• 한국전산유체공학회지
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• 제16권3호
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• pp.8-14
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• 2011

Although the geometry of circular cylinder is simple, the flow is complicate because of the flow separation and vortex shedding. In spite of many numerical and experimental researches, the flow around a circular cylinder has not been clarified even now. It has been known that the unsteady vortex shedding from a circular cylinder can vibrate and damage a structure. Lock-on phenomenon is very important in the flow around an oscillating circular cylinder. The lock-on phenomenon is that when the oscillation frequency of the circular cylinder is at or near the frequency of vortex shedding from a stationary cylinder, the vortex shedding synchronizes with the cylinder motion. This phenomenon can be recognized by the spectral analysis of the lift coefficient history. At the lock-on region the vortex is shedding by the modulated frequency to the body frequency. However, the vortex is shedding by the mixed frequencies of natural shedding and forced body frequency in the region of non-lock-on. In this paper, it was analyzed the relation between the frequency of rotary oscillating circular cylinder and the vortex shedding frequency.

• 대한기계학회논문집B
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• 제22권5호
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• pp.573-583
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• 1998

Effects of the oscillating incoming flow on vortex shedding and lock-on behind a square cylinder are investigated using numerical simulations at a Reynolds number of 100. Vortex shedding occurred at low forcing frequencies of the incoming flow similar to the natural vortex shedding. As the forcing frequency further increases, the shedding frequency decreases to the half of the forcing freqnency. For a sufficiently large frequency, vortex shedding returns to the natural vortex shedding irrespective of the forcing amplitude. Also, the lock-on region becomes wider with higher forcing amplitudes. The phase diagram between the drag and lift shows a simple periodic behavior in the lock-on region, while a complicated periodic phase relation is observed when there is no lock-on.

• Wind and Structures
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• 제7권4호
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• pp.265-280
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• 2004

This paper presents some fundamental results on the dynamics of the periodic Karman wake behind a circular cylinder. The wake is treated like a dynamical system. External forcing is then introduced and its effect investigated. The main result obtained is the following. Perturbation of the wake, by controlled cylinder oscillations in the flow direction at a frequency equal to the Karman vortex shedding frequency, leads to instability of the Karman vortex structure. The resulting wake structure oscillates at half the original Karman vortex shedding frequency. For higher frequency excitation the primary pattern involves symmetry breaking of the initially shed symmetric vortex pairs. The Karman shedding phenomenon can be modeled by a nonlinear oscillator. The symmetrical flow perturbations resulting from the periodic cylinder excitation can also be similarly represented by a nonlinear oscillator. The oscillators represent two flow modes. By considering these two nonlinear oscillators, one having inline shedding symmetry and the other having the Karman wake spatio-temporal symmetry, the possible symmetries of subsequent flow perturbations resulting from the modal interaction are determined. A theoretical analysis based on symmetry (group) theory is presented. The analysis confirms the occurrence of a period-doubling instability, which is responsible for the frequency halving phenomenon observed in the experiments. Finally it is remarked that the present findings have important implications for vortex shedding control. Perturbations in the inflow direction introduce 'control' of the Karman wake by inducing a bifurcation which forces the transfer of energy to a lower frequency which is far from the original Karman frequency.

• Journal of Advanced Marine Engineering and Technology
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• 제25권4호
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• pp.846-856
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• 2001

An experimental study has been made to investigate the effect on wake flow and vortex shedding frequency by vortex stabilizer in Karman vortex type air flow sensor. The conditions investigated include 3 types of shapes and 3 types of separation distances of the vortex stabilizer. The phase averaged technique and smoke-wire flow visualization method are used to understand the detail information. The rolling up position of shear layer is fixed by the influence of the vortex stabilizer. Especially, the convex type vortex stabilizer has shown the more stable repeatability and linearity regarding the vortex shedding frequency compared to the other types.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.245-248
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• 2002

Vortex lock-on or resonance in the flow behind a circular cylinder is visualized by a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K{\'{a}}rm{\'{a}}n$ vortices in the wake-transition regime at the Reynolds number 360. Basically, natural shedding state is observed to compare with lock-on state. Wake motion by the change of the shedding frequency of lock-on state is investigated. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. The physical flow phenomena of natural shedding and lock-on states are analyzed with physical parameters of recirculation and vortex formation region. Consequently, it is found that the change of wake bubble plays an important role in the flow at the lock-on state. Vortex formation region is also actively changed like recirculation region as the lock-on occurs. Therefore, it is deduced that the recirculation region is closely related with the vortex formation region.

• Korea-Australia Rheology Journal
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• 제21권1호
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• pp.27-37
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• 2009

In this work, we numerically investigate the effect of viscoelasticity on 2D laminar vortex dynamics in flows past a single rotating cylinder for rotational rates $0{\leq}{\alpha}{\leq}5$ (the rotational rate ex is defined by the ratio of the circumferential rotating velocity to free stream velocity) at Re=100, in which the vortex shedding has been predicted to occur in literature for Newtonian fluids. The objective of the present research is to develop a promising technique to fully suppress the vortex shedding past a bluff body by rotating a cylinder and controlling fluid elasticity. The predicted vortex dynamics with the present method is consistent with the previous works for Newtonian flows past a rotating cylinder. We also verified our method by comparing our data with the literature in the case of viscoelastic flow past a non-rotating cylinder. For $0{\leq}{\alpha}{\leq}1.8$, the frequency of vortex shedding slightly decreases but the fluctuation of drag and lift coefficient significantly decreases with increasing fluid elasticity. We observe that the vortex shedding of viscoelastic flow disappears at lower ${\alpha}$ than the Newtonian case. At ${\alpha}$=5, the relationship between the frequency of vortex shedding and Weissenberg number (Wi) is predicted to be non-monotonic and have a minimum around Wi=0.25. The vortex shedding finally disappears over critical Wi number. The present results suggest that the vortex shedding in the flow around a rotating cylinder can be more effectively suppressed for viscoelastic fluids than Newtonian fluids.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2007년도 추계 학술대회논문집
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• pp.115-119
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• 2007

In this paper, a flow visualization analysis has been carried out on an oscillating square section cylinder, numerically, using a commercially available code CFD-ACE. In this study, the square cylinder is forced to oscillate at different frequencies of excitation, viz., fe/fo=0.5, 1.0 and 2.0 (where, fe is the excitation frequency provided to the cylinder and fo is the natural vortex shedding frequency from the stationary cylinder at a particular Reynolds number (=5200). In all the cases, the peak-to-peak amplitude of oscillation is kept at 32% of the side dimension of the square cylinder. These studies are conducted to understand the influence of frequency of oscillation on the flow field features around the cylinder, particularly the mode of vortex shedding. Results indicate that, the flow field around a square cylinder is very much influenced by the excitation frequency, in particular the vortex shedding mode. It is also found that, the vortex street parameters are significantly influence by the oscillation frequency. Comparison with earlier reported experimental studies has also been attempted in this paper. In appears that, such a numerical exercise (as performed in this paper) is first of its kind. It is believed that, these studies would enable one to understand the mechanisms underlying the flow-induced vibrations of a square section cylinder.

• 한국항공우주학회지
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• 제41권1호
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• pp.40-47
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• 2013

하이브리드 로켓은 고체로켓과 매우 흡사한 내부유동 특성을 가지고 있으므로 와류흘림(vortex shedding)은 하이브리드 로켓에서도 연소안정성에 영향을 미치는 중요한 인자로 판단된다. 본 연구에서는 예연소실에서 와류를 발생시켜 연료 표면와류와의 간섭과 연소특성 변화를 관찰하였다. 기본 형상과 디스크를 장착한 실험 결과, 5개의 주파수 특성이 관찰되었으며 이들은 각각 추진제의 열적지연에 의한 주파수, 디스크, 연료 단면, 후연소실 단면 변화에 의한 와류흘림과 관련된 주파수임을 확인하였다. 특히 디스크 1과 3의 결과를 비교해 보면 디스크 위치에 따라 변화하는 와류흘림이 연소실의 특정 주파수 특성과 상호 간섭하여 공진이 발생하기도 하지만, 또 다른 경우에는 어떠한 간섭도 일어나지 않는 것을 확인하였다. 이러한 선택적 공진현상은 하이브리드 로켓의 비선형 연소안정성 발생을 설명할 수 있는 중요한 메카니즘으로 판단된다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제2권2호
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• pp.112-118
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• 2010

The numerical study about the vortex-induced vibration and vortex shedding in the wake has been presented. Prior to the numerical simulation of flexible riser systems concerning engineering conditions, efficiency validating of the proposed FSI solution method have been performed. The comparison between numerical simulation and published experimental data shows that the CFD method designed for FSI solution could give acceptable result for the VIV prediction of flexible riser/pipe system. As meaningful study on VIV and vortex shedding mode with the focus on flexible riser model systems, two kinds of typical simulation cases have been carried out. One was related to the simulation of vortex visualization in the wake for a riser model subject to forced oscillation, and another was related to the simulation of fluid-structure interaction between the pipes of coupled multi-assembled riser system. The result from forced oscillation simulation shows that the vortex-induced vibration with high response frequency but small instantaneous vibration amplitude contributes to vortex conformation as much as the forced oscillation with large normalized amplitude does, when the frequency of forced oscillation was relatively high. In the multi-assembled riser systems, it has been found that the external current velocity and the distance between two pipes are the critical factors to determine the vibration state and the steady vibration state emerging in quad-pipe system may be destroyed more easily than dual-pipe system.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2004년도 추계학술대회 논문집
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• pp.18-21
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• 2004

Lock-on characteristics of the flow around a circular cylinder performing a rotationally oscillation with a relatively high forcing frequency have been investigated experimentally using flow visualization and hot-wire measurements. Dominant parameters are Reynolds number (Re), amplitude of oscillation $(\theta_A)$, and frequency ratio $F_R=f_f\;/\;f_n$, where $f_f$ is the forcing frequency and if is the natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14\times10^3,\;\pi/15\leq\theta_A\leq\pi/3$, and $F_R=1.0$. The effects of this active control technique on the lock-on flow regime of the cylinder wake were evaluated through wake velocity measurements and spectral analysis of hot-wire signals. The rotary oscillation modified the flow structure of near wake significantly. The lock-on phenomenon was found to occur in the range of frequency encompassing the natural vortex shedding frequency. In addition, when the amplitude of oscillation is less than a certain value, the lock-on phenomenon was occurred only at $F_R=1.0$. The lock-on range expanded and vortex formation length decreased as the amplitude of oscillation increases. The rotary oscillation generated small-scale vortex structure just near the cylinder surface.