Cylinder Oscillation Frequency Research Articles

Simulation of flow across a row of transversely oscillating square cylinders

A numerical study of flow across a row of transversely oscillating square cylinders (of diameter d) has been undertaken using the lattice Boltzmann method, for a better understanding of fluid–structure interaction problems. The effects of cylinder oscillation frequency ratio (fe/fo, where fe is the cylinder oscillation frequency and fo is the corresponding vortex shedding frequency for stationary row of cylinders), amplitude ratio (A/d), non-dimensional spacing between the cylinders (s/d) and Reynolds number (Re) on ensuing flow regimes and flow parameters have been studied to understand the flow physics. Six different flow regimes observed in this study are the quasi-periodic non-lock-on-I, synchronous lock-on, quasi-periodic lock-on, quasi-periodic non-lock-on-II, synchronous non-lock-on and chaotic non-lock-on. It is observed that the range of the lock-on regime depends upon the relative dominance of incoming flow and cylinder motion. Although the lock-on regime in the case of Re = 80, s/d = 4 and A/d = 0.2 is substantially larger as compared to that for a single oscillating cylinder, the range of the lock-on regime shrinks with a reduction in the cylinder spacing, increase in the Reynolds number or decrease in the oscillation amplitude. It is also observed that the wake interaction behind the cylinders weakens with an increase in fe/fo, Re, A/d or s/d, leading to the formation of independent wakes and synchronous nature of the flow. For fe/fo ≥ 1.2, independent and intact oscillating wakes are noted and an additional frequency (wake oscillation frequency) is obtained in the time series of the lift coefficient. Although it was expected that the complexity in the wake interaction would increase with cylinder oscillation or amplitude ratio, an opposite effect (that is, formation of independent wakes) is noted from the results.

Characterization of long time fluctuations of forces exerted on an oscillating circular cylinder at KC=10

Flow dynamics, in-line and transverse forces exerted on an oscillating circular cylinder in a fluid initially at rest are studied by numerical resolution of the two-dimensional Navier–Stokes equations. The Keulegan–Carpenter number is held constant at KC=10 and Re is increased from 40 to 500. For the different flow regimes, links between flow spatio-temporal symmetries and force histories are established. Besides simulations of long duration show that in two ranges of Re, forces exhibit low frequency fluctuations compared to the cylinder oscillation frequency. Such observations have been only mentioned in the literature and are more deeply examined here. In both ranges, force fluctuations correspond to oscillations of the front and rear stagnation points on the cylinder surface. However, they occur in flow regimes whose basic patterns (V-shaped mode or diagonal mode) have different symmetry features, inducing two distinct behaviors. For 80 ≤ Re ≤ 100 , fluctuations are related to a spectral broadening of the harmonics and to a permutation between three vortex patterns (V-shaped, transverse and oblique modes). In the second range 150 ≤ Re ≤ 280 , amplitude fluctuations are correlated to the appearance of low frequency peaks interacting with harmonics of the cylinder frequency. Fluctuations are then a combination of a wavy fluctuation and an amplitude modulation. The carrier frequency corresponding to the wavy fluctuation depends on Re and is related to a fluid characteristic time; the modulation frequency is independent of Re and equal to 1/4 of the cylinder oscillation frequency.

Numerical simulation of an elementary Vortex-Induced-Vibration problem by using fully-coupled fluid solid system computation

Numerical simulation of Vortex-Induced-Vibrations (VIV) of a rigid circular elastically-mounted cylinder submitted to a fluid cross-flow has been extensively studied over the past decades, both experimentally and numerically, because of its theoretical and practical interest for understanding Flow-Induced-Vibrations (FIV) problems. In this context, the present article aims to expose a numerical study based on fully-coupled fluid-solid computations compared to previously published work [34], [36]. The computational procedure relies on a partitioned method ensuring the coupling between fluid and structure solvers. The fluid solver involves a moving mesh formulation for simulation of the fluid structure interface motion. Energy exchanges between fluid and solid models are ensured through convenient numerical schemes. The present study is devoted to a low Reynolds number configuration. Cylinder motion magnitude, hydrodynamic forces, oscillation frequency and fluid vortex shedding modes are investigated and the “lock-in” phenomenon is reproduced numerically. These numerical results are proposed for code validation purposes before investigating larger industrial applications such as configurations involving tube arrays under cross-flows [4].

Generation, propagation, and breaking of an internal wave beam

We report upon an experimental study of internal gravity waves generated by the large-amplitude vertical oscillations of a circular cylinder in uniformly stratified fluid. Quantitative measurements are performed using a modified synthetic schlieren technique for strongly stratified solutions of NaCl or NaI. The oscillatory forcing leads to the development of turbulence in the region bounding the cylinder. This turbulence is found to be the primary source of the observed quasimonochromatic wave beams, whose characteristics at early times differ from theoretical predictions and experimental investigations of waves generated by small-amplitude cylinder oscillations. In particular, their wavelength is set by the Ozmidov scale rather than the size of the cylinder. The wave frequency is set by the buoyancy frequency N if the cylinder frequency is larger or much less than N. Otherwise it is set by the cylinder oscillation frequency. Over long times the finite-amplitude waves that have propagated away from their source are observed to break down and the process is examined quantitatively through conductivity probe measurements and qualitatively through unprocessed synthetic schlieren images. From an analysis of the location of wave breakdown we determine that the likely mechanism for breakdown is through parametric subharmonic instability. This conclusion is supported by fully nonlinear numerical simulations of the evolution of a temporally, although not spatially, monochromatic internal wave beam.

LAMINAR FLOW PAST AN OSCILLATING CIRCULAR CYLINDER IN CROSS FLOW

The present study numerically investigates the twodimensional laminar flow past a circular cylinder forced to oscillate transverse to the free-stream. The numerical simulations are performed at a various range of cylinder oscillation frequency ranged between 0.8 and 1.2 of the natural vortex shedding frequency, and the oscillation amplitude extended up to 50% of the cylinder diameter at one Reynolds number of 185 showing the typical two-dimensional vortex shedding. The immersed boundary method is used to handle the oscillating cylinder in a rectangular grid system using the finite volume method. The primary vortex shedding frequency has the same value with the exciting frequency. When the exciting frequency exceeds the natural vortex shedding frequency, the secondary vortex shedding frequency appeared with the value less than the natural shedding frequency. The time sequence of the wake structures near the cylinder at the extreme upper position reveals that a single vortex or a pair of vortices appears according to the time. A pair of vortices is composed of two saddle points and two centers of vortices, showing the occurrence of vortex switching phenomenon. The quantitative information about the flow variables such as the distribution of wall vorticity on the cylinder surface, drag and lift coefficients is highlighted.

Heat transfer enhancement in a slot channel via a transversely oscillating adiabatic circular cylinder

Heat transfer enhancement in a uniformly heated slot channel due to vortices shed from a transversely oscillating adiabatic circular cylinder is investigated. Effects of the cylinder motion and vortex shedding on heat transfer are systematically assessed by varying the cylinder oscillation frequency from 75% to 125% of the natural vortex shedding frequency of a fixed cylinder within the same domain. Numerical simulations at Re = 100 and 0.1 ⩽ Pr ⩽ 10 are performed using spectral element discretization of Navier–Stokes and energy equations in a moving domain based on an arbitrary Lagrangian–Eulerian formulation. Results within the thermally developing flow region show heat transfer enhancement due to the placement of a stationary cylinder compared to the straight channel case. Transverse oscillations of the cylinder further increase the wall heat transfer coefficient. Pumping power in the channel and the power necessary to oscillate the cylinder is also provided for comparisons. Cylinder oscillations with 75% of the natural vortex shedding frequency is shown to yield the best results with only 10% more power to pump the fluid, compared with the fixed cylinder case.

Flow past an oscillating circular cylinder in a channel with an upstream splitter plate

A transversely oscillating circular cylinder confined in a channel has the potential to promote mixing and heat transfer at moderate Reynolds number flows. In the present study, simulation results for flow past a circular cylinder subjected to forced cross-flow oscillations in a straight channel with an upstream splitter plate are presented in a wide range of cylinder oscillation frequencies, including the subharmonic, superharmonic, and primary lock-in regimes. Simulations are performed at Re=100, with cylinder oscillation amplitude of 0.4 diameters and a blockage ratio of 1/3. A spectral element algorithm based on the arbitrary Lagrangian Eulerian formulation is utilized. The numerical method exhibits spectral accuracy and allows large mesh deformation in the computational domain without mesh refinements. The main objective of this study is systematic investigations of the cylinder oscillation on the vortex shedding mechanism, downstream vortex patterns, and forces exerted on the cylinder.

Beating of a circular cylinder mounted as an inverted pendulum

This paper contains temporally and spatially resolved flow visualization and DPIV measurements characterizing the frequency–amplitude response and three-dimensional vortex structure of a circular cylinder mounted like an inverted pendulum. Two circular cylinders were examined in this investigation. Both were 2.54 cm in diameter and ~140 cm long with low mass ratios, m* = 0.65 and 1.90, and mass–damping ratios, m*ζ = 0.038 and 0.103, respectively. Frequency–amplitude response analysis was done with the lighter cylinder while detailed wake structure visualization and measurements were done using the slightly higher-mass-ratio cylinder. Experiments were conducted over the Reynolds number range 1900≤Re≤6800 corresponding to a reduced velocity range of 3.7 ≤ U* ≤ 9.6. Detailed examination of the upper branch of the synchronization regime permitted, for the first time, the identification of short-time deviations in cylinder oscillation and vortex-shedding frequencies that give rise to beating behaviour. That is, while long-time averages of cylinder oscillation and vortex-shedding frequencies are identical, i.e. synchronized, it is shown that there is a slight mismatch between these frequencies over much shorter periods when the cylinder exhibits quasi-periodic beating. Data are also presented which show that vortex strength is also modulated from one cylinder oscillation to the next. Physical arguments are presented to explain both the origins of beating as well as why the quasi-periodicity of the beating envelopes becomes irregular; it is believed that this result may be generalized to a broader class of fluid–structure interactions. In addition, observations of strong vertical flows associated with the Kármán vortices developing 2–3 diameters downstream of the cylinder are presented. It is hypothesized that these three-dimensionalities result from both the inverted pendulum motion as well as free-surface effects.

Two-degree-of-freedom vortex-induced vibration of a pivoted cylinder below critical mass ratio

In this study, we investigated two-degree-of-freedom (2d.f.) vortex-induced vibrations (VIVs) of a circular cylinder with a pinned attachment at its base; it had identical mass ratios and natural frequencies in both streamwise and transverse directions. The cylinder had a mass ratio, m * of 0.45, and a mass damping, ( m * + C A ) ζ , equal to 0.0841. Laser-induced fluorescence flow visualization and digital particle image velocimetry experiments were conducted over a Reynolds number range, 820≤ Re ≤6050 (corresponding to the reduced velocity range, 1.1≤ U * ≤8.3). Measurements and visualization studies were made in a fixed plane at the cylinder mid-height, providing a two-dimensional picture of a highly three-dimensional system. However, significant insights can be gained from these experiments and form the basis of this paper. A large transverse amplitude response, (or four diameters peak-to-peak), in the upper branch was observed. The streamwise amplitude response exhibits an even higher peak amplitude, , which is approximately 125% of peak . Results show that there is no lower branch for this system and the transverse upper branch exhibits asymptotic behaviour, i.e. a wide regime of resonance. For Re >3000, the Strouhal number for the vortex shedding was 0.16 (±9%). Both the transverse cylinder oscillation and vortex-shedding frequencies, f OS, Y and f VS , respectively, were virtually identical throughout this range. While the streamwise oscillation frequency is typically twice the transverse oscillation frequency for a 2d.f. system, this is not the case at the lowest reduced velocities where oscillations first occur. Under these conditions the streamwise and transverse oscillation frequencies were identical. Finally, we observed that the cylinder wake exhibits both the P+S vortex-shedding mode and a desynchronized vortex pattern, which are uncommon for flow past a cylinder experiment. Very interestingly, the wide U * range over which resonance occurs is dominated by a desynchronized vortex pattern. These results clearly demonstrate the differences that arise in 2d.f. VIV occurring below the critical mass ratio.

Vortex Flow Around an In-Line Forced Oscillating Circular Cylinder

In this study, the flow features of vortex shedding from a circular cylinder oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fK=0-6.2, amplitude ratio 2a/d=0.0625-1.0, and Reynolds number Re=570-1700. The variations of mean vortex shedding frequency were investigated and the existence range of the lock-in was shown. It was obtained that although the cylinder oscillation frequency is lower than the natural Karman vortex's frequency; the lock-in phenomenon arises. It was found that the distribution of the lock-in form in the lock-in region was shown and that it became the tendency where the range with the frequency which becomes the lock-in form of fVK/f=1/1 with the increase of amplitude ratio expands. The flow patterns were classified into five kinds by the configuration of vortex shedding and the direction of vortex shedding. The structure of the vortex flow which depends on the frequency ratio was clarified by observing flow from both to the axial direction and the direction of span. In the case of lock-in state fVK/f=1/1, two kinds of vortex structure were obtained. One of which was three-dimensional characteristic on the side of low oscillation frequency, and the other was two-dimensional characteristic on the side of high oscillation frequency. In the case of lock-in state fVKf=1/2, even if the lock-in had occurred, it was shown that the phase difference sometimes occurred to the direction of the span in vortex shedding.

Vortex Shedding from a Pair of In-Line Forced Oscillating Parallel Arranged Circular Cylinders (In the Case of Equal Diameter)

In this study, the flow features of vortex shedding from a pair of parallel arranged circular cylinders oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fk=0-7, amplitude ratio 2a/d=0.125, 0.25, 0.5 and 0.75, gap ratio G/d=0.25, 0.75 and 1.75, and Reynolds numbers Re=470-670 (towing water tank test) and 700-750 (closed circuit water channel test). The variations of mean vortex shedding frequency from each oscillating cylinder were investigated. As a result of the experiments, typical flow patterns of lock-in state or un-lock-in state were shown every gap ratio G/d. Although the cylinder oscillation frequency f is lower than the natural Karman vortex's frequency fk, the lock-in phenomenon can be seen. It was found that the measurement result depended on the observation point because the rearrangement of vortex formation had changed the characteristics of the flow. When the cylinders were oscillated to the direction of flow, the flow pattern of bias gap flow became not seen. The map of lock-in state in the lock-in range was obtained. It was found that the lock-in range and the lock-in form were more different from the case of single oscillating cylinder.

Experimental Investigation of Added Mass Coefficient with a Free-Oscillating Circular Cylinder

The shell and circular cylinder heat exchanger are widely used for various devices, such as kettle reboilers, feed water heaters, steam generator, evaporator in power, generation and so on. In late years, taking measures for an anti-vibration of the heat exchanger, it begins to be important problem to investigate flow induced vibration. The added mass is one of the essential parameters about flow induced vibration. Though the added mass coefficient is same as the displaced mass in the conventional understanding, recent study with the forced oscillation experiments revealed the added mass coefficient varies depending on the reduced velocity Vr. In this study, the added mass coefficients of the circular cylinder oscillation were measured by free oscillation experiments in a water channel. The frequency of circular cylinder oscillation and the flow oscillation around the circular cylinder were measured simultaneously with DynamicPIV, no intrusive to the flow and high spatial and time resolved, measurement techniques. The result showed the added mass coefficient varies depending on the reduced velocity Vr in free oscillation also.

가상경계법을 사용한 횡단 진동하는 실린더 주위의 유동 해석

Numerical calculations are carried out for flow past a circular cylinder forced oscillating normal to the free-stream flow at a fixed Reynolds number equal to 185. The cylinder oscillation frequency ranged from 0.8 to 1.2 of the natural vortex-shedding frequency, and the oscillation amplitude extended up to 20% of the cylinder diameter. IBM (Immersed Boundary Method) with direct momentum forcing was adopted to handle both of a stationary and an oscillating cylinder Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. The instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios showed the synchronized wakes pattern in the lock-in region and vortex switching phenomenon at higher frequency ratio than the critical frequency ratio.

Control of flow over a cylinder using rotational oscillations

In this work, we focus on two-dimensional, incompressible viscous channel flow over an infinitely long cylinder and examine the effect of controlled rotational cylinder oscillation in reducing the drag exerted on the cylinder. Based on an analysis of the open-loop simulations, a control system is designed that automatically determines the frequency of cylinder oscillation, based on the Reynolds number, to consistently reduce the drag exerted on the cylinder for Reynolds numbers in the range 100–500.

1628 流れ方向に強制振動している迎え角 45°の正方形柱からの渦放出の可視化観測

In this study, the flow features of vortex shedding from a square cylinder forced-oscillating in the in-line direction were investigated by use of flow visualization technique at the Reynolds number Re=1004,with six kinds of amplitude ratio and the range of oscillation frequency ratio from 0.0 to 12.7. As a result of the experiments, six kinds of representative flow patterns were observed. In the case of the smallest amplitude ratio 2a/h=0.0353,there was not an influence of the cylinder oscillation to the mean vortex shedding frequency. It was found that when the oscillation amplitude ratio 2a/h becomes large, the frequency ratio which the lock-in occurs to becomes small and even if the cylinder oscillation frequency is smaller than the natural Karman vortex shedding frequency, The lock-in phenomenon arises. The flow pattern map in the lock-in region was shown.

A NUMERICAL SIMULATION OF VORTEX SHEDDING FROM AN OSCILLATING CIRCULAR CYLINDER

Vortex shedding from an oscillating circular cylinder is studied by numerical solutions of the two-dimensional unsteady Navier–Stokes equations. A physically consistent method is used for the reconstruction of velocity fluxes which arise from discrete equations for the mass and momentum balances. This method ensures a second-order accuracy. Two phenomena are investigated and, in both cases, the cylinder oscillation is forced. The first is the flow induced by the harmonic in-line oscillation of cylinder in water at rest. The Reynolds number is equal to 100 and the Keulegan–Carpenter number is equal to 5. A comparison of phase-averaged velocity vectors between measurements and predictions is presented. Applying the widely used model of Morison to the computed in-line force history, the drag and the inertia coefficients are calculated and compared for different grid levels. Using these to reproduce the force functions, deviations from those originally computed are revealed. The second problem is an investigation of a transversely oscillating cylinder in a uniform flow at fixed Reynolds number equal to 185. The cylinder oscillation frequency ranges between 0·80 and 1·20 of the natural vortex-shedding frequency, and the oscillation amplitude is 20% of the cylinder diameter. As the frequency of excitation of the cylinder increases relative to the inherent vortex formation frequency, the initially formed concentration of vorticity moves closer to the cylinder until a limiting position is reached. At this point, the vorticity concentration abruptly switches to the opposite side of the cylinder. This process induces distinct changes of the topology of the corresponding streamline patterns.

Finite element analysis of bubbly flow around an oscillating square-section cylinder

The bubbly flow around a square-section cylinder, which is forced to oscillate normal to a uniform flow, is simulated by an incompressible two-fluid model. The model is solved by the Arbitrary Lagrangian–Eulerian (ALE) finite element method. The width of the cylinder is 30 mm . A bubbly flow is assumed, and the bubble diameter is set as 1 mm . The volumetric fraction of the gas-phase upstream of the cylinder is 0.05, where the Reynolds number defined by the volumetric velocity of the liquid-phase is 200. With an increment in the cylinder oscillation frequency Sc, the vortex shedding frequency St approaches the Sc value. When the Sc is increased beyond a critical value, the lock-in occurs, in which the vortex shedding is synchronized with the cylinder oscillation. The time-averaged drag coefficient and the root-mean-square of the lift coefficient for the cylinder take their maximum values at the lock-in state. The volumetric fraction of the gas-phase α g is higher in the wake of the cylinder, irrespective of the Sc value. The α g value is lower below the lock-in state and higher in the lock-in state than the value for the stationary cylinder.

NUMERICAL INVESTIGATION OF A ROTATIONALLY OSCILLATING CYLINDER IN MEAN FLOW

Vortex shedding and the development of a wake behind a rotationally oscillating circular cylinder was investigated using a hybrid vortex method at a Reynolds number of 1000 over a wide range of forcing frequency and amplitude. The normalised peripheral velocity–oscillation amplitude of the cylinder ranged from 0 to 3 while the ratio of forcing frequency to the vortex-shedding frequency from a stationary cylinder varied from 0 to 10. The time-dependent pressure, lift and drag forces exerted on the cylinder were studied together with the flow patterns in the wake. Some behaviours of vortex shedding are revealed and the lock-on range for vortex shedding is obtained. It is found that, in the case of a very low frequency ratio, vortices are shed at a frequency close to that from a stationary cylinder when the amplitude is small; however, the vortices are shed at cylinder-oscillation frequency when the amplitude is large. When the frequency ratio is close to 1, the form of vortex shedding and lock-on exhibit a particularly strong resonance between the flow perturbations and the vortex wake, and the mean value of the drag coefficients increases remarkably. Its maximum value increases with increasing amplitude within the lock-on range and shifts towards the lower frequency end of the lock-on range. When the frequency ratio is greater than a certain value beyond the lock-on range, small-scale vortices are shed at the forcing frequency in the near wake. Subsequently, these vortices coalesce and result in a large-scale antisymmetrical structure in the far wake similar to the Kármán vortex street past a stationary cylinder. The mean value of the drag coefficients decreases in the post lock-on frequency range. The larger the amplitude, the more distinct is the drag coefficient decrease, and the minimum value is lower than that for flow past a stationary cylinder. After the minimum is reached, the drag coefficient increases again with further increase in cylinder-oscillation frequency and approaches the value for the stationary cylinder.

CROSS-FLOW PAST AN OSCILLATING CIRCULAR CYLINDER: SYNCHRONIZATION PHENOMENA IN THE NEAR WAKE

Cross-flow past a circular cylinder in a water tunnel undergoing forced harmonic oscillations in the transverse plane was investigated experimentally at Reynolds numbers ranging from 1250 to 1500, by means of dye visualization of the formation region and measurement of wake spectra. Cylinder oscillation frequencies feranging from 13-subharmonic excitation to 3-superharmonic excitation were considered, with the amplitude of oscillation fixed at 0·22 diameters. As feis incremented within the fundamental lock-in regime, the structure of the formation region undergoes two distinct transitions. The first transition, which is accompanied by an abrupt reduction in vortex formation length, corresponds to a switch from the ‘2P’ wake vortex pattern to the ‘2S’ wake vortex pattern of Williamson & Roshko. After a subsequent transition, the phase of vortex shedding relative to cylinder displacement switches by about π. Between the two transitions, the vortex formation length is a minimum. A similar minimum seems to be next attained only when feis increased to achieve 3-superharmonic synchronization. Finally, while for both the 13-subharmonic excitation and the12-subharmonic excitation the topology of the formation region is periodic with every cycle of cylinder oscillation, a synchronized wake pattern forms only in the former case.

Streamwise oscillations of a cylinder in a steady current. Part 1. Locked-on states of vortex formation and loading

Streamwise oscillations of a circular cylinder in a steady uniform flow are investigated experimentally using a technique of high-image-density particle image velocimetry, in conjunction with instantaneous force measurements. This approach allows insight into the relationship between the loading and the patterns of vorticity and streamline topology in the near wake.In analogy with the classical locked-on state arising from transverse oscillations of a cylinder in uniform flow, it is possible to attain either locked-on or quasi-locked-on states due to streamwise oscillations. In these cases, however, the repetitive signature of the transverse force is not sinusoidal; rather, it is strongly modulated and the corresponding spectra can exhibit several sharply defined peaks. The predominant peak can vary over a remarkably wide range, extending from the subharmonic to the third harmonic of the cylinder oscillation frequency; for certain locked-on states of the transverse force signature, the spectral peak at the cylinder oscillation frequency is actually suppressed. Corresponding instantaneous traces and spectra of the in-line force simply show dominance of the spectral peak at the cylinder oscillation frequency. Further interpretation of the loading is provided in terms of Lissajous patterns of the transverse and in-line force coefficients.All of these features are related to the instantaneous patterns of vortex formation in the near wake. During a typical cycle of the cylinder oscillation, these patterns can be divided into two broad categories: Kármán-like shedding; and a nearly ‘frozen’ array of shed vortices. The order of occurrence of these basic patterns during an oscillation cycle dictates the instantaneous signatures and time-averaged spectra of the transverse force.