• Title/Summary/Keyword: 후류 제어

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Skin-Friction Drag Reduction in Wake Region by Suction Control on Horseshoe Vortex in front of Hemisphere (반구 전방에 생성된 말굽와류 흡입제어에 의한 후류영역 마찰저항 감소에 관한 연구)

  • Koo, Bonguk;Kang, Yong-Duck
    • Journal of the Korean Society of Marine Environment & Safety
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    • v.25 no.6
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    • pp.795-801
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    • 2019
  • The aim of this study was to investigate the possibility of the skin-friction reduction by vortex control. A vortical system such as a horseshoe vortex, a hairpin vortex, and a wake region was induced around a hemisphere attached on a Perspex flat plate in the circulating water channel. Hairpin vortices were developed from the wake region and horseshoe vortices were formed by an adverse pressure gradient in front of the hemisphere. The horseshoe vortices located on the flank of the hemisphere induced a high momentum flow in the wake region by the direction of their vorticity. This process increased the frequency of the hairpin vortices as well as the frictional drag on the surface of the wake region. To reduce the skin-friction drag, suction control in front of the hemisphere was applied through a hole. Flow visualization was performed to optimize the free-stream velocity, size of the hemisphere, and size of the suction hole. Once the wall suction control mitigated the strength of the horseshoe vortex, the energy supplied to the wake region was reduced, causing the frequency of the hairpin vortex generation to decrease by 36.4 %. In addition, the change in the skin-friction drag, which was measured with a dynamometer connected to a plate in the wake region, also decreased by 2.3 %.

A Study on Frequency Characteristics of Wake Flow a Circular Cylinder with Control Cylinder (제어봉이 부착된 원관 후류의 주파수특성에 관한 연구)

  • Choe, Sang-Bom;Han, Won-Hui;Cho, Dae-Hwan;Gim, Ok-Sok
    • Proceedings of KOSOMES biannual meeting
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    • 2007.11a
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    • pp.177-183
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    • 2007
  • Flow characteristics of the cylinder wake controlled with a small control cylinder were experimentally investigated by the PN (Particle Image Velocimetry) technique. Flow visualization of the flow around a circular cylinder was conducted in the Circulating Water Channel. The control cylinder having diameter of d=5mm, 10mm and 20mm was installed behind a circular cylinder of D=50mm. And the Reynolds number were $Re=4.9{\times}10^3,\;Re=9.9{times}10^3$ and $Re=1.9{\times}10^4$. In this study, the frequency characteristics of the controlled wake were analyzed by using spectral analysis of the measured wake velocity signals. As a result, the controlled wake had smaller vortex shedding frequency than that of circular cylinder wake by the effect of the control cylinder. Governing parameters of the flow control were d/D, and Reynolds number and they largely influenced the frequency characteristics of the cylinder wake. And vortex shedding frequency appeared most lowly at d=0.2D

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Flow Characteristics around a Circular Cylinder with a Spiral Strake of Fixed Offshore Platform (고정식 해양구조물의 원형지지각 주위 와유기진동에 관한 연구)

  • Gim, Ok-Sok;Lee, Gyoung-Woo
    • Proceedings of the Korean Institute of Navigation and Port Research Conference
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    • 2012.06a
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    • pp.289-290
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    • 2012
  • 본 연구에서는 고정식 해양구조물의 지지각으로 사용되는 원형실린더에 나선형 판을 부착하여 와류유기진동에 미치는 영향을 고찰하였다. 연구의 목적을 달성하기 위해 $Re=6.5{\times}10^3$의 균일유입유동에서 2-프레임 그레이레벨 상호상관 PIV기법을 이용한 실험적인 방법을 적용하였다. 모델의 직경은 40mm이며 후류 유동구조, 난류강도, 응력분포에 대한 통계적 유동정보를 계측하였다. 실험결과로는 원형실린더와 뱃전판을 부착한 실린더와의 비교를 통해 뱃전판의 유동특정을 평가하였으며, 뱃전판의 영향은 후류에서 와의 생성과 소멸 메커니즘의 변화에 큰 영향을 미치고 후류 칼만 와열의 안정적인 제어를 통해 와류유기진동을 억제하였다.

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Research of stability about aerodynamic levitation using jet (제트를 이용한 구체 부양 안정성 연구)

  • Han, Song-Lee;Yun, Tae-Gyun;Gu, Jun-Mo
    • Proceeding of EDISON Challenge
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    • 2013.04a
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    • pp.305-309
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    • 2013
  • 본 연구에서는 유체역학적인 후류 효과를 제어하고 항력을 이용하여 구체를 안정적으로 부양하는 방법을 해석 및 실험적으로 고찰하였다. 물체를 공중에 부양할 때는 물체가 받는 중력과 제트에 의하여 공급되는 항력이 평형을 이루어야 한다. 이 때 공기의 흐름의 레이놀즈 수에 따라 공기가 구체를 지나가며 구체후면에 생기는 후류에 의하여 부양상태가 불안정하게 될 수 있다. 구체의 운동이 제트에 의하여 안정되는 현상은 베르누이 및 코안다 효과로 설명할 수 있다. 에디슨을 이용하여 레이놀즈 수에 따른 실린더에 작용하는 항력과 후류에 의한 불안정화 힘을 해석적으로 구하였으며 이를 실험적으로 고속카메라를 이용하여 부양 거리, 안정도 등을 측정하여 비교하였다. 고찰 결과 레이놀즈 수가 작을수록 더 안정됨을 확인하였다.

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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.

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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CONTROL OF CIRCULAR CYLINDER WAKE USING PLASMA ACTUATION (플라즈마 가진에 의한 원형 실린더 후류의 제어)

  • Kim, Dong-Joo
    • Journal of computational fluids engineering
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    • v.17 no.2
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    • pp.71-77
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    • 2012
  • Numerical simulations are carried out for flow over a circular cylinder controlled by the momentum forcing which is generated by a pair of plasma actuators symmetrically mounted on the cylinder surface. A popular and empirical plasma model is used for the spatial distribution of momentum forcing. In this study, we consider two different types of actuation, i.e., steady and unsteady (or pulsed) actuation. In the unsteady actuation, the actuation is turned on and off periodically, its frequency being a control parameter. The objective of this study is to investigate the effects of actuator location and actuation frequency on the flow structures and the forces on the cylinder. Results show that the cylinder wake can be effectively controlled by proper actuator location. For example, when the actuators are located at $120^{\circ}$ from the stagnation point, vortex shedding is completely suppressed with the boundary layer almost fully attached to the surface, resulting in drag reduction and lift elimination.

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

  • Lee S. B.;Baek S.-J.;Sung H. J.
    • 한국전산유체공학회:학술대회논문집
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    • 2005.04a
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    • pp.51-56
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    • 2005
  • A new feedback control system based on system identification is proposed and preliminarily tested on Van der Pol equation which has a similar characteristic to circular cylinder. The same principle is applicable to circular cylinder in a uniform flow for suppresing the vortex shedding. The feedback controller is designed to impose feedback signal at the phase which is located outside the range of lock-on. The lift coefficient (CL) is employed as a feedback signal and the control forcing is given by a rotational oscillation of the cylinder. By applying the feedback control system, the lift coefficient is reduced.

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Experimental Study of Manipulating the Vertical Fence Wake using the Vortex Generator (와류 생성기를 이용한 수직벽 후류 제어의 실험적 연구)

  • Lee, Sang-Hyuk;Kang, In-Su;Cha, Jae-Eun;Kim, Hyoung-Bum
    • Journal of the Korean Society of Visualization
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    • v.7 no.2
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    • pp.12-16
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    • 2010
  • The effect of vortex generators to manipulate the separated flow region behind the vertical fence was experimentally investigated. The experiments were performed in the circulating water channel. The parameters used in this study were the distance between the fence and vortex generators and size of vortex generators. Digital PIV method was applied to measure the instantaneous velocity fields around the fence. And the obtained flow properties were compared with those of fence How without the vortex generators. The obtained results quantitatively shows the specific size and distance of vortex generators which were effective to reduce the mean reattachment length of separation bubble behind the fence.

An Experimental Study on Wake Cavity Flow Characteristics of Two-dimensional Wedge Shaped Control Fins (2차원 쐐기형 제어핀 후류의 공동유동 특성에 대한 실험연구)

  • Jeong, So-Won;Ahn, Byoung-Kwon
    • Journal of the Society of Naval Architects of Korea
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    • v.53 no.3
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    • pp.180-187
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    • 2016
  • As distinct from a slender body, the separation of the boundary layer on a bluff body give rise to complex wakes in which various kinds of vortices form, develop and interact with each other. In this paper, we investigate cavitation wake field behind wedge shaped two-dimensional fin models. Eight different models are tested at the Chungnam National University Cavitation Tunnel (CNU-CT). First, we measure wake cavity shapes and compare with numerical results, which shows the good agreement with each other. In addition, we demonstrate that wake flow characteristics of the control fin are clearly identified by the correlation analysis of high-speed camera images and pressure fluctuation measurements.