• Title/Summary/Keyword: 자연쉐딩

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Suppression of Wake Transition and Occurrence of Lock-on Downstream of a Circular Cylinder in a Perturbed Flow in the A-mode Instability Regime (A-mode 불안정성 영역에서 교란유동장에 놓인 원형실린더 후류의 천이지연과 유동공진의 발생)

  • Kim, Soo-Hyeon;Bae, Joong-Hun;Yoo, Jung-Yul
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.31 no.8
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    • pp.702-710
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    • 2007
  • Direct numerical simulation (DNS) is performed to investigate suppressed wake transition and occurrence of lock-on in the wake of a circular cylinder disturbed by sinusoidal perturbation at the Reynolds number of 220 (A-mode instability regime). The sinusoidal perturbation, of which the frequency is near twice the natural shedding frequency, is superimposed on the free stream velocity. It is shown that the wake transition behind the circular cylinder can be suppressed due to the perturbation of the free stream velocity. This change causes a jump in the Strouhal number from the value corresponding to A-mode instability regime to the value corresponding to retarded wake transition regime (extrapolated from laminar shedding regime) in the Strouhal-Reynolds number relationship. As a result, vortex shedding frequency is locked on the perturbation frequency depending not on the natural shedding frequency but on the modified shedding frequency.

Direct Numerical Simulation of the Lock-on Phenomena in the Wake behind a Circular Cylinder in a Perturbed Flow at Re=360 (Re=360에서 교란유동장에 놓인 원형실린더 후류의 유동공진 현상에 대한 직접수치해석)

  • Park, Ji-Yong;Kim, Soo-Hyeon;Bae, Joong-Hun;Park, No-Ma;Yoo, Jung-Yul
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.31 no.9
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    • pp.780-789
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    • 2007
  • Lock-on phenomenon in the wake of a circular cylinder is investigated at the Reynolds number of 360 using direct numerical simulation (DNS). To induce lock-on, a streamwise velocity perturbation with a frequency of twice the natural shedding frequency is superimposed on the free stream velocity. The Reynolds stress distributions are investigated to analyze the streamwise force balance acting on the recirculation region and the results are compared with the previous experimental result. When the lock-on occurs, the pressure force on the recirculation region is shown to increase mainly due to the reversal of the Reynolds shear stress distribution, which is consistent with our previous results using PIV measurement. It is also shown that, with the lock-on, the strength of the primary vortices increases whereas that of the secondary vortices decreases significantly. Further, under the lock-on condition the wavelength of the secondary vortices increases by as much as 2.5 times.

Lock-on states of a circular cylinder in the oscillatory flow (진동 유동장에서 원형 실린더의 lock-on 해석)

  • Kim Wontae;Sung Jaeyong;Yoo Jung Yul
    • Proceedings of the KSME Conference
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    • 2002.08a
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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.

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