On vibrations induced in a cylinder in the wake of another due to vortex shedding

Summary Purpose of this work is to describe the results obtained in a wind-tunnel on a model consisting of two cylinders, one in the wake of the other, with both fixed or with the downstream cylinder allowed to vibrate.The fundamental characteristics of the fluid in the wake and the forces acting on the downstream cylinder are also reported. The value of energy introduced by the fluid on a vibrating cylinder in the wake is also determined.

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Sommario Vengono riportati i risultati sperimentali ottenuti in galleria del vento su un modello costituito da due cilindri, di cui uno è posto in scia dell'altro, nei casi in cui siano entrambi fissi o che il cilindro in scia possa vibrare.Sono pure riportate le caratteristiche fondamentali del fluido nella scia e le forze che agiscono sul cilindro a valle. Nel caso in cui il cilindro in scia possa vibrare viene pure riportato il valore dell'energia introdotta dal fluido.

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This work is the outcome of a collaboration between ENEL, Salvi S.p.A. and the Institute of Applied Mechanics, Polytechnic of Milan. The Institute of Applied Mechanics received a CNR subsidy to take part in this research.     

Phase-resolved characterization of vortex shedding in the near wake of a square-section cylinder at incidence

The vortex formation and shedding process in the near wake region of a 2D square-section cylinder at incidence has been investigated by means of particle image velocimetry (PIV). Proper orthogonal decomposition (POD) is used to characterize the coherent large-scale flow unsteadiness that is associated with the wake vortex shedding process. A particular application of the POD analysis is to extract the vortex-shedding phase of individual velocity fields, which were acquired at asynchronous low rate with respect to the vortex shedding cycle. The phase of an individual flow field is determined from its projection on the first pair of POD modes, allowing phase averaging of the measurement data to be performed. In addition, a low-order representation of the flow, constructed from the mean and the first pair of POD modes, is found to be practically equivalent to the phase-averaged results. It is shown that this low-order representation corresponds to the basic Fourier component of the flow field ensemble with respect to the reconstructed phase. The phase-averaged flow representations reveal the dominant flow features of the vortex-shedding process and the effect of the angle of incidence upon it.     

Effect of novel synthetic jet on wake vortex shedding modes of a circular cylinder

A novel actuator signal achieved by changing the ratio of the suction duty cycle to the blowing duty cycle is adopted to enhance the control effect of the synthetic jet for the flow around a circular cylinder. The suction duty cycle factor k defined as the ratio between the time duration of the suction cycle and the blowing cycle and the equivalent momentum coefficient C_μ are introduced as the determining parameters. The synthetic jet is positioned at the rear stagnation point in order to introduce symmetric perturbations upon the flow field. The proper orthogonal decomposition (POD) technique is applied for the analysis of the spanwise vorticity field. Increasing the suction duty cycle factor, the momentum coefficient is enhanced, and thus a stronger and larger scale synthetic jet vortex pair with a higher convection velocity is generated. The synthetic jet vortex pair interacts with the spanwise vorticity shear layers behind both sides of the cylinder, resulting in the variations of the wake vortex shedding modes at Re=950: for k=0.25, C_μ=0.148, vortex synchronization at the subharmonic excitation frequency with antisymmetric shedding mode; for 0.50≤k≤1.00, 0.213≤C_μ≤0.378, vortex synchronization at the excitation frequency with the symmetric or antisymmetric shedding modes; for 2.00≤k≤4.00, 0.850≤C_μ≤2.362, vortex synchronization at the excitation frequency with symmetric shedding mode. Hence, the control effect of the synthetic jet upon the wake vortex of a circular cylinder can be enhanced by increasing the suction duty cycle factor so as to increase the momentum coefficient. This is also validated at a higher Reynolds number Re=1600.     

The turbulent wake of a circular cylinder rotating about the streamwise axis

This paper presents measurements in the turbulent wake of a circular cylinder rotating with its axis normal to the free-stream velocity; in other words, the axis of rotation was parallel to the streamwise direction. All three mean velocities and six Reynolds stresses were obtained at three positions downstream of the cylinder, with and without rotation of the free-stream. Most emphasis is given to the latter results because of the better flow quality. The ratio of the circumferential velocity of the cylinder to the free-stream velocity — the swirl number — had a maximum value of 0.6. Measurements for two combinations of the free-stream and angular velocities showed the velocity deficit in the wake to be a multi-valued function of the swirl number, implying that the rotation affected the separation of the cylinder's boundary layer in a complex manner. In the turbulent wake, the rotation did not significantly alter the magnitudes of the normal stresses, but caused large changes to the shape of the profiles of the axial and cross-stream normal stresses. Eventually, the primary (cross-stream) shear stress became almost entirely positive, but there was no corresponding change to the (cross-stream) gradient of the streamwise mean velocity. Despite these alterations to the turbulence, the rotationally-activated generation terms in the Reynolds transport equations never dominated the terms that are common to the wakes of rotating and non-rotating cylinders.This work was supported by the Australian Research Council. Most of the data acquisition software was written by Mr J. J. Smith.     

Feedback control of vortex shedding from a circular cylinder

This paper presents an experimental study on the suppressing of vortex shedding from a circular cylinder by feedback sound. Experiments were performed in a wind tunnel, and the feedback sound was generated inside the cylinder and locally introduced into the flow through a thin slit on the cylinder surface. In this way, the shear flow nearest to the slit was directly manipulated during the control. The results show that the suppression of vortex shedding can be achieved at Reynolds numbers ranging from 4×10³ to 1.3× 10⁴, according to signals from a hot-wire checking throughout the wake and signals from a remote microphone. This local and one-sided feedback, being different from other control techniques, allows a better understanding of the control mechanism, which in this case probably causes a destructive interaction between two shear flows separated from both sides of the cylinder. The technique has been useful to deepen our understanding to the wake instabilities behind the cylinder.This work was done when the author was a visiting Scientist at the Institute of Technical Acoustics, Technical University Berlin. The author wishes to express his gratitude to Prof. M. Heckl and Prof. M. Möser for the arrangement and encouragement in this research. The help from Mr. M. Hansen in the experiment is acknowledged. Thanks also go to the German Science Foundation for the financial support.     

Further study on vortex shedding flow by a circular cylinder

In the present paper the mechanism involved in vortex shedding flows is investigated in detail. In the early stage of the unsteady separated flow the interaction between secondary vortices and primary vortices is quite strong. In the later stage of the flow, corresponding to the vortex shedding the recirculating flow region on each side of the aft body goes through such a cycle: growth-contraction-growth, the secondary separation occurs periodically rather than continuously. The reduction of circulation is taken into account in three cases with different decay factors to study its influence on the prediction of main flow characteristics. Results show that to simulate vortex shedding flow it is necessary to include the reduction of circulation to bring the calculated results into good agreement with experiments. An improved discrete vortex model is suggested in which both the secondary separation and the reduction are incorporated. The processes of vortex shedding, the forces prediction and other flow characteristics are given and some discussions are made. Porject is supported by National Natural Science Foundation of China.     

Feedback control of vortex shedding from a circular cylinder by cross-flow cylinder oscillations

Feedback control of vortex shedding from a circular cylinder in a uniform flow at moderate Reynolds numbers is studied experimentally with the cylinder subjected to feedback cylinder oscillations in cross-flow direction. The cylinder oscillation is digitally phase shifted with respect to the shedding vortex and is controlled by velocity feedback from the shear layer of the cylinder wake. Possible attenuation of vortex shedding is demonstrated by hot-wire measurements of the flow field and its mechanisms are studied by simultaneous data sampling and flow visualization with the smoke wire method and a laser-sheet illumination technique. Measurement results reveal substantial reduction in the fluctuating reference velocity at the optimum phase control. Flow visualization study indicates that the shear layer roll-up and the eventual vortex formation are dynamically attenuated under the control which results in a modification of the near wake.List of symbols A amplitude of cylinder oscillation - D cylinder diameter - E _u power spectrum function for fluctuating velocity u - frequency - R radius of circular cylinder - t time - u streamwise mean velocity - u streamwise fluctuating velocity - U streamwise mean velocity of main flow - u _r mean reference velocity - u _r fluctuating reference velocity - u _rf fluctuating reference velocity after filtering - y _c cylinder displacement - x, y, z coordinates from the cylinder center (Fig. 1) - feedback coefficient - phase lag The authors would like to express thanks to Professor K. Nagaya for his advice for designing electromagnetic actuators in the present experiments.     

Vortex shedding from a circular cylinder near a moving wall

Flow over a circular cylinder near a moving plane wall is simulated numerically. The influence of the moving wall on the vortex shedding from the cylinder is demonstrated and the corresponding mechanism is illustrated using instability theory. A critical gap ratio between the circular cylinder and the moving wall is defined, and a precise method for determining the critical gap ratio is proposed. The drag and lift forces and the pressure coefficient are presented as a function of the gap ratio. The scaling of the Strouhal number is discussed.     

A large eddy simulation of the near wake of a circular cylinder

Viscous flow around a circular cylinder at a subcritical Reynolds number is investigated using a large eddy simulation (LES) coupled with the Smagorinsky subgrid-scale (SGS) model. A fractional-step method with a second-order in time and a combined finite-difference/spectral approximations are used to solve the filtered three-dimensional incompressible Navier-Stokes equations. Calculations have been performed with and without the SGS model. Turbulence statistical behaviors and flow structures in the near wake of the cylinder are studied. Some calculated results, including the lift and drag coefficients, shedding frequency, peak Reynolds stresses, and time-average velocity profile, are in good agreement with the experimental and computational data, which shows that the Smagorinsky model can reasonably predict the global features of the flow and some turbulent statistical behaviors. The project supported by the National Science Fund for Distinguished Scholars (10125210), the Special Funds for Major State Basic Research Project (G1999032801) and the National Natural Science Foundation of China (19772062)     

A study of flow properties near critical points in the near wake of a circular cylinder

By applying the phase-plane technique to velocity data in the near-wake of a circular cylinder, three types of critical points are identified. Foci and saddle points occur most frequently, but a significant number of nodes is also found. Flow topology and properties associated with these points are examined in some detail. While foci and saddle points are associated with maxima of local vorticity and strain rate respectively, nodes are associated with a strong local divergence, indicating significant local three-dimensionality. The relative probability of time delay between critical points is also discussed.     

The velocity field of the turbulent very near wake of a circular cylinder

Hot-wire measurements were conducted in the very near wake (x/d10) of a circular cylinder at a Reynolds number based on cylinder diameter, Re _d of 3900. Measurements of the streamwise velocity component with the use of single sensor hot-wire probes were found to be inaccurate for such flowfields where high flow angles are present. An X-array probe provided detailed streamwise and lateral velocity component statistics. Frequency spectra of these two velocity components are also presented. Measurements with a 4-sensor hot-wire probe confirmed that the very near wake region is dominantly two-dimensional, thus validating the accuracy of the present X-array data.This study has been funded by the NASA-Ames University Consortium Cooperative Agreement, NCC2-5003. We wish to thank Patrick Beaudan for providing us with the LES results for comparison and Parviz Moin for his interest in and encouragement of this experiment to provide validation data for the LES. We also wish to thank loseph Murray for his help with the look-up-table data reduction program.     

The effect of air-water interface on the vortex shedding from a vertical circular cylinder

The flow past an interface piercing circular cylinder at the Reynolds number Re=2.7×10⁴ and the Froude numbers Fr=0.2 and 0.8 is investigated using large-eddy simulation. A Lagrangian dynamic subgrid-scale model and a level set based sharp interface method are used for the spatially filtered turbulence closure and the air-water interface treatment, respectively. The mean interface elevation and the rms of interface fluctuations from the simulation are in excellent agreement with the available experimental data. The organized periodic vortex shedding observed in the deep flow is attenuated and replaced by small-scale vortices at the interface. The streamwise vorticity and the outward transverse velocity generated near the edge of the separated region, which enforces the separated shear layers to deviate from each other and restrains their interaction, are primarily responsible for the devitalization of the periodic vortex shedding at the interface. The lateral gradient of the difference between the vertical and transverse Reynolds normal stresses, increasing with the Froude number, is the main source of the streamwise vorticity and the outward transverse velocity at the interface.     

Suppression of vortex shedding for flow around a circular cylinder using optimal control

Adjoint formulation is employed for the optimal control of flow around a rotating cylinder, governed by the unsteady Navier–Stokes equations. The main objective consists of suppressing Karman vortex shedding in the wake of the cylinder by controlling the angular velocity of the rotating body, which can be constant in time or time‐dependent. Since the numerical control problem is ill‐posed, regularization is employed. An empirical logarithmic law relating the regularization coefficient to the Reynolds number was derived for 60?Re?140. Optimal values of the angular velocity of the cylinder are obtained for Reynolds numbers ranging from Re=60 to Re=1000. The results obtained by the computational optimal control method agree with previously obtained experimental and numerical observations. A significant reduction of the amplitude of the variation of the drag coefficient is obtained for the optimized values of the rotation rate. Copyright © 2002 John Wiley & Sons, Ltd.     

Flow around a circular cylinder—structure of the near wake shear layer

The separated shear layer in the near wake of a circular cylinder was investigated using a single hot wire probe, with special attention given to the shear layer instability characteristics. Without end plates to force parallel vortex shedding, the critical Reynolds number for the onset of the instability was 740. The present data, together with all previously published data, show that the ratio of the instability frequency f_sl to the vortex shedding frequency f_v varies as Re^0.65, which is in agreement with the Re^0.67 dependence obtained by Prasad and Williamson [1997, J Fluid Mech 333:375–402]. However, the distribution of f_sl/f_v and the spectra of the longitudinal velocity fluctuation (u) suggest that, on either side of Re=5,000, the shear layer exhibits lower and upper subcritical regimes, in support of the observations by Norberg [1987, publication no. 87/2, Chalmers University of Technology, Sweden] and Prasad and Williamson [1997, J Fluid Mech 343:235–265]. The spectra of u provide strong evidence for the occurrence of vortex pairing in wake shear layers, suggesting that the near wake develops in a similar manner to a mixing layer.     

Reduction in drag and vortex shedding frequency through porous sheath around a circular cylinder

A numerical study on the laminar vortex shedding and wake flow due to a porous‐wrapped solid circular cylinder has been made in this paper. The cylinder is horizontally placed, and is subjected to a uniform cross flow. The aim is to control the vortex shedding and drag force through a thin porous wrapper around a solid cylinder. The flow field is investigated for a wide range of Reynolds number in the laminar regime. The flow in the porous zone is governed by the Darcy–Brinkman–Forchheimer extended model and the Navier–Stokes equations in the fluid region. A control volume approach is adopted for computation of the governing equations along with a second‐order upwind scheme, which is used to discretize the convective terms inside the fluid region. The inclusion of a thin porous wrapper produces a significant reduction in drag and damps the oscillation compared with a solid cylinder. Dependence of Strouhal number and drag coefficient on porous layer thickness at different Reynolds number is analyzed. The dependence of Strouhal number and drag on the permeability of the medium is also examined. Copyright © 2009 John Wiley & Sons, Ltd.     

PIV investigation of the shear layer vortices in the near wake of a circular cylinder

Particle image velocimetry measurements are performed in the near wake of a circular cylinder at a Reynolds number of 12,500. Attention is focused on the shear layer that develops just downstream of the separation point from the cylinder surface to investigate the possible existence of a preferred spatio-temporal organization in this flow region and the possible occurrence of the vortex pairing phenomenon. Eddy structures are identified in instantaneous velocity maps in order to investigate their spatial relationships. For that purpose a vortex extraction procedure is designed, based on the wavelet transform of instantaneous maps of the swirling strength. This algorithm allows not only the detection of the vortical structures from the instantaneous velocity fields, giving access to their instantaneous location, but also the estimation of their main characteristics such as their radius, intensity and convection velocity. The vortex population detected in the shear layer is found to be of small diameter compared to that of the von Kármán vortex and of rather high intensity, in agreement with the existence of a thin shear layer. The strong flapping motion of the shear layer and its complex spatial development is also confirmed. By employing conditional analysis of the computed data and their proper scaling, the surrounding of the detected vortex cores is investigated. A preferred spatial vortex separation is detected and is shown to vary with the longitudinal distance from the origin of the shear layer, in agreement with the qualitative behavior of a turbulent plane mixing layer. Evidence of the occurrence of the vortex pairing or amalgamation mechanisms in the shear layer is also demonstrated.     

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.     

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.     

Simultaneous measurement of fluctuating velocity and pressure in the near wake of a circular cylinder

Simultaneous measurement of fluctuating velocity and pressure by a static-pressure probe and a hot-wire probe was performed in the near wake of a circular cylinder, in order to strengthen reliability of the measurement technique. Effect of geometry of the static-pressure probe was systematically investigated, and validity of the measurement results was addressed by quantitative comparison with reference data by a large-eddy simulation. Interference between the probes was found to mainly depend on the diameter of the pressure probe and only weakly on the length. A certain time lag between the velocity and pressure signals was detected in the experiment, and the measurement results of velocity–pressure correlation $\overline{up}$ and $\overline{vp}$ obtained with the correction of the time lag were in good agreement with the computational results. It was also found that the measurement of $\overline{vp}$ is extremely sensitive to a small time lag between the velocity and pressure signals, while that of $\overline{up}$ is not.     

Regimes of vortex shedding from an in-line oscillating circular cylinder in the uniform flow

A finite volume method for the time dependent viscous incompressible flow around an in-line oscillating circular cylinder at Reynolds number of 200, 855 is presented in this paper. The Navier-Stokes equations in a finite volume form are solved with a moving grid system, based on a time dependent coordinate transformation. To investigate the vortex-shedding characteristics behind the circular cylinder and the effects of Reynolds number and other non-dimensional parameters such as reduced amplitude and reduced frequency， several numerical schemes have been tested with different amplitude and frequency close to Sto and a harmonic at each Reynolds number. Present numerical results indicate several types of vortex shedding mode which is known mainly depending on the reduced frequency and also the reduced amplitude, which is called synchronization or lock-on.