Oscillation Of Cylinder Research Articles

Numerical investigation of synthetic jet effects on vortex-induced vibrations in tandem circular cylinders

ABSTRACT The influence of symmetric synthetic jets, on the upstream cylinder's (Cylinder 1's) leeward side on VIVs of tandem circular cylinders, was numerically investigated at Re = 150, m* = 2.0. The amplitude responses, frequency responses, hydrodynamic forces and wake patterns were analysed in the reduced velocity U* range of 3.0–14.0. The results indicated that synthetic jets with sufficient momentum coefficients (C u = 2.0 and 4.0) could suppress transverse oscillations of both cylinders, resulting in a symmetric +2S wake pattern. While the standard deviations of lift coefficients for Cylinder 1 decreased accordingly, the time-averaged drag coefficients of both cylinders didn't always follow suit. Complex wake patterns, including 2S + 2S, 2P + 2S, 2P + 2P, 2(P + S) + 2P and 2(P + S) + 2(P + S), were observed when synthetic jets can't dominate Cylinder 1's vortex shedding. Galloping responses of both cylinders can also be initiated.

Numerical Simulation and Investigation of Flow Patterns Around Three Side-by-Side Cylinders Arranged in Line with an Oscillating Central Cylinder

A flow past three cylinders arranged in a vertical row in a side-by-side configuration in which both side cylinders were kept fixed and an oscillatory movement provided to the middle cylinder was simulated. This particular configured setup holds significant importance in engineering and presents a compelling resemblance to numerous real-world scenarios especially in electricity production using oscillatory motion. In the present study, free-stream flow across three side-by-side cylinders with a transversely oscillating middle cylinder, was simulated using the commercial CFD software tool ANSYS 2021 R2 at a low Reynolds number (Re) = 100, with non-dimensional cylinder spacing (s/d) = 2;. For the middle cylinder, non-dimensional amplitude (amplitude ratio), A/d = 0.5 and a frequency ratio of fe/fo=0.8, 1 and 1.2 were considered (fe/fo, where fe is the cylinder oscillation frequency and fo is the corresponding vortex shedding frequency for stationary cylinders). In this study, re-meshing was done using the dynamic mesh technique, and pre-defined oscillatory motion was provided with the help of an additionally hooked User Defined Function (UDF) in the commercial CFD software tool. The objective of the present study was to investigate the effect of governing parameters on flow physics and wake nature, as well as the effect of the flow transition on engineering parameters. The interesting finding to report here that for oscillation frequency fe/fo= 1 there exist synchronous flow confirmed from wake diagram having comparatively more drag with that of other two higher fe/fo= 1.2 and lower oscillation frequency ratio fe/fo= 0.8 for cylinder spacing (s/d) = 2.

Improving light availability and creating high-frequency light–dark cycles in raceway ponds through vortex-induced vibrations for microalgae cultivation: a fluid dynamic study

Limited light availability due to insufficient vertical mixing strongly reduces the applicability of raceway ponds (RWPs). To overcome this and create light–dark (L/D) cycles for enhanced biomass production through improved vertical mixing, vortex-induced vibration (VIV) system was implemented by the authors in a previous study to an existing pilot-scale RWP. In this study, experimental characterization of fluid dynamics for VIV-implemented RWP is carried out. Particle image velocimetry (PIV) technique is applied to visualize the flow. The extents of the vertical mixing due to VIV and the characteristics of L/D cycles were examined by tracking selected particles. Pond depth was hypothetically divided into three zones, namely dark, light Iimited and light saturated for detailed analysis of cell trajectories. It has been observed that VIV cylinder oscillation can efficiently facilitate the transfer of cells from light-limited to light-saturated zones. Among the cells that were tracked, 44% initially at dark zone entered the light-limited zone and 100% of initially at light-limited zone entered the light-saturated zone. 33% of all tracked cells experienced high-frequency L/D cycles with an average frequency of 35.69 s−1 and 0.49 light fraction. The impact of VIV was not discernible in the deeper sections of the pond, due to constrained oscillation amplitudes. Our findings suggest that the approximately 20% increase in biomass production reported in our previous study can be attributed to the synergistic effects of enhanced L/D cycle frequencies and improved light availability resulting from the transfer of cells from dark to light-limited zones. To further enhance the effectiveness of VIV, design improvements were developed. It was concluded that light availability could be significantly improved with the presented method for more effective use of RWPs.

Interference Effects on Flow-Induced Oscillation of Three Tandem Cylinders With Passive Turbulence Control

Abstract The characteristics of flow-induced oscillation (FIO) of multiple tandem cylinders with passive turbulence control (PTC) change due to interference between PTC cylinders. Identifying positive and negative interferences is vital for designing marine structures involving PTC cylinders on elastic supports. Experiments on a single PTC cylinder, two PTC cylinders and three PTC cylinders in tandem were conducted to study the interferences due to placing an identical oscillating PTC cylinder upstream, downstream, and both upstream and downstream. Critical parameters (damping, stiffness) and Reynolds number (Re) are varied in the tests. The onset inflow velocity of oscillation is reduced and back-to-back vortex-induced vibration (VIV) and galloping is achieved, when placing interference cylinders in the vicinity of the basic oscillator. The interference due to the downstream cylinder enhances FIO of the basic oscillator. In the VIV region, one interference downstream cylinder is more effective than two interference downstream cylinders. In the galloping region, the enhancement improves for two interference cylinders for lower stiffness and damping ratios. The interference upstream cylinder tends to suppress FIO of the basic oscillator for higher stiffness. The suppression of one upstream cylinder is more vigorous than that of two upstream cylinders in the VIV region, and the opposite is true in the galloping region. When two interference cylinders are placed both downstream and upstream of the basic oscillator, its FIO is enhanced for the lower stiffness whilst it is suppressed for the higher stiffness.

Effect of thermodynamic-dynamic parameters on the oscillation and performance of a free piston Stirling engine

This paper presents an analysis of the oscillation and parametric performance of a gamma-type free piston Stirling engine using a thermodynamic-dynamic nonlinear model. Firstly, the concept of oscillation and impact cylinder points is defined and a method for determining the stable oscillation zone of the free piston Stirling engine is proposed. Besides, the effects of various thermodynamic parameters on the system’s performance, such as charging pressure, hot-end and cold-end temperatures, and dynamic parameters (such as the masses and spring stiffness of the displacer and piston), are investigated. The oscillation zone takes on a “V” shape when the piston and displacer masses or stiffness, and hot-end temperature and displacer stiffness, are varied. Finally, the output power and operating frequency of the impact cylinder line are determined. The results show that maximum output power and operating frequency of 75.36 W at 58.21 Hz or 76.73 W at 58.61 Hz are achieved at the “V” bottom when either the piston and displacer masses are adjusted or their stiffness properties are varied. Additionally, consistent output power and operating frequency of 125.35 W and 75.08 Hz, and 104.63 W and 68.59 Hz, are obtained regardless of changes in both displacer mass and stiffness.

Characterizing vibrations and associated wake structures of tandem square cylinders at different angles of incidence

Finite element computations were conducted to investigate the transverse vibrations of three identical tandem square cylinders and the associated wake patterns at a Reynolds number Re = 150. The reduced velocities ranged from U*=3 to 20, and the angles of incidence were set at α=0°, 22.5°, and 45°. The streamwise gaps for these three α were Lx=5H, 3.8H, and 3.5H, respectively, where H represents the projected dimension of the cylinder normal to the freestream. The mass ratio of the cylinders was fixed at m*=2, and damping was neglected to allow the cylinders to attain maximum amplitudes. In the presence of primary vortices being shed from all three cylinders, the upstream cylinder at α=22.5° and 45° exhibits three distinct vibration regimes: initial excitation and upper and lower response regions. On the other hand, due to interaction with the upstream vortices, the two downstream cylinders display four response regions. In the case of α=0°, the dynamic response of the upstream cylinder appears in only two regimes, but with a higher peak amplitude compared to α=22.5° and 45°. Vibration and shedding frequencies closely synchronize with the natural frequency of the spring-mass system in the second regime, leading to high amplitude oscillations for the most upstream cylinder with α=22.5° and 45°. The third response regime for the two downstream cylinders is associated with the lock-in phenomenon. In α=0° and 22.5° configurations, the shedding mode is 2S in all response regimes, while at α=45°, the shedding mode shifts to P + S during the second regime. Up to the second regime, lift and vortex forces are in-phase with the cylinder's oscillation for α=0° and 22.5°, but they go out of phase beyond that. In the case of α=45°, although lift remains in-phase with the displacement in the second regime, the vortex force is found to be out of phase. This study is expected to enhance the understanding of fluid–structure interaction phenomena involved in multiple structures and can aid in the design of stable structures in civil engineering, offshore engineering, and development of energy harvesting devices.

Harmonic oscillations of a piezoceramic functional-graded sectional cylinders with account of energy dissipation

Harmonic oscillations of piezoceramic functional-gradient sectioned hollow cylinders are studied, taking into account energy dissipation. A cylinder of finite length is considered, consisting of an even number of sections polarized in the circular direction, which are connected to each other by similar sides. The material is considered to be functionally heterogeneous in the direction of the previous polarization. The cylinder is loaded by the potential difference applied to the flat faces of the sections. The calculation is performed by the finite element method. The forming of damping matrix and determining the coefficients corresponding to the four ways of introducing Rayleigh damping into the FEM is described. Amplitude-frequency responses taking into account damping are built for displacements, charge on the electrodes, electrical admittance and its logarithm. The consideration of energy dissipation according to the Rayleigh damping model is related to the experimental data due to the Q factor of the piezo element. The frequencies of electrical resonances, anti-resonances and the corresponding electromechanical coupling coefficients for functionally inhomogeneous and homogeneous cylinders are determined. For the functional-gradient piezoelectric element of the considered configuration, the maximum CEMC occurs at the frequency of the second electrical resonance. At this frequency, the dynamic stress-strain state of the cylinder is investigated. Forms of oscillations and distribution of stress amplitude values are constructed. The largest normal stresses according to the Coulomb-Mohr strength theory are determined and compared with the von Mises strength theory. FEA allows three-dimensional calculation of harmonic oscillations of functional-gradient piezo elements of complicated geometry. The determined frequencies of electrical resonance and anti-resonance and the corresponding CEMC make it possible to choose the most effective operating mode of the piezo element. The FEM mathematical apparatus makes it possible to quickly and qualitatively evaluate the strength, determine the charge on the electrodes, the current in the circuit, build the amplitude-frequency characteristics of admittance and impedance, and evaluate the efficiency of energy conversion.

Feel the force: From local surface pressure measurement to flow reconstruction in fluid–structure interaction

Drawing inspiration from the lateral lines of fish, the inference of flow characteristics via surface-based data has drawn considerable attention. The current approaches often rely on analytical methods tailored exclusively for potential flows or utilize black-box machine learning algorithms to estimate a specific set of flow parameters. In contrast to a black-box machine learning approach, we demonstrate that it is possible to identify certain modes of fluid flow and then reconstruct the entire flow field from these modes. We use dynamic mode decomposition to parametrize complex, dynamic features across the entire flow field. We then leverage deep neural networks to infer the dynamic modes of the pressure and velocity fields within a large, unsteady flow domain, employing solely a time series of pressure measurements collected on the surface of an immersed obstacle. Our methodology is successfully demonstrated to diverse fluid–structure interaction scenarios, including cases with both free oscillations in the wake of a cylinder and forced oscillations of tandem cylinders, demonstrating its versatility and robustness.

Multiple regimes of lock-in and hysteresis in free vibration of a rotating cylinder

Flow-induced vibration of a rotating cylinder that is free to oscillate in the stream-wise and cross stream directions is studied in the laminar flow regime via linear stability analysis (LSA) and direct time integration (DTI). LSA reveals that the instability can arise from fluid-, elastic-, or coupled fluid-elastic modes depending on the rotation rate of the cylinder, α, and reduced speed, U*. Beyond α=2, the U*-range of lock-in increases exponentially with an increase in α. DTI brings out the multiple regimes of lock-in at various α. Each lock-in regime extends for a certain range of U* and is associated with a different mode of vortex shedding. The modes differ in terms of the number of pair of vortices shed during each cycle of cylinder oscillation. The amplitude of cylinder oscillation increases with an increase in the number of shed vortices. With an increase in Re, the number of vortex shedding modes increase. Vortices are generally shed during the upstream movement of cylinder, while the shear layer wraps around it resulting in large lift during the downstream motion. The flow as well as the oscillation amplitude is found to be sensitive to the initial condition for a certain range of α and U*. A flow regime is discovered where three distinct response states can be realized depending on the initial condition. Hysteresis in response to the cylinder and flow, with respect to increase and decrease in U*, occurs near the transition between lock-in and desynchronization and during the switch in the mode of vortex shedding.

Application of singular spectrum analysis to nonstationary time series in flow-induced vibrations of a circular cylinder

In numerical simulations of flow-induced vibrations (FIV) of a circular cylinder, abundant time series data are available, including cylinder displacement and acting forces. Singular spectrum analysis (SSA) is employed to deal with nonstationary multi-component time series produced in two FIV cases with proper interpretation in physics. In the first case, the cylinder displacement time series is decomposed into two oscillatory components using SSA. The instantaneous frequencies of these two components are obtained by Hilbert transform (HT) and found to be in agreement with the wavelet transform of the cylinder displacement. In the second case, three oscillatory components are extracted from the cylinder displacement time series by SSA. The dominant component is characterized by steady oscillations at the vortex shedding frequency, which suggests a relatively steady vortex shedding process behind the rear cylinder. In contrast, the second component, which is closely associated with the alternate boundary layer separations from the front cylinder, features in the increasing amplitude with time. This implies that the unsteady flow field in the gap might be attributed to the nonstationary cylinder oscillations. This work demonstrates that SSA, in conjunction with HT, enables a comprehensive time-frequency analysis of nonstationary time series obtained in FIV.

Significant influence of fluid viscoelasticity on flow dynamics past an oscillating cylinder

This study focuses on numerically investigating the impact of fluid viscoelasticity on the flow dynamics around a transversely forced oscillating cylinder operating in the laminar vortex shedding regime at a fixed Reynolds number of $Re = 100$ . Specifically, we explore how fluid viscoelasticity affects the boundary between the lock-in and no lock-in regions and the corresponding wake characteristics compared with a simple Newtonian fluid. Our findings reveal that fluid viscoelasticity enables the synchronization of the vortex street with the cylinder motion at lower oscillation frequencies than those required for a Newtonian fluid. Consequently, the lock-in region boundary for a viscoelastic fluid differs from that of a Newtonian fluid and expands in the non-dimensional cylinder oscillation amplitude and frequency parameter space. In the primary synchronization region, the wake of a Newtonian fluid exhibits ‘2S’ (two single vortices) and ‘P+S’ (a pair of vortices and a single vortex) shedding modes. In contrast, a ‘2P’ (two pairs of vortices) vortex mode is observed for a viscoelastic fluid within the same region. To gain a deeper understanding of the differences in the coherent flow structures and their associated frequencies between the two fluids, we employ the data-driven reduced-order modelling technique, known as the dynamic mode decomposition (DMD) technique. Utilizing this technique, we successfully extract and visualize the two competing fundamental frequencies (cylinder oscillation and natural vortex shedding frequencies) and their associated flow structures in the case of the no lock-in state, whereas only the dominant cylinder oscillation frequency and associated flow structure in the case of the lock-in state. Furthermore, we propose that the presence of excess strain resulting from the stretching of polymer molecules in viscoelastic fluids leads to a distinct difference in the wake structure compared with Newtonian fluids. This observation aligns with the findings obtained from the $Q$ -criterion and vorticity transport analysis of the wake.

Suppression of the hydroelastic responses of a composite very large floating structure by a submerged elastic ring

With the aid of the method of matched eigenfunction expansions, the hydroelastic dynamics of a composite very large floating structure on a two-layer fluid is analytically investigated in the framework of linear potential flow theory. The composite structure consists of a semi-immersed rigid cylinder fixed with a floating elastic ring and protected by a submerged flexible ring for mitigating the hydroelastic responses. A new dispersion relation for the hydroelastic waves in the two-layer fluid containing a submerged plate is derived to obtain the wave numbers with a given frequency. The coupling effect among the oscillation of rigid cylinder, the deformation of the floating and submerged plates and the motion of wave is considered by dividing the problem into scattering and radiation parts. An inner product is introduced to deduce the unknown expansion coefficients of the velocity potentials. The amplitudes of heave and pitch motions for the rigid cylinder are obtained and shown graphically. The effects of the physical parameters are explored to confirm the mitigative effect of the submerged ring on the response of the composite very large floating structure. A submerged porous ring can also eliminate the amplitudes of waves in the surrounding water region.

Reinforcement learning-based active flow control of oscillating cylinder for drag reduction

This study explores the use of a reinforcement learning (RL)-based active flow control strategy to reduce the drag of a transversely oscillating cylinder confined between two walls. We incorporate wake kinematic information of the oscillating cylinder from direct numerical solution into the agent to actively adjust the oscillating amplitude. Our findings reveal two strategies that significantly improve drag reduction. First, the oscillating frequency should be chosen within the lock-in region. Second, placing probes in the active region of turbulent kinetic energy enables the full utilization of physical information for achieving more efficient and stable control. The results show that an effective control strategy can reduce the drag by 8.4% compared to the case of stationary cylinder at Re = 200. We identify three characteristics of the controlled flow that contribute to drag reduction: an elongated recirculation zone, an increased energy transport coefficient, and asymmetric cylinder oscillation. We observed that this well-behaved controlled flow can be obtained through real-time RL-based control, as it consistently attenuates the asymmetric dynamic mode decomposition modes of the flow. Furthermore, we find that the asymmetry cylinder oscillation synchronizes with the vortex shedding, indicating that the RL-based control can expand the lock-in region.

An analytical study on vortex-induced vibrations of cylinders with variable cross-section in the range of low Reynolds numbers

In the current paper, vortex-induced oscillations of circular cylinders with variable have been investigated. Considering the von-Kármán nonlinear strain-displacement relations and using the displacement coupling model to simulate the system and fluid-structure interaction, the nonlinear differential equations governing the motion have been derived using Hamilton’s principle. To apply the effect of vortices on the system vibrations, the nonlinear Van der Pol oscillator equation has been used. Then, using the Galerkin method, differential equations with partial derivatives are discretized, and nonlinear differential relations are solved by the Runge-Kutta approach. Finally, the response of the system, the phase curves, and the changes in the maximum amplitude according to the fluid velocity are obtained for various parameters, and the results have been discussed. Regarding the obtained results, changes in the cross-section of the beam have a significant effect on the lock-in region and also the maximum deflection of the beam. The maximum dynamic deflection for θ = 2° occurs at the velocity u = 2 , which is equal to 0.927. Also, the lock-in phenomenon is created for the beam with θ = 2.5° and θ = 3° at the velocities u = 2.78 and u = 4.23 , respectively.

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.

Influence of time-dependent wall temperature fluctuation on conjugate mixed convection inside a chamber with an oscillating cylinder

The current work is an attempt to examine the mixed convective heat flow in a square chamber subjected to time-dependent sinusoidal wall temperature containing a heat-conducting rotationally oscillating circular cylinder at the center. The right wall is kept at a cold temperature and the left wall is subjected to time-varying wall temperature, whereas the remaining walls are thermally insulated. A sinusoidal oscillating velocity is applied on the surface of the oscillating cylinder. This problem has practical relevance for several applications, notably solar collectors, microelectronic cooling systems, chemical reactors, radiators, and heaters. The main goal of this study is to promote heat transfer by investigating optimum system characteristics. The pressure-velocity formulation of the Navier-Stokes and heat energy equations is used to model the fluid domain inside the chamber, and heat transfer inside the cylinder is modeled by the two-dimensional heat conduction equation. Those equations are discretized using the Galerkin finite element method after converting them into non-dimensional forms. The instantaneous and time-averaged Nusselt numbers of the heated wall are examined for three different frequencies of the cylinder oscillation and sinusoidal wall temperature variation. Parametric simulation is carried out for three different cases within a mixed convection regime through a combination of Reynolds (31.62–316.23), Grashof (103–105), and Richardson (0.1–10) numbers. For each different combination, the optimum values of cylinder frequency are determined. Due to the thermal boundary layer's repeated contraction and expansion, the instantaneous Nusselt number demonstrates an oscillating pattern over time. Moreover, the maximum wall temperature frequency is found to maximize the heat transfer process.

Control of flow-induced vibration of a circular cylinder using a splitter plate

A circular cylinder attached by a rigid splitter plate of different lengths was tested to examine its effects on the control of flow-induced vibration. Tests were carried out in a closed-loop water channel. A cylinder of diameter D = 20 mm and a mass ratio m* ≈ 50 was installed to oscillate in the transverse direction. A wide range of splitter length was considered, i.e., L/D = 0–3.5, at a range of reduced velocity Ur = 1–25 and the Reynolds number Re = 800–11 000. Numerical simulations were also conducted to reveal the flow structures associated with the vibration modes observed in the experiment. It is found that, as L/D increases from 0 to 0.25, the peak value of cylinder oscillation amplitude increases and appears at higher reduced velocities. When the splitter length continues to rise, galloping-type oscillations occur at L/D = 0.5 and 0.75. The transition stage has been found at L/D = 1.0. Oscillation is then significantly suppressed when the splitter length is larger than L/D = 1.5.

Three-dimensional numerical investigation of a transversely oscillating slotted cylinder and its applications in energy harvesting

The present paper numerically investigates the fluid dynamics associated with the flow over an elastically mounted circular cylinder at a Reynolds number of 500. A slit normal to the flow direction is placed at the cylinder's centre. The study covers the large parametric set of calculations for the combined mass-damping ratio $m^{*}\zeta = 0.2$ , including the slit offset for six different offset angles ( $-30^\circ \leq \alpha \leq +30^\circ$ ) measured from the vertical axis at the centre point of the cylinder and the effect of slit widths ( $0.1 \leq s/D \leq 0.3$ ) on the aerodynamic loading, vibration response and associated flow characteristics. Furthermore, a wide range of reduced velocities ( $3\leq U_r \leq 7$ ) are examined for the complete closure of studying the effect on the vortex-induced vibration response. The results demonstrate that adding the normal slit increases the periodic suction-blowing phenomena, strengthening the vortex shedding. The results suggest that the normal slit can suppress or increase vortex-induced vibration depending on the slit-offset angle. Placing it toward the front stagnation point results in the increased oscillation amplitude (with a wider wake behind the cylinder) while shifting it toward the rear stagnation point diminishes the cylinder's oscillations. The paper reveals that this dual behaviour of the normal slit (based on its offset placement) is closely related to the phase difference between the lift force and the oscillation amplitude. Various vortex-shedding patterns associated with the different slit-offset angles are duly reported in the paper. Furthermore, a magnet is attached to the slit cylinder, and its effect on the energy harvesting capability via a coil-magnet arrangement is explored in detail for a wide range of reduced velocities.

Vortex-induced vibration and hydrodynamic characteristics of a round-ended cylinder

A round-ended section possesses the advantages of both circular and square sections, i.e. a large span and smooth transition at both ends. Therefore, the two-degree-of-freedom VIV response of a round-ended cylinder is numerically investigated in this paper in the reduced velocity (Ur) range of 2–16, comparing with the response of a circular cylinder of the same mass-damping ratio. The numerical results indicate that the development and separation of boundary layers determine the appearance of the minimum pressure coefficient, the vortex shedding mode and the characteristic sizes of wake structure. The reattachment and second separation of boundary layers are observed in the round-ended cylinder when Ur ≥ 6, contributing to a much downstream final separation point as compared to the circular cylinder. Both the hydrodynamic forces and the VIV response are closely associated with the vortex shedding mode, which is further illustrated by the variations of the added mass coefficient, the phase difference and the transferred energy coefficient. The turning points of the curves of hydrodynamic coefficients and response amplitudes correspond to the transition of vortex shedding mode as well as the switching of VIV branch. Compared to the circular cylinder, the oscillation of the round-ended cylinder presents an elongated lower branch and much smaller vibration amplitudes.

Suppression of vortex shedding in the wake of a circular cylinder through high-frequency in-line oscillation

This paper presents a new flow control approach to suppress the vortex shedding in the wake of a circular cylinder through high-frequency oscillation. The circular cylinder is forced to oscillate in the streamwise direction at high-frequency and low amplitude, corresponding to a high Stokes number (β = 100–1000) and low Keulegan–Carpenter number (KC = 0.001–4). Two-dimensional (2-D) and three-dimensional (3-D) direct numerical simulations of an oscillating circular cylinder in steady current have been carried out in the parameter space of KC, Rec, and β. Our numerical results show that when the flow remains in the two-dimensional vortex shedding regime, the cylinder wake sequentially experiences transitions from the vortex shedding regime to the suppression of the vortex shedding regime and finally to the symmetry breaking regime, with increasing KC. Corresponding wake characteristics and variations of hydrodynamic forces over the three wake regimes are explored. Three quantities that represent shear-layer characteristics, including the length of separating shear layers, the circulation of shear layers and wake recirculation length, reach maxima at the onset of suppression. The physical mechanisms for the suppression of vortex shedding and occurrence of symmetry breaking are also explained. Once the flow becomes 3-D, vortex shedding from the cylinder cannot be suppressed, primarily because the outer shear layers induced by the steady approaching flow are enhanced in 3-D flows. The cylinder oscillation over the frequency range investigated in the present study delays wake transition to 3-D. The cylinder oscillation alters the 3-D vortical structure and its spanwise wavelength significantly.