• 해양환경안전학회지
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• 제20권4호
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• pp.443-450
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• 2014

본 연구에서는 수중익 배치와 종모멘트 특성이 전몰형 수중익선의 종방향 운동 안정성에 미치는 영향에 대하여 Tandem 수중익 모형의 시험을 통하여 고찰한다. 먼저, Canard 날개배치를 가지는 Tandem 수중익 모형을 제작하고 수중익의 양력 및 항력 계측 실험을 수행하여 양력 및 항력 특성을 평가하였다. 또한 종방향 운동의 초기 안정성과 자기 안정성을 가지는 Tandem 수중익 모형의 날개 배치를 결정하였다. Tandem 수중익 모형의 종안정성 시험에서는 선수 및 선미 수중익의 종방향 간격 변화와 종모멘트의 자기안정성 변화에 따른 Pitch 및 Heave 운동 특성, 익주 상태의 종안정성을 평가하였다. 먼저, Canard 날개배치를 가지는 Tandem 수중익 모형을 제작하고 수중익의 양력 및 항력 계측 실험을 수행하여 양력 및 항력 특성을 평가하였다. 또한 종방향 운동의 초기 안정성과 자기 안정성을 가지는 Tandem 수중익 모형의 날개 배치를 결정하였다. Tandem 수중익 모형의 종안정성 시험에서는 선수 및 선미 수중익의 종방향 간격 변화와 종모멘트의 자기안정성 변화에 따른 Pitch 및 Heave 운동 특성, 익주 상태의 종안정성을 평가하였다. 실험 수행 결과 본 연구의 주요 결론의 다음과 같다. 종 모멘트 자기 안정성이 클수록 주어진 고정 pitch 각에서 pitch운동에 대한 자기안정성이 높다. 다음 종모멘트 자기 안정성이 클수록, pitch각도 변화에 따른 pitch운동 및 heave운동의 변동 폭이 상대적으로 작으며 종방향 안정성이 우수하다. 마지막으로 종모멘트 자기안정성이 클수록, 수중익 양력이 동일한 경우, Tandem 수중익의 안정적인 익주를 위해서는 충분한 종모멘트의 자기안정성을 확보하는 것이 필수적인 것을 시험을 통하여 확인하였다.

• Wind and Structures
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• 제21권1호
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• pp.25-40
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• 2015

Oscillating motions of flexible cylinders are associated to some extent with the aerodynamic response of plants. Tandem motions of reeds with flexible stems in a colony are experimentally investigated using an array of flexible cylinders made of polydimethylsiloxane (PDMS). Consecutive images of flexible cylinders subjected to oncoming wind are recorded with a high-speed camera. To quantify oscillating motions, the average bending angle and displacement of flexible cylinders are evaluated using point-tracking method and spectral analysis. The tandem motions of flexible cylinders are closely related to the flow characteristics around the cylinders. Thus, the dynamic motions of a tandem arrangement of flexible cylinders are investigated with varying numbers of cylinders arranged in-line, numbers of cylinders in a group (behaving like a single body), and Reynolds numbers (Re). When the number of cylinders in a group increases, the damping effect caused by the support of downstream cylinders is pronounced. These results would be provide useful information on the tandem-arranged design of complex structures and energy harvesting devices.

• 한국가시화정보학회지
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• 제18권3호
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• pp.52-60
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• 2020

When a bubble reaches a free surface, a bursting of the bubble produces a high speed jet. Despite its practical importance, significant effort has been devoted to investigate a bursting jet by a single bubble near a free surface. In the present study, we perform numerical simulations of bubbles in a tandem arrangement at Bo=0.05. The configuration of the tandem bubbles is systematically varied by changing a radius of a following bubble (RF) and the gap distance between two bubbles (L). Compared to a single bubble case, we show that the bursting bubble in the tandem arrangement accelerates, and the jet velocity increases. Moreover, we find that a critical gap distance at which the jet velocity unexpectedly changes exists in the tandem case.

• 대한기계학회논문집B
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• 제30권5호
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• pp.480-488
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• 2006

Numerical simulations of flow over two spheres placed in a tandem arrangement are conducted to investigate the effect of the inter-sphere spacing on the flow characteristics. The Reynolds numbers considered are 100, 250, 300 and 425, corresponding to steady axisymmetric, steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric flows, respectively, in the case of a single sphere. For small inter-sphere spacings, the flow past two spheres is more stable than that past a single sphere. For example, with the spacing of the sphere radius, the flow is steady axisymmetric up to Re=300. However, for relatively large spacings, the flow past two spheres becomes unstable and vortex shedding takes place even at Re=250. The drag coefficient of the rear sphere decreases significantly with decreasing inter-sphere spacing due to reduction of the stagnation pressure, thus being smaller than that of the front sphere. Also, the rear sphere shows large fluctuations of the lift force as compared to the front one in the case of unsteady flow.

• Journal of Advanced Marine Engineering and Technology
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• 제25권4호
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• pp.857-868
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• 2001

The aerodynamic forces and wake structures of the non-rotating downstream cylinder which is located behind the spinning upstream cylinder in tandem and staggered arrangement have been investigated by experimental method at Re= $1.32{\times}10^4$. The measurements of wake flow and pressure distributions of downstream cylinder are carried out in various spin parameters by combination of both longitudinal spacing rations L/d=1.5, 3.0, 4.5 and transverse spacing ratios T/d =0.0, -0.5, 0.5. For the present experiment, it has been found that the spin parameter of spinning upstream cylinder affect more easily the downstream cylinder in tandem arrangement than that in staggered arrangement.

• 대한기계학회논문집B
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• 제29권11호
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• pp.1199-1208
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• 2005

The flow fields including velocities, turbulence intensities, Reynolds shear stress and turbulent kinetic energy were investigated using particle image velocimetry(PIV) to study the flow characteristics around two square cylinders in a tandem arrangement. The experiments were carried out in the range of the spacing from 1.0 to 4.0 widths of cylinder, Reynolds number of 5.3$\times$10$^{3}$ and 1.6$\times$10$^{4}$ respectively. Discontinuous jumping at the drag coefficient variation was found for two cylinders simultaneously when the spacing between two cylinders is varied. This phenomenon is attributed to a sudden change of the flow pattern which depends on the reattachment of the shear layer separated from the upstream cylinder. Near such a critical spacing, the changes of the flow fields as well as the effect of Reynolds number were studied in detail.

• 대한기계학회논문집B
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• 제37권4호
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• pp.359-366
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• 2013

본 연구는 두 원기둥의 유동에 기인한 진동에 관한 실험적 연구이다. 두 원기둥이 직렬 배열(${\alpha}=0^{\circ}$), 대각선 배열(${\alpha}=5^{\circ}$, $10^{\circ}$, $15^{\circ}$, $25^{\circ}$, $45^{\circ}$, $60^{\circ}$), 병렬 배열(${\alpha}=90^{\circ}$) 됐을 때 두 원기둥의 간격(L/D)과 유속을 변화 시켰을 때의 진동 특성을 조사했다. 그 결과 다음과 같은 결과를 얻었다. (i)직렬, 대각선, 병렬 배열된 두 원기둥에서 발생하는 유동에 기인한 진동의 패턴은 7 개 패턴이다. (ii)복수로 존재하는 두 원기둥의 유동에 기인한 진동은 서로 진동을 유발시킨다. (iii) 두 원기둥의 직렬, 대각선, 병렬 배열 중에 유동에 기인한 진동을 유발하기 가장 쉬운 배열은 병렬 배열이다. (iv) 모든 배열에서 두 원기둥의 간격변화에 따른 최대 진폭의 변화는 변동 양력 계수의 변화와 유사하다.

• Journal of Advanced Marine Engineering and Technology
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• 제27권1호
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• pp.49-56
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• 2003

The hydrodynamic interference between two circular cylinders in tandem and side by side arrangements was investigated by measuring of lift and drag on each cylinder. The time variations of interference lift and drag coefficients in each arrangement were observed at center-to-center pitch ratios of P/D=1.25 and 2.5 and Reynolds number of $Re=1.5\times10^4$. Average interference lift and drag coefficients were also observed at pitch ratios from P/D=1.25 to 2.5 and Reynolds number from $Re=1.5\times10^4$ to $1.5\times10^4$. The hydrodynamic interference between two circular cylinders differed with the shape of the arrangement and the pitch ratio, but the characteristics were revealed by measuring of lift and drag on each cylinder.

• Wind and Structures
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• 제16권2호
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• pp.179-192
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• 2013

The flow around two rectangular cylinders with aspect ratio of 0.5 in a tandem arrangement, was investigated using pressure measurements (in a wind tunnel) and flow visualizations (in a water tunnel) in the range of P/h from 0.6 to 4.0. Four flow patterns were identified, and processes of shear layers wrapping around, the shear layer reattachment, vortices wrapping around and vortices impingement, were observed. Mean and rms pressure distributions, flow visualizations and Strouhal numbers were presented and discussed. The paper revealed that the variations of Strouhal numbers were associated with the shear layers or vortex interference around two cylinders.

• Wind and Structures
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• 제29권1호
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• pp.65-75
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• 2019

A heated square cylinder (with height $A^*$) is kept parallel to the cold wall at a fixed gap height $0.5A^*$ from the wall. Another adiabatic rectangular cylinder (of same height $A^*$ and width $0.5A^*$) is placed upstream in an inline tandem arrangement. The spacing between the two cylinders is fixed at $3.0A^*$. The inlet flow is taken as Couette-Poiseuille flow based non-linear velocity profile. The conventional fluid (also known as base fluid) is chosen as water (W) whereas the nanoparticle material is selected as $Al_2O_3$. Numerical simulations are performed by using SIMPLE algorithm based Finite Volume approach with staggered grid arrangement. The dependencies of hydrodynamic and heat transfer characteristics of the cylinder on non-dimensional parameters governing the nanofluids and the fluid flow are explored here. A critical discussion is made on the mechanism of improvement/reduction (due to the presence of the upstream cylinder) of heat transfer and drag coefficient, in comparison to those of an isolated cylinder. It is observed that the heat transfer increases with the increase in the non-linearity in the incident velocity profile at the inlet. For the present range studied, particle concentration has a negligible effect on heat transfer.