Vortex Shedding Of Cylinder Research Articles

Hydrodynamic characteristics of hyperbolic cylinder under periodic pulsating flow at moderately high Reynolds number (Re=3900)

In this paper, the hydrodynamic characteristics of a hyperbolic cylinder under periodic pulsating flow are studied. The non-uniform turbulent flow is formed by a periodically oscillating wing structure placed upstream of the hyperbolic cylinder. The influence of different oscillation velocities and oscillation amplitudes of the upstream wing structure and the horizontal distance between the wing structure and the hyperbolic cylinder on the hydrodynamic characteristics of the downstream hyperbolic cylinder is studied, and the results indicate that the hydrodynamic characteristics of the hyperbolic cylinder are mainly determined by leading-edge vortices and trailing-edge vortices of the upstream wing structure and the vortex shedding of cylinder itself. Further, the distance between the wing and cylinder has an obvious influence on the hydrodynamic characteristics of the cylinder owing to the difference in the diffusion range of the vortex. Also, the different oscillation velocities of the upstream wing lead to a difference in the attachment vortices on the hyperbolic cylinder, which indicates the positive or negative drag coefficient. Moreover, a critical value of the oscillation amplitude exists for each oscillation velocity value, so the hyperbolic cylinder is subjected to a minimum force under the periodic pulsating incoming flow.

Numerical simulation of flow past an 8:1 oscillating rectangular cylinder at Re = 22 000

To investigate the influence of structure’s oscillatory motion on flow, the present study employs the arbitrary Lagrangian–Eulerian method in k–ω shear stress transport (SST) turbulence model to simulate the flow past an oscillating rectangular cylinder at Re= 22 000. The cylinder undergoes reciprocating sinusoidal motion at a specified frequency f e, and the frequency ratio fr (defined as the ratio of cylinder oscillation frequency f e to the stationary cylinder vortex shedding frequency f 0), ranges from 0 to 4. The results demonstrate that, in the synchronization region (0.8 ⩽ fr⩽ 1.2), the drag coefficient shows the most notable variation and reaches its maxima at fr= 1.1 and the root mean square (r.m.s.) of the lift coefficient is proportional to the square of fr (, R 2 = 0.99). Moreover, the present study compares the similarities and differences of vortex shedding morphology between stationary and oscillating cylinders. With the increase of fr, the wake vortex gradually transforms from a single-row arrangement on the wake centerline to a parallel double-row arrangement, with the main vortex modes in the wake region observed as ‘2S’, ‘P + S’, ‘2P’ and ‘C’. Furthermore, spectral analysis, including amplitude spectrum analysis and wavelet analysis, in addition to probability density function statistical methods, are employed to comprehensively understand the velocity characteristics of the wake region. The results indicate that the oscillation of the cylinder reduces the correlation of the wake velocity. Those are beneficial in understanding the interaction between turbulence and structural fluid-induced motion.

Flow behavior downstream of two side-by-side circular cylinders in shallow water

Investigations of bistable flow structure past a pair of cylinders positioned side-by-side in shallow water is conducted experimentally applying dye observation and the particle image velocimetry (PIV) method. For the gap ratio of G/D = 1.25, the jet-like flow between cylinders deflects asymmetrical flow structures forming a large-scale wake as well as a small-scale wake downstream of cylinders. The small vortices around the right cylinder get closer to each other forming a larger vortex in the large-scale wake region, which leads the jet-like flow to changeover side to side. The small frequency (f = 0.352 Hz) associated with frequency of vortex shedding of cylinder with wider wake and the higher frequency (f = 0.793 Hz) which depicts the smaller wakes frequency.

Numerical Study of Flow Patterns in a Tandem Array Cylinder System

The spectral element method, a direct numerical technique, is used to study the behavior of flow past two cylinders in tandem array. A control cylinder is employed in front of the main cylinder to study the drag reduction performance and flow patterns. Three major flow patterns are found, including single cylinder vortex shedding, two cylinders vortex shedding and suppression. The flow patterns are affected by the distance between two cylinders, Reynolds number and the diameter ratios of cylinders. In a bistable regime, when there is a critical distance between cylinders, drag is reduced dramatically.

Feedback control of vortex shedding using a full-order optimal compensator

In the present study the linear feedback control of the unsteady cylinder wake is numerically investigated at low Reynolds numbers. The classical small-gain or minimal control energy (MCE) solution of the optimal control and estimation problems is used to design a full-dimensional stabilising compensator of the linearised Navier–Stokes equations, thus bypassing the open-loop model reduction of the fluid plant. For such high-dimensional system, both the feedback and the observer gains are efficiently computed based on the knowledge of the unstable global modes only. The derived control technique provides us with a theoretical analysis tool to investigate the best performance achievable by a ‘perfect’ MCE compensator, i.e. a MCE compensator free from model-reduction errors, on the actual flow field. In our setup, a single-input-single-output (SISO) configuration is considered, the vortex-shedding being controlled by means of the unsteady angular rotation of the cylinder surface with a single velocity sensor located in the wake for the state estimation. For Re=50 the MCE compensator is able to completely suppress the cylinder vortex shedding, driving the flow from the natural limit cycle to the unstable basic state, which is finally restored. The effects of sensor placement on the compensator performance are then investigated and finally, as Re is increased, the upper bound on the delay of the instability threshold is assessed.

Interaction of an off-surface cylinder with separated flow from a bluff body leading edge

Time resolved PIV measurements are performed on a cylinder of diameter D placed in the vicinity of the leading edge of an elongated bluff body at ReD=2.6×104. The off-surface cylinder is placed at three different wall-normal positions and two streamwise positions, upstream and downstream the bluff body leading edge. Topological, statistical and transient analyses are performed on the resulting flowfield. The streamwise position of the cylinder relative to the bluff body leading edge significantly affects the mean flow topology and the vortex shedding dynamics of both the cylinder and the bluff body separation bubble. When the cylinder is positioned upstream of the bluff body leading edge, the cylinder shedding becomes phase-locked to the bluff body bubble shedding, while the low frequency bubble fluctuation is not significantly affected. When the cylinder is positioned downstream the bluff body leading edge, the pressure gradient eliminates the separation bubble and regular cylinder vortex shedding occurs. The StD number of the cylinder shedding is reduced when the separation bubble is eliminated as the latter causes blockage that reduces the effective cylinder diameter. POD analysis captures the separation bubble and cylinder shedding dynamics, demonstrating that when the cylinder is placed near the wall, low frequency fluctuation modes associated with the separation bubble dominate over the cylinder shedding modes.

On the controversy regarding the effect of flow shear on the Strouhal number of cylinder vortex shedding

High lift devices generate asymmetric wakes that impact the aerodynamic and aeroacoustic performance of airplanes. Asymmetric wakes are also produced by a shear flow around a cylinder if the shear direction is perpendicular to the cylinder axis. Most of the work devoted to asymmetric wakes focuses on the latter scenario. In the literature there is no consensus about whether the shear reduces or increases the Strouhal number of the vortex shedding. The current work is concerned with this apparent controversy. The work presents a linear stability analysis of the asymmetric wake profiles, which shows that the dominant Strouhal number decreases with the asymmetry. The results are validated by direct numerical simulations. The literature shows that the shear affects the cylinder drag. These results are normalized by cylinder diameter and free-stream velocity, whereas it is recognized that wake scalings are more appropriate to describe the flow. Such a wake scaling is not available in the literature data; the drag is used here to re-scale the experimental data proportionally to the wake width, collapsing virtually all the data available. The re-scaled data shows that the shear decreases the dominant Strouhal number in qualitative agreement with the analysis.

Interference Between Two Stationary or Elastically Supported Rigid Circular Cylinders of Unequal Diameters in Tandem and Staggered Arrangements

Analysis of model test results was carried out to investigate the hydrodynamic interaction between pairs of fixed or elastically supported rigid cylinders of dissimilar diameters in a water flume. The two cylinders are placed with one situated in the wake of the other. The spacing between the cylinders ranges from 1 to 15 times the larger cylinder diameter. The Reynolds numbers are within the subcritical range. For the vibrating cylinders which are free to oscillate in both the in-line and the cross-flow directions, the reduced velocity ranges from 1 to 13 and the low damping ratio of the test setup at 0.006 gives a combined mass-damping parameter of 0.02. For the fixed cylinders, the downstream cylinder experiences a drag reduction and it was found that this drag reduction also depends upon the diameter ratio. The lift on the fixed downstream cylinder has the frequency components derived from the upstream cylinder's vortex shedding as well as from its own vortex shedding, and the relative importance of the two sources is influenced by the spacing between the two cylinders. This is reflected in the downstream cylinder's vortex induced vibration (VIV) response which appears to be dependent upon the actual reduced velocities of both the cylinders.

An experimental study of the spanwise correlation of vortex shedding in the towing tank

Although there are many correlation studies dedicated to the vortex shedding of cylinders in wind tunnels, the experiments conducted in water have not been carried out well before, especially for the flexible cylinder. In order to investigate this area, the present experiment was conducted in a towing tank and both the rigid and flexible cylinders were used. Various results are obtained including the correlation coefficients, correlation lengths and probability distribution, which exhibit several valuable features. Firstly, the correlation coefficients of the flexible cylinder are more irregular than that of the rigid cylinder. Secondly, the correlation lengths have visual peaks at Re ≈ 5.0 ∼ 6.0 × 103 for both the rigid and flexible cylinders, and a sharp decrease after this region should involve the inception of vortex dislocations. In addition, the analysis of root mean square and probability density function of the phase drift gives more insights of the vortex shedding along the spanwise different cross-sections.

Some observations of two interfering VIV circular cylinders of unequal diameters in tandem

Analysis of model test results was carried out to investigate the hydrodynamic interaction between a pair of elastically-supported rigid cylinders of dissimilar diameters in a water flume. The two cylinders are placed in tandem with one situated in the wake of the other. The diameter of the upstream cylinder is twice as large as that of the downstream cylinder. The spacing between the two cylinders ranges from 1 to 10 times the larger cylinder diameter. The Reynolds numbers are within the sub-critical range. The cylinders are free to oscillate in both the in-line and the cross-flow directions. The reduced velocity ranges from 1 to 10 and the low damping ratio of the model test set-up at 0.006 gives a combined mass-damping parameter of 0.02. It is found that the lift on and the cross-flow motion of the downstream cylinder have the frequency components derived from the upstream cylinder's vortex shedding as well as from its own vortex shedding, and the relative importance of the two sources of excitation is influenced by the spacing between the two cylinders. The downstream cylinder's VIV response appears to be largely dependent upon the actual reduced velocity of the cylinder.

Defining a universal and continuous Strouhal–Reynolds number relationship for the laminar vortex shedding of a circular cylinder

The existence of a discontinuity in the Strouhal–Reynolds number relationship for the laminar vortex shedding of a cylinder is found to be caused by a change in the mode of oblique shedding. By ‘‘inducing’’ parallel shedding (from manipulating end conditions) the resulting Strouhal curve becomes completely continuous and agrees very well with the oblique-shedding data, if it is transformed by S0=Sθ/cos θ (where Sθ is the Strouhal number corresponding with the oblique-shedding angle θ). The curve also agrees with data from a completely different facility. This provides evidence that this Strouhal curve (S0) is universal (for a circular cylinder).