On vortex shedding from low aspect ratio dual step cylinders

A dual-step cylinder is comprised of two cylinders of different diameters. A large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). The present study investigates the effect of Reynolds number (Re_D) and L/D on dual step cylinder wake development for D/d=2, 0.2≤L/D≤3, and two Reynolds numbers, Re_D=1050 and 2100. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from both the large and small diameter cylinders for 1≤L/D≤3 at Re_D=2100 and 2≤L/D≤3 at Re_D=1050. At these conditions, large cylinder vortices predominantly form vortex loops in the wake and small cylinder vortices form half-loop vortex connections. At lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. At these lower aspect ratios, the presence of the large cylinder induces periodic vortex dislocations. Increasing L/D increases the frequency of occurrence of vortex dislocations and decreases the dominant frequency in the large cylinder wake. The identified changes in wake topology are related to substantial variations in the location of boundary layer separation on the large cylinder, and, consequently, changes in the size of the vortex formation region. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.     

Experimental study on wake flow structures of screen cylinders using PIV

Experiments were conducted in a water flume using Particle Image Velocimetry (PIV) to study the evolution of the vortical structures in the wakes of four types of screen cylinders at a Reynolds number of about 3200. The results were compared with that of a bare cylinder. The screen cylinders were made of stainless steel screen meshes of various porosities (37%, 48%, 61% and 67%) rolled into cylindrical shapes. Smoke wire flow visualisations in a wind tunnel were also conducted in support of the PIV tests. Depending on the porosity of the screen mesh, two vortex formation mechanisms for the screen cylinder wakes were identified. One was associated with wake instability and the other was associated with shear-layer (Kelvin-Helmholtz) convective instability which involved merging through pairing and tripling of small-scale vortices within the shear layers. The former was responsible for the formation of large-scale vortices in the bare cylinder and the screen cylinder wakes with 37% and 48% porosities, while the latter was responsible for the screen cylinder wakes with 61% and 67% porosities. The results also showed that with increasing porosity, the vortex formation region was extended farther downstream and the Reynolds shear stress, the Turbulent Kinetic Energy (TKE) and vortex intensity were decreased constantly.     

Simultaneous CFD evaluation of VIV suppression using smaller control cylinders

Two-degree-of-freedom vortex-induced vibrations (VIV) of a circular cylinder with and without two smaller control cylinders are investigated numerically by computational fluid dynamics (CFD) models coupling with a fluid–structure interaction (FSI) computational method. The numerical model is validated against experimental data of VIV of an isolated cylinder in uniform current. The study is aimed to investigate the effect of smaller control cylinders on VIV suppression. The trajectories of cylinder motion, amplitude response, and temporal evolution of vortex shedding and streamlines are obtained by conducting a series of simulations. And the effect of Reynolds number, located angle and rotational rate of small control cylinders are discussed in detail. It is found that placing small cylinders at 45° to the downstream vector can achieve a good suppression effect, but the effect is different at different Re. Rotating control cylinders with a reasonable rotation velocity can further enhance the VIV suppression by injecting enough momentum into the boundary layer of the main cylinder. The best effect is found at U_c=10, which has a 64.56% reduction in the transverse vibration response.     

Numerical investigation on flow around circular cylinders in tandem arrangement at a subcritical Reynolds number

Three-dimensional fluid computations have been performed to investigate the flows around two circular cylinders in tandem arrangements at a subcritical Reynolds number, Re=2.2×10⁴. The center-to-center space between the cylinders was varied from twice the cylinder diameter to five times that, and the flows and fluid-dynamic forces obtained from the simulations are compared with the experimental results reported in the literature. Special attention is paid to the characteristics of the vortices shed from the upstream cylinder such as the convection, the impingement onto the downstream cylinder and the interaction with the vortices from the downstream cylinder. The effects of the vortices from the upstream cylinder on the fluid-dynamic forces acting on the downstream cylinder are discussed.     

Vortex shedding and surface pressures on a square cylinder at incidence to a uniform air stream

This article describes results of experiments on vortex-shedding frequencies and surface pressures of a square cylinder at non-zero angle of incidence. The range of Reynolds numbers was 2000–21 000, but the lower range was emphasized. For Reynolds numbers greater than 5300, the Strouhal number shows a similar trend with changing angle of incidence; that is, a rapid rise in Strouhal number occurs at an angle of around 13°. The occurrence of such a jump in Strouhal number was found to be associated with onset of the flow reattachment, bringing in a strong pressure recovery on the lower side face of the cylinder. For lower Reynolds numbers Re=2000–3300, the maximum Strouhal number occurs at a relatively higher angle of 17°. Around this angle, the pressure measurements exhibit a rather weak pressure recovery, suggesting a less firm shear-layer reattachment to the side face of the cylinder. The nature of the reattaching flow was further examined by spectral analysis of the fluctuating pressure coefficients measured on the lower side face of the cylinder.     

Flow behavior around a square cylinder subject to modulation of a planar jet issued from upstream surface

The flow behavior in the up- and downstream regions of a square cylinder subject to the modulation of a planar jet issued from the cylinder׳s front surface was studied using the laser-assisted smoke flow visualization method and hot-wire anemometer measurement. Reynolds numbers were from 1628 to 13 000. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the up- and downstream faces. The temporally evolving smoke flow patterns in the up- and downstream regions were synchronously revealed through the smoke flow visualization. The frequency characteristics of the instability waves in the up- and downstream regions were synchronously detected by the two hot-wire anemometers. Four characteristic flow modes were observed within the different ranges of the injection ratios. At the low injection ratios (IR<1), the ‘swinging jet’ appeared. The jet swung periodically leftward and rightward and formed a fluid bubble on the front surface. The fluid bubble contained a pair of counter-rotating vortices and presented a periodic variation in its height. At moderately low injection ratios (1<IR<4.3), the ‘deflected oscillating jet’ appeared. The jet was deflected in either the left or the right direction and wrapped around one of the edges of the square cylinder. Both the swinging and oscillating motions of the jet in the swinging jet and deflected oscillating jet modes were induced by the periodic feedback pressure signals generated by the vortex shedding in the wake. At the moderately high (4.3<IR<8.3) and high (IR>8.3) injection ratios, the ‘deflection jet’ and ‘penetrating jet’ appeared. The jet detached from the cylinder׳s front surface and penetrated a long distance into the upstream region due to large jet momentum. Neither periodic jet oscillation in the upstream region nor vortex shedding in the wake was observed. The drag coefficient was found to be decreasing quickly with increasing the injection ratio.     

Numerical investigation of steady suction control of flow around a circular cylinder

This paper numerically investigates the effectiveness of the control of steady suction on a stationary circular cylinder with several isolated suction holes on the surface at a subcritical Reynolds number. The control effectiveness as a function of the azimuthal position, spanwise spacing and suction flow rate of the suction holes on the control of the aerodynamic forces on the cylinder and the suppression of alternate vortex shedding are taken into account. The study of the azimuthal location of the suction holes indicates that azimuthal angles of θ=90° and 270°, which are close to the separation point, provide the most substantial decreases in the aerodynamic forces. When restricted to the most effective azimuthal angle, a remarkable control effectiveness can be achieved when the axial spacing between two neighboring suction holes is less than a minimum value even under a small suction momentum coefficient. However, if the axial spacing exceeds the minimum spacing, the control effectiveness will not be saturated even under a very large suction momentum coefficient. Thus, the cause of the effective aerodynamic force control is suggested to be a result of obvious three-dimensional phenomenon in the near wake, which is characterized by the generation of a convergent flow between two neighboring suction hole sections and a stronger, larger three-dimensional vortex pair adjacent to the convergent flow. It has been suggested that this strongly three-dimensional flow pattern is induced by the strong interaction between two neighboring but counter-rotating three-dimensional vortices separately produced by two neighboring suction holes. Moreover, the effects of such three-dimensional flow patterns are investigated in detail based on variations in the flow field and sectional aerodynamic forces in different cross sections. Finally, the upper limit of the axial spacing between two neighboring suction holes to form such a three-dimensional flow pattern is suggested to be between 0.75 D and 1.5 D when the suction flow rate exceeds a certain value.     

The effect of a splitter plate on the flow around a finite prism

The effect of a wake-mounted splitter plate on the flow around a surface-mounted finite-height square prism was investigated experimentally in a low-speed wind tunnel. Measurements of the mean drag force and vortex shedding frequency were made at Re=7.4×10⁴ for square prisms of aspect ratios AR=9, 7, 5 and 3. Measurements of the mean wake velocity field were made with a seven-hole pressure probe at Re=3.7×10⁴ for square prisms of AR=9 and 5. The relative thickness of the boundary layer on the ground plane was δ/D=1.5–1.6 (where D is the side length of the prism). The splitter plates were mounted vertically from the ground plane on the wake centreline, with a negligible gap between the leading edge of the plate and rear of the prism. The splitter plate heights were always the same as the heights of prisms, while the splitter plate lengths ranged from L/D=1 to 7. Compared to previously published results for an “infinite” square prism, a splitter plate is less effective at drag reduction, but more effective at vortex shedding suppression, when used with a finite-height square prism. Significant reduction in drag was realized only for short prisms (of AR≤5) when long splitter plates (of L/D≥5) were used. In contrast, a splitter plate of length L/D=3 was sufficient to suppress vortex shedding for all aspect ratios tested. Compared to previous results for finite-height circular cylinders, finite-height square prisms typically need longer splitter plates for vortex shedding suppression. The effect of the splitter plate on the mean wake was to narrow the wake width close to the ground plane, stretch and weaken the streamwise vortex structures, and increase the lateral entrainment of ambient fluid towards the wake centreline. The splitter plate has little effect on the mean downwash. Long splitter plates resulted in the formation of additional streamwise vortex structures in the upper part of the wake.     

Vortex-induced vibration of two elastically coupled cylinders in side-by-side arrangement

Vortex-induced vibration (VIV) of two elastically coupled circular cylinders in side-by-side arrangement is investigated numerically. The Reynolds-averaged Navier–Stokes equations are solved by the finite element method for simulating the flow and the equation of motion is solved for calculating the vibration. The mass ratio (the ratio of the mass of the cylinder to the displaced fluid mass) is 2 and the Reynolds number is 5000 in the simulations. Simulations are carried out for one symmetric configuration (referred to be Case A) and one asymmetric configuration (referred to be Case B). In both Case A and Case B, the primary response frequencies of the two cylinders are found to be the same both inside and outside the lock-in regimes. Five response regimes are found in both cases and they are the first-mode lock-in regime, the second-mode lock-in regime, the sum-frequency lock-in regime and two transition regimes. When the vibration is transiting from the first- to the second-mode lock-in regimes, the vibration of each cylinder contains both first- and the second-mode natural frequencies, and the vibrations are usually irregular. In the transition regime between the second-mode lock-in and the sum-frequency lock-in regimes, the response frequencies of both cylinders increases with an increase in the reduced velocity until they are close to the sum of the two natural frequencies. In both cases, the lower boundary reduced velocity of the total lock-in regime (the sum of the five lock-in regimes) is about 3 and the upper boundary reduced velocity is about 11 times the first-to-second-mode natural frequency ratio.     

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。     

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.     

Flow around four cylinders arranged in a square configuration

This paper presents an experimental study of the flow around four circular cylinders arranged in a square configuration. The Reynolds number was fixed at Re=8000, the pitch-to-diameter ratio between adjacent cylinders was varied from P/D=2 to 5 and the incidence angle was changed from α=0° (in-line square configuration) to 45° (diamond configuration) at an interval of 7.5°. The flow field was measured using digital Particle Image Velocimetry (PIV) to examine the vortex shedding characteristics of the cylinders, together with direct measurement of fluid dynamic forces (lift and drag) on each cylinder using a piezoelectric load cell. Depending on the pitch ratio, the flow could be broadly classified as shielding regime (P/D≤2), shear layer reattachment regime (2.5≤P/D≤3.5) and vortex impinging regime (P/D≥4). However, this classification is valid only in the case that the cylinder array is arranged nearly in-line with the free stream (α≈0°), because the flow is also sensitive to α. As α increases from 0° to 45°, each cylinder experiences a transition of vortex shedding pattern from a one-frequency mode to a two-frequency mode. The flow interference among the cylinders is complicated, which could be non-synchronous, quasi-periodic or synchronized with a definite phase relationship with other cylinders depending on the combined value of α and P/D. The change in vortex pattern is also reflected by some integral parameters of the flow such as force coefficients, power spectra and Strouhal numbers.     

Flow around three rectangular cylinders arranged in connected and separated Y-shape

Characteristics of cross flow around three rectangular cylinders with two aspect ratios of breadth to width arranged in connected and separated Y-shape at various angles of incident flow were studied by means of force measurement in a wind tunnel. Flow visualizations with smoke-wire technique for typical cases were also given. Different types of flow patterns were formed for individual models at different angles of incident flow. From the results of fluctuating velocity measurement in the wake, features of vibration were determined. It shows that as the wind blows along the lines of one limb or rectangular cylinder of the model, oscillation is weak, whereas when the wind blows along the bisector lines of two limbs or cylinders, strong vibration is observed. It is associated with the regular vortex shedding.The project supported by the National Natural Science Foundation of China (10172008)The English text was polished by Keren Wang     

Vortex shedding around a near-wall circular cylinder induced by a solitary wave

This study developed a two-dimensional generalized vortex method to analyze the shedding of vortices and the hydrodynamic forces resulting from a solitary wave passing over a submerged circular cylinder placed near a flat seabed. Numerical results for validation are compared with other numerical and experimental results, and satisfactory agreement is found. A series of simulations were performed to study the effects of gap-to-diameter ratio and incident wave height on vorticity pattern as well as the forces exerted on the cylinder. The range of the heights of incident waves is from 0.3h to 0.7h, where h is the still water depth. The range of the gap-to-diameter ratios is from 0.1 to 0.8. The results indicate that the flow pattern and the pressure distribution change significantly because of the close proximity of the seabed where the vorticity flux on the seabed-side surface of the cylinder is suppressed. Placing the cylinder nearer the seabed increases the drag and the positive lift on the cylinder. When the gap-to-diameter ratio increases, the pattern of vortices changes because of the interaction between the main recirculation zone and the shear layers separated from the gap. The maxima of drag, lift and total force increase linearly with the height of the incident wave.     

Scale-adaptive simulation of flow past wavy cylinders at a subcritical Reynolds number

The Xu & Yan scale-adaptive simulation (XYSAS) model is employed to simulate the flows past wavy cylinders at Reynolds number 8 × 10 3.This approach yields results in good agreement with experimental measurements.The mean flow field and near wake vortex structure are replicated and compared with that of a corresponding circular cylinder.The effects of wavelength ratios λ/D m from 3 to 7,together with the amplitude ratios a /D m of 0.091 and 0.25,are fully investigated.Owing to the wavy configuration,a maximum reduction of Strouhal number and root-meansquare (r.m.s) fluctuating lift coefficients are up to 50% and 92%,respectively,which means the vortex induced vibration (VIV) could be effectively alleviated at certain larger values of λ/D m and a /D m.Also,the drag coefficients can be reduced by 30%.It is found that the flow field presents contrary patterns with the increase of λ/D m.The free shear layer becomes much more stable and rolls up into mature vortex only further downstream when λ/D m falls in the range of 5-7.The amplitude ratio a /D m greatly changes the separation line,and subsequently influences the wake structures.     

Wake modes of rotationally oscillating cylinders at Re=150

The wake structure of a circular cylinder undergoing forced rotary oscillations is studied at Re=150. Cylinder forcing conditions range between 0.17≤θ_PP≤5.58, where θ_PP is the peak-to-peak oscillation amplitude in radians and 0.67≤F_R≤3.5, where F_R is the ratio of forcing frequency to natural shedding frequency. DPIV and spectral analysis are used to obtain quantitative data about the wake structure. Wake modes and their dependence on forcing conditions are identified. These include previously unknown wake modes present at higher frequencies. Wake mode boundaries are identified on the parameter space. These lead to a better understanding of wake formation at low Re.