirculation region in the cylinder wake.     

Re-examination of the effect of a plane boundary on force and vortex shedding of a circular cylinder

The hydrodynamic forces and vortex shedding of a smooth circular cylinder immersed in different boundary layers were experimentally investigated at Reynolds numbers from 1.30×10⁴ to 1.45×10⁴. The effects of the bed proximity, the thickness of the boundary layer, and the velocity gradient in the boundary layer on the pressure distribution, the hydrodynamic forces and the vortex shedding behavior were examined. The experimental results show that both the drag and lift coefficients strongly depend on the gap ratio, and are affected by the boundary layer. A downward lift is observed at certain gap ratios in rod-generated boundary layers, and an explanation of this downward lift is given. Two different criteria for calculating the Strouhal number in the literature are discussed in this paper. It is found that the variation of the root-mean-square (RMS) lift coefficient reveals the onset or suppression of the vortex shedding. A quantitative method for identifying the vortex shedding suppression point is proposed. The observations show that the vortex shedding is suppressed at a gap ratio of about 0.2–0.3, depending on the thickness of the boundary layer. This critical gap ratio decreases as the thickness of the boundary layer increases.     

Vortex-induced vibration of stay cable under profile velocity using CFD numerical simulation method

Vortex-induced vibration (VIV) of a stay cable subjected to a wind profile is numerically simulated through combining computational fluid dynamics (CFD) code CFX 10.0 and computational structural dynamics (CSD) code ANSYS 10.0. A stay cable with the inclined angle of 30° is used as the numerical model. Under a profile of mean wind speed, unsteady aerodynamic lift coefficients of the cable have been analyzed in both time domain and frequency domain when VIV occurs. The results indicate that the lift coefficient wave response of the stay cable under a wind profile is different from that of an infinitely long cable under a uniform flow in water (i.e., without consideration of profile) obtained by direct numerical simulation. Cable oscillations can severely affect the unsteady aerodynamic frequencies, change flow field distribution near the cable and affect the vortex shedding in the wake.     

Vortex-induced vibration of stay cable under profile velocity using CFD numerical simulation method

Vortex-induced vibration (VIV) of a stay cable subjected to a wind profile is numerically simulated through combining computational fluid dynamics (CFD) code CFX 10.0 and computational structural dynamics (CSD) code ANSYS 10.0. A stay cable with the inclined angle of 30° is used as the numerical model. Under a profile of mean wind speed, unsteady aerodynamic lift coefficients of the cable have been analyzed in both time domain and frequency domain when VIV occurs. The results indicate that the lift coefficient wave response of the stay cable under a wind profile is different from that of an infinitely long cable under a uniform flow in water (i.e., without consideration of profile) obtained by direct numerical simulation. Cable oscillations can severely affect the unsteady aerodynamic frequencies, change flow field distribution near the cable and affect the vortex shedding in the wake.     

An analytical study of the across-wind response of cylinders due to vortex shedding

The across-wind response of tapering cylinder-like structures, such as chimneys, due to periodic shedding of vortices is obtained under lock-in conditions. The method of analysis incorporates aerodynamic effects and describes the lift forces by a linear model monoharmonic in nature. Under lock-in conditions, the critical wind speed and the lift and aerodynamic coefficients are assumed to be uniform throughout the height of the chimney, although its outside diameter varies. The aerodynamic and lift coefficients are derived from a lift oscillator model which adequately describes the experimental results of the vortex-induced oscillation (at lock-in) of an elastically-supported cylinder. With the help of an example problem, the paper shows how the response of the chimney is sensitive to the choice of the critical diameter used for the determination of critical wind velocity and the lift and aerodynamic coefficients. It also emphasizes the importance of the lock-in condition in the second mode in relation to bending stresses.     

桥梁箱型吊杆涡振与驰振耦合振动的数值模拟

为研究桥梁箱型吊杆涡振与驰振耦合状态下的风致振动机理,基于某大跨度钢桁架拱桥大长细比箱型吊杆的节段模型风洞试验参数与试验结果,采用计算流体动力学（CFD）软件FLUENT对模型的耦合振动进行了数值模拟,通过FLUENT的后处理功能进一步研究分析了不同振动状态下的气动力和尾流旋涡特征信息。研究结果表明:数值计算得到的风振曲线与试验的实测结果吻合良好,在涡振与驰振的耦合振动过程中,随着振动幅值的不断增大,尾流旋涡脱落由小振幅阶段规则的卡门旋涡形态转化为大振幅阶段的复合模态特征,气动升力也由小振幅阶段的单频特征逐步转化为大振幅阶段的多频特征。     

Effect of rotational oscillation upon fluid forces about a circular cylinder

The present work aims at a computational study of the effect of various parameters that influence fluid-force behavior when a circular cylinder is subjected to rotational oscillations. A numerical simulation is conducted using the unsteady form of Reynolds-averaged Navier-Stokes equations combined with the k-ε model of turbulence. The study is carried out to examine the influence of various flow parameters, such as oscillation non-dimensional frequencies (0.1-2), rotational non-dimensional amplitudes (0.25-3) and Reynolds numbers (2000-30,000). Special attention is focused on the resonance condition at lower frequency and the subsequent drag reduction at higher frequency. It is found that the peak value of fluid forces on the circular cylinder increases abruptly when the forcing frequency is closer to the vortex shedding frequency. This is followed by a drag reduction at higher forcing frequencies, which becomes very strong at a non-dimensional oscillation frequency around 1 and at rotational amplitudes larger than 1. The former phenomenon is found to intensify with an increase in Reynolds numbers, but the latter almost preserves its strength in the range of Reynolds numbers considered here.     

Drag reduction on a circular cylinder using dual detached splitter plates

Drag reduction on a circular cylinder using dual detached splitter plates is numerically studied. Two splitter plates with the same length as the cylinder diameter (d) are placed along the horizontal centerline; one is upstream of the cylinder and the other is in the near-wake region, respectively. Their positions are described by the gap ratios G₁/d, G₂/d, where G₁ represents the gap between the cylinder stagnation point and the rear edge of the upstream splitter plate, and G₂ denotes the gap between the cylinder base point and the leading edge of the downstream splitter plate. The drag varies with the two gap ratios; it has the minimum value at a certain set of gap ratios for each Reynolds number. The upstream splitter plate reduces the stagnation pressure by friction, while the downstream one increases the base pressure by suppressing vortex shedding. This combined effect causes a significant drag reduction on the cylinder. In particular, the drag sharply increases past the optimum G₂/d; this is related to the restarted vortex shedding in the wake region.     

Aerodynamic stability of the downstream of two tandem square-section cylinders

The work reported in the present paper consists of three parts. In part one, the velocity distribution in the wake of a square cylinder at different distances from it (2⩽x/D⩽12) are measured and reported. Analytical expressions for the wake velocity distribution and for the correlation between wake half-width and downstream distance are obtained. The above expressions make it possible to estimate the wake velocity distribution without the availability of the actual experimental data. In part two, the lift and drag acting on the downstream of two cylinders are measured. The results are found to be in reasonable agreement (except in the range L/D=3–4 and T/D=2–3) with previous measurements, and are presented as contours of constant quantities, which make them useful to other researchers for quick information retrieval or estimation. Based on these steady flow results, the region where the downstream cylinder will become unstable to transverse galloping (static instability) are estimated and reported. In the next part, data are acquired with the downstream cylinder undergoing transverse oscillation. From the measurement of the phase angle between the body frequency component of the lift force and the cylinder displacement, the region where the downstream cylinder will be (dynamically) unstable to transverse galloping is estimated, and is found to be in good agreement with the estimation based on the steady flow results in the range L/D⩽4. The variations of the mean drag as well as the Strouhal number and fluctuating lift and drag of the downstream cylinder with reduced velocity are also measured at different L/D and A/D, and possible explanations for the behaviour of the data are offered.     

Aerodynamic forces on bluff structures in a wake

An experimental investigation into mean aerodynamic forces on a cylindrical structure in a wake of another parallel cylinder is carried out. Measurements of the aerodynamic response were taken for various configurations of streamwise and transverse spacings over a range of 12 000–73 000 of subcritical Reynolds number. On the basis of the geometric parameters and the flow Reynolds number dealt with in this study, empirical correlations of the normalized symmetric variations of drag coefficients as well as of the anti-symmetric behaviour of lift coefficients are presented. The surface pressure distributions lead to additional information about the roles of the main parameters involved in this complex flow situation. The data presented yield further information about the effects of turbulent shear flows on bluff structures and the formation of associated wakes.     

On the vortex shedding forcing on suspension bridge deck

Vortex-induced vibrations are not negligible while evaluating the aerodynamic and aeroelastic behavior of a long-span suspension bridge. This kind of forcing, that can produce high vibration levels, occurs at very low reduced velocities concerning low wind speed and high-frequency modes. This work presents results of experimental tests performed on the multibox deck shape of the Messina Strait bridge investigating vortex shedding phenomena and developing a numerical model to reproduce the vortex shedding forcing. The experimental tests, focused on low reduced velocities, highlight the typical non-linear pattern of the vortex shedding.     

深圳京基金融中心气动抗风措施试验研究

深圳京基金融中心高439m,风荷载是该超高层建筑的控制荷载。采用高频底座力天平方法对该建筑模型进行了风洞试验,考察了利用其顶部设备和避难层进行开敞形成不同的风走廊(气动措施)对结构风荷载和风致响应的影响。试验结果表明：在重现期100a敏感风向作用下结构漩涡脱落频率明显低于结构基阶固有频率;气动措施可显著抑制和削弱脱落漩涡的强度,当没有受到明显干扰影响时,气动措施显示出良好的抗风效果。不同气动措施可使重现期100a结构基底弯矩减少8.2%~21.2%,使重现期10a峰值加速度减少5.3%~16.0%;受到来自于地王大厦的干扰效应的影响,在所关注重现期风速范围内的结构风振响应为上游地王大厦的尾流所控制,影响了气动措施的控制效果,但在发生涡激共振的临界风速时气动控制措施效果显著。     

Experimental investigation of wake structure around an external rear view mirror of a passenger car

In order to investigate the wake structure around the external rear view mirror of a passenger car, velocity vector fields and spectra of velocity fluctuations in the mirror wake were measured by hot wire anemometry and laser Doppler velocimetry in a blow down wind tunnel at Re=200,000. Time-averaged velocity fields were measured on one streamwise vertical plane and three cross-sectional planes. In early wake, an alternating vortex was found along the upper and the lower edge line of the mirror housing, and in the downstream, a vortex wrap like a tip vortex was also found. A reverse flow region was observed at up to 1.2 times the mirror vertical width. In the moment measurements on the cross-sectional planes, peak values of the root-mean-square fluctuation velocity appeared near the tip edge of the mirror due to significant vortex shedding. Corresponding skewness and kurtosis factors clearly deviated from the Gaussian distribution, which shows mirror wake boundaries. Spectral characteristics of the mirror wake obtained on the cross-sectional planes showed that the frequencies of most of the vortex shedding in the mirror wake was less than 50 Hz, and that the peaks of the vortex shedding energy were near the outside edge of the mirror housing.     

Flow around two parallel circular cylinders in a linear shear flow

The flow around two parallel circular cylinders, subjected to a free stream linearly sheared in the spanwise direction was experimentally investigated in the subcritical range of Reynolds numbers. The study describes the effect of mutual interference on pressure distributions, force distribution and vortex shedding frequency along the span of each cylinder. In the tandem arrangement, it was found that there is an abrupt change in these distributions at $\text{L/D = 3.5}$. For $\text{L/D < 3.5}$, there is no vertex shedding from the upstream cylinder and vortex shedding from the downstream one breaks down into spanwise cells. For $\text{L/D > 3.5}$, vortices are shed from the two cylinders such that the Strouhal number, based on local shedding frequency and local free stream velocity, is constant along the span of each cylinder. In the side-by-side arrangement, it was found that for $\text{1.1 < T/D < 1.8}$, the flow around the two cylinders is asymmetrical and bistable. It was also found that at any value of $\text{T/D}$, the Strouhal number based on local vortex shedding frequency and local free stream velocity, is constant along the span.     

Flow patterns and vortex shedding behavior behind a square cylinder

The flow structures, wake-flow characteristics and drag coefficients of a square cylinder at various Reynolds numbers (Re) and incidence angles (θ) were experimentally studied in an open-loop wind tunnel. The cross section of square cylinder is characterized by the aspect ratio (AR) and blockage ratio (BR) of 25% and 4%, respectively. The Reynolds number is changed from 4000 to 36,000 and the incidence angle is adjusted from 0° to 45°. The flow patterns near/behind the square cylinder were determined using the smoke-wire scheme. The global velocity fields and streamline patterns were analyzed using the particle image velocimetry (PIV). Additionally, the flow-topology method was applied to analyze the flow patterns by calculating the separatrices, alleyways and critical points. Experimental results showed that the flow structures around the square cylinder exhibit three modes—leading-edge separation, separation bubble and attached flow. The surface-pressure profile, drag coefficient (C_D), lift coefficient (C_L) and vortex shedding frequency were detected/calculated using a pressure transducer and hot-wire anemometer. The lift coefficient did not significantly vary with Re. The minimum C_D occurs at θ=12°, whereas the minimum C_L occurs at θ=13°. The minimum projected-Strouhal-number (St_d) occurs at θ=0° while the maximum St_d occurs at θ=15°.     

Across-wind vibrations of structures of circular cross-section. Part I. Development of a mathematical model for two-dimensional conditions

A model is presented for predicting the across-wind response of constant-diameter circular cylinders vibrating in a mode of uniform amplitude and subject to uniform flow. A key feature of the model is the representation of all motion-dependent phenomena by a nonlinear aerodynamic damping force. This force coexists with the fluctuating force which arises from vortex shedding on a stationary cylinder, and the two forces are assumed to be uncorrelated.The ability of the device used in representing the motion-induced force to model certain aeroelastic characteristics associated with vibrating cylinders is demonstrated. The device is shown to be capable of successfully reproducing two effects; namely, the increase of the spanwise correlation of forces with increasing amplitude, and the phenomenon of “lock in” where the shedding frequency is apparently dictated by the vibration frequency.The model is developed within the framework of random-vibration theory, and a number of simplifying assumptions are necessary to incorporate the nonlinear aerodynamic damping force and also to account for the influence of turbulence. Numerical experiments, undertaken to examine the nature of the approximations involved in the assumptions adopted, are described. The results of the numerical experiments are very encouraging and justify the simplifications made in the modelling process.     

Interference effects on aerodynamic and wind-induced responses for an upstream interfering building with small section size

上游来流经小截面施扰建筑可产生频率较高的漩涡，在较低的风速下会使受扰建筑发生涡激共振而受到较大的干扰作用。为此，通过同步测压风洞试验和风振响应计算，详细分析上游施扰建筑与下游受扰主建筑的截面宽度比为0.4时的受扰建筑基底气动弯矩、基底峰值弯矩响应以及结构顶部峰值加速度的干扰效应。结果表明：在B类和C类地貌风场下受扰建筑基底平均和脉动气动弯矩均表现为遮挡效应和弱放大效应，最大干扰系数仅为1.05；顺风向、横风向基底峰值弯矩响应和顶部峰值加速度的干扰效应都较基底平均和脉动气动弯矩的强；结构顶部峰值加速度的干扰效应明显强于基底峰值弯矩响应，其中在B类和C类地貌风场下的横风向峰值加速度均在串列位置(2b, 0)附近处呈现强放大效应，最大包络干扰系数分别为4.7和3.03。进一步对尾流涡激共振的干扰机理研究表明，基底峰值弯矩响应和结构顶部峰值加速度响应的放大效应干扰机理是一致的。对于受小截面施扰建筑影响的情况，仅分析气动力的干扰效应是不够的，必须考虑不同折算风速下的荷载响应包络干扰效应。     

小截面建筑干扰下的高层建筑气动力及风致响应研究

上游来流经小截面施扰建筑可产生频率较高的漩涡,在较低的风速下会使受扰建筑发生涡激共振而受到较大的干扰作用。为此,通过同步测压风洞试验和风振响应计算,详细分析上游施扰建筑与下游受扰主建筑的截面宽度比为0.4时的受扰建筑基底气动弯矩、基底峰值弯矩响应以及结构顶部峰值加速度的干扰效应。结果表明：在B类和C类地貌风场下受扰建筑基底平均和脉动气动弯矩均表现为遮挡效应和弱放大效应,最大干扰系数仅为1.05;顺风向、横风向基底峰值弯矩响应和顶部峰值加速度的干扰效应都较基底平均和脉动气动弯矩的强;结构顶部峰值加速度的干扰效应明显强于基底峰值弯矩响应,其中在B类和C类地貌风场下的横风向峰值加速度均在串列位置(2b, 0)附近处呈现强放大效应,最大包络干扰系数分别为4.7和3.03。进一步对尾流涡激共振的干扰机理研究表明,基底峰值弯矩响应和结构顶部峰值加速度响应的放大效应干扰机理是一致的。对于受小截面施扰建筑影响的情况,仅分析气动力的干扰效应是不够的,必须考虑不同折算风速下的荷载响应包络干扰效应。     

Experimental investigation on the connection between flow fluctuations and vorticity dynamics in the near wake of a triangular prism placed vertically on a plane

Experiments are carried out to investigate on the connection between flow fluctuations and dynamics of different vorticity structures in the wake of an equilateral triangular prism placed vertically on a plane with its apex edge against the incoming flow. Fluctuations at three frequencies are found to dominate in different wake regions; the highest frequency is caused by the alternate vortex shedding from the lateral vertical edges of the prism, while the lowest frequency is connected with an oscillation of the streamwise vorticity structures detaching from the free-end. The fluctuations at the intermediate frequency are shown to originate from an oscillation of the transversal vorticity sheet bounding the recirculation region behind the body. Geometrical modifications are then introduced to successively interfere with the dynamics of the different vorticity structures and thus to identify possible interactions between the various fluctuations. Indentations along the prism vertical edges produce an increase of the mean wake width and a consequent decrease of both the vortex shedding and intermediate frequencies, whereas the low-frequency fluctuations are not affected. The latter remain unaltered even when irregularities are added to the free-end edges and the streamwise vortices are seen to roll-up with a different formation process.