• Title/Summary/Keyword: Vortex Shedding

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Fluidelastic Instability of Flexible Cylinders in Tube Bundle Subjected to Cross Air-flow (공기-횡 유동장에 놓인 유연성 실린더 관군의 유체탄성 불안정)

  • Sim, Woo-Gun
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.17 no.6 s.123
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    • pp.498-506
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    • 2007
  • Using wind tunnel, experimental approaches were employed to investigate fluidelastic instability of tube bundles, subjected to uniform cross flow. There are several flow-induced vibration excitation mechanisms, such as fluidelastic instability, periodic wake shedding resonance, turbulence-induced excitation and acoustic resonance, which could cause excessive vibration in shell-and tube heat exchanges. Fluidelastic is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to cross flow. The system comprised of cantilevered flexible cylinder(s) and rigid cylinders of normal square array, In order to see the characteristics of flow in tube bundles, particle image velocimetry was used. From a practical design point of view, Fluidelastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping. The threshold flow velocity for dynamic instability of cylinder rows is evaluated and the data for design guideline is proposed for the tube bundles of normal square array.

Flow Characteristics According to Velocity Conditions of Cylinder Boundary Under Low Reynolds Number (저 레이놀즈 수에서 실린더 경계 유속조건에 따른 흐름 특성)

  • Song, Chang Geun;Seo, Il Won;Kim, Tae Won
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.33 no.6
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    • pp.2267-2275
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    • 2013
  • Existing conventional model for analysis of shallow water flow just assumed the internal boundary condition as free-slip, which resulted in the wrong prediction about the velocity, vorticity, water level, shear stress distribution, and time variation of drag and lift force around a structure. In this study, a finite element model that can predict flow characteristics around the structure accurately was developed and internal boundary conditions were generalized as partial slip condition using slip length concept. Laminar flow characteristics behind circular cylinder were analyzed by varying the internal boundary conditions. The simulation results of (1) time variations of longitudinal and transverse velocities, and vorticity; (2) wake length; (3) vortex shedding phenomena by slip length; (4) and mass conservation showed that the vortex shedding had never observed and laminar flow like creeping motion was occurred under free-slip condition. Assignment of partial slip condition changed the velocity distribution on the cylinder surface and influenced the magnitude of the shear stress and the occurrence of vorticity so that the period of vortex shedding was reduced compared with the case of no slip condition. The maximum mass conservation error occurred in the case of no slip condition, which had the value of 0.73%, and there was 0.21 % reduction in the maximum mass conservation error by changing the internal boundary condition from no slip to partial slip condition.

Behavior of Oil-Water Interface between Tandem Fences (이중 유벽 사이의 기름과 물의 계면의 거동)

  • Kang Kwan Hyoung;Lee Choung Mook
    • Journal of the Korean Society for Marine Environment & Energy
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    • v.2 no.2
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    • pp.70-77
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    • 1999
  • The disturbance of oil-water interface confined between tandem fences caused by a sequence of traveling vortices below the interface is investigated. The traveling vortices are assumed to be those detached from the tip of the fore fence. The potential flow is assumed and the density interface is replaced as a sheet of vortex. The shape of the interface is predicted by tracing a finite number of marker particles placed at the interface. The velocity of the marker particles is determined by the Biot-Savart integral along the vortex sheet plus the contribution from the traveling point vortices. The rate of change of vortex-sheet strength is predicted by using an evolution equation for vorticity. The calculated results obtained for various conditions demonstrate that the large amplitude of interfacial wave following the moving vortek can be generated by the vortices.

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Feedback Control of a Circular Cylinder Wake with Rotational Oscillation (주기적 회전을 이용한 원봉 후류의 되먹임 제어)

  • Baek, Seung-Jin;Seong, Hyeong-Jin
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.26 no.9
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    • pp.1234-1240
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    • 2002
  • A new feedback control law is proposed and tested for suppressing the vortex shedding from a circular cylinder in a uniform flow. The lift coefficient ( $C_{L}$) is employed as a feedback control signal and the control forcing is given by a rotational oscillation of the cylinder. The influence of the feedback transfer function on the $C_{L}$ reduction is examined. The main rationale of the feedback control is that a feedback control forcing is imposed at a phase which is located outside the range of lock-on. By applying the feedback control law, $C_{L}$ is reduced significantly. Furthermore, the reduction mechanism of $C_{L}$ is analyzed by showing the vortex formation modes with respect to the forcing phase.e.ase.e.

A study on temporal accuracy of OpenFOAM

  • Lee, Sang Bong
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.9 no.4
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    • pp.429-438
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    • 2017
  • Cranke-Nicolson scheme in native OpenFOAM source libraries was not able to provide 2nd order temporal accuracy of velocity and pressure since the volume flux of convective nonlinear terms was 1st accurate in time. In the present study the simplest way of getting the volume flux with 2nd order accuracy was proposed by using old fluxes. A possible numerical instability originated from an explicit estimation of volume fluxes could be handled by introducing a weighting factor which was determined by observing the ratio of the finally corrected volume flux to the intermediate volume flux at the previous step. The new calculation of volume fluxes was able to provide temporally accurate velocity and pressure with 2nd order. The improvement of temporal accuracy was validated by performing numerical simulations of 2D Taylor-Green vortex of which an exact solution was known and 2D vortex shedding from a circular cylinder.

Feedback Control of a Circular Cylinder Wake with Rotational Oscillation (주기적 회전을 이용한 원봉 후류의 되먹임 제어)

  • Baek, Seung-Jin;Sung, Hyung-Jin
    • Proceedings of the KSME Conference
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    • 2001.06e
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    • pp.265-270
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    • 2001
  • This study presents a feedback control methodology for suppression of the vortex shedding from a circular cylinder in a uniform flow. A rotational oscillation is applied as a controlled forcing and the lift coefficient ($C_L$) is used as a feedback signal. A feedback control concept is made based on the phase relation between the rotation velocity and $C_L$ at 'lock-on', The phase between the forcing and the vortex formation is changed $180^{\circ}$ from the phase of enhancing the lock-on state. This concept is examined by solving the Van del Pol equation. The results are satisfactory.

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A Study on the Vortical patterns of a Heaving Foil (히빙익 후류의 유동패턴에 관한 연구)

  • Yang Chang-Jo
    • Journal of Advanced Marine Engineering and Technology
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    • v.29 no.8
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    • pp.899-906
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    • 2005
  • It is known that an oscillating airfoil can Produce a driving force through the generation of a reversed $K\'{a}rm\'{a}n$ vortex street, and this can be expected to be a new highly effective propulsion system. The wake formation behind the heaving airfoil was visualized and was measured using PIV systems We have been examined various conditions such as frequency number, amplitude in NACA 0010. As Strouhal number is greater than 0.08. wake profile with velocity deficit can be transformed into the wake with velocity excess After evaluating vortex center flow patterns in the wake investigated using tracking trajectories in temporal evaluation of the shedding vortices. We also Presented the experimental results on the unsteady vortices structure of the heaving airfoil at various parameters.

Numerical Study on Vortex Structures in a Two-dimensional Bluff-Body Burner in the Transitional Flow Regime

  • Kawahara, Hideo;Nishimura, Tatsuo
    • Journal of the Korean Society of Combustion
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    • v.7 no.1
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    • pp.31-36
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    • 2002
  • Vortical structures are investigated numerically for both cold and combusting flows from a two-dimensional bluff-body burner in the transitional flow regime from steady to unsteady state. The Reynolds number of the central fuel flow is varied from 10 to 230 at a fixed air Reynolds number of 400. The flame sheet model of infinite chemical reaction and unit Lewis number are assumed in the simulation. The temperature dependence of the viscosity and diffusivity of the gas mixture is also considered. The vortex shedding is observed depending on the fuel flow. For cold flow, four different types of vortical structure are identified. However, for combusting flow of methane-air system the vortical structures change significantly due to a large amount of heat release during the combustion process, in contract to cold flow.

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Comparison of the Side-Jets and Rear-Jet Effects on the Controllability of Flow-Induced Vibrations

  • HONG Jun-Ho;ARAI Norio
    • 한국전산유체공학회:학술대회논문집
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    • 2003.10a
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    • pp.164-165
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    • 2003
  • The problem of a bluff body oscillating in a fluid flow has been receiving a great deal of attention. When a bluff body is placed in a flow, it experiences fluctuating hydraulic forces in both transverse and stream-wise directions. It is caused by the formation of vortices behind the body, which could cause large damages of structures. It is called the flow-induced vibrations. In this article, it is investigated the effects of that side-jets and rear-jet, which is applied to control the vortex shedding. The rear-jet is available to control the flow-induced vibrations according as the body shapes and the velocity of fluid flow in which the galloping phenomena is not appeared.

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THREE DIMENSIONAL CHARACTERISTICS OF WAKES OVER A CIRCULAR CYLINDER IN THE B-MODE USING HIGH ORDER SCHEME (고해상도 수치기법에 의한 B-Mode 영역에서 원형실린더 주위의 3차원 후류 유동특성 연구)

  • Kim, T.S.;Lee, S.S.;Kim, J.S
    • 한국전산유체공학회:학술대회논문집
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    • 2008.03a
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    • pp.50-56
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    • 2008
  • 본 논문에서는 실린더 주위유동의 B-mode에 해당하는 레이놀즈 수 300${\sim}$1000범위에서 고차 고해상도기법(OHOC Scheme)을 이용하여 원형 실린더 주위의 유동장 및 음향장 특성에 대해서 연구하였다. B-mode 레이놀즈 수 범위에서 스팬방향 길이에 따른 3차원 원형실린더의 스트롤 수, 양 항력계수의 상관관계에 대해 수치계산 및 실험 결과와 비교 분석한 결과 매우 잘 일치하는 것을 보였다. 그리고 이 결과를 토대로 하여 B-mode 불안정성 영역에서 보다 정확한 2차 와류 모사를 위한 적절한 스팬방향을 찾고, 3차원 후류유동의 불안정성이 음향장의 변화에 미치는 영향을 정성적으로 고찰하여, 3차원 원형실린더의 공력소음 대한 기초적인 연구를 수행하였다.

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