• 대한기계학회논문집
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• 제17권6호
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• pp.1518-1528
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• 1993

본 연구는 단일 원주의 와여기 진동특성에 관한 연구의 연속연구로서 풍동내 에 직렬로 배열된 두 원주의 직교유동에 의한 두 가지 경우의 동적 거동을 조사하였다. 첫째는 상류측 탄성지지 원주의 후류내에 가깝게 놓여진 하류측 고정원주의 간섭에 의한 상류측 원주의 진동(proximity-induced vibration)이며, 둘째는 상류측의 고정원 주의 후류내에 가깝게 놓여진 하류측 탄성지지 원주의 공기역학적 진동(wake-induced vibration)이다. 본 연구의 목적으로 직렬로 배열된 동일 직경의 두 원주에 있어서 어느 한 쪽의 원주가 탄성지지가 되어 있을 경우, 두 원주사이의 간격과 유속변화에 따른 와여기 진동과 유력탄성 불안정 진동의 진동특성 및 후류내에서의 와유출 특성을 명확히 하기 위해 실험적으로 조사, 연구하였다.

• Wind and Structures
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• 제19권4호
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• pp.389-404
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• 2014

The flow characteristics of a circular cylinder surrounded by an outer permeable cylinder were experimentally investigated using Particle Image Velocimetry Technique in deep water flow. In order to consider the effects of diameter and porosity of the outer cylinder on flow structures of the inner cylinder, five different outer cylinder diameters (D=37.5, 52.5, 60, 75 and 90 mm) and eight different porosities (${\beta}$=0.4, 0.5, 0.6, 0.65, 0.7, 0.75, 0.8 and 0.85) were selected. During the experiments, the diameter of inner cylinder was kept constant as d=30 mm. The depth-averaged free-stream velocity was adjusted as U=0.156 m/s, which corresponds to the Reynolds number of Re=5000 based on the inner cylinder diameter. It has been concluded that both the outer permeable cylinder diameter and the porosity have important influences on the attenuation of vortex shedding in the wake region. The presence of outer permeable cylinder decreases the magnitude of Reynolds shear stress and turbulent kinetic energy compared to the bare cylinder case. Moreover, the spectral analysis of vortex shedding frequency has revealed that the dominant frequency of vortex shedding downstream of the cylinder arrangement also reduces substantially due to the weakened Karman shear layer instability.

• 대한조선학회논문집
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• 제43권1호
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• pp.59-67
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• 2006

Evolution of the unsteady three-dimensional tip vortex in the wake field of a rectangular NACA 0012 hydrofoil in pitching motion is investigated. Measurements were made in CWC using PIV. A hydrofoil has an aspect ratio of 5 with chord length of 1 Oem. Pitching angle and mean angle of attack were set to $\pm$ $5^{\circ}$ and $10^{\circ}$, respectively. Frequency of oscillation was varied from 0.1 Hz to 1 Hz in order to study the effect of unsteadiness imposed by various frequencies, which correspond to the reduced frequency of K=0.1, 0.21, 0.52 and 1.05. Reynolds number based on chord length and free-stream velocity was $30\times$$10^{4}$ Phase-averaging technique was employed. Unsteadiness and variation of the size and characteristics of tip vortex at different reduced frequency were discussed.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2006년도 추계학술대회
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• pp.261-264
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• 2006

This paper describes the effects to wind turbine blade aerodynamics due to interaction between blade and tower on upwind type HAWT. In order to analyze effects of blade-tower interact ion, the analyst s program WINFAS which is based on VLM(Vortex Lattice Method), Free wake and FVE model is used. In this study, the changes of wind turbine blade aerodynamics caused by blade-tower interact ion are Investigated with various parameters windshear, yaw error, TSR and tower diameter.

• 한국항공우주학회지
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• 제34권6호
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• pp.1-9
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• 2006

본 연구의 목적은 여러 가지 비행 모드 상의 로터 성능을 효율적으로 예측하는 것이다. 헬리콥터의 공력 특성을 예측하기 위한 비정상 source-doublet 패널 기법 기반의 수치 기법을 개발하였다. 후류의 형상 예측에는 시간 전진 자유후류모델이 사용되었다. 점성에 의한 확산을 고려한 후류의 roll-up 모사를 위하여 후류의 doublet 패널은 같은 강도의 와류고리로 대체하여 계산하였다. 후류와 양력면의 충돌 문제는 표면격자 내부에 들어간 와류고리의 포텐셜값을 제거하여 해결하였다. 제자리비행의 해석 시에 나타나는 와류 불안정성의 해결에는 slow starting과 vortex core growth 모델을 사용하였다. 로터 공력 해석 프로그램은 제자리비행과 전진비행에 대한 실험 결과와 비교하여 검증하였으며, 실험치와 일치하는 결과를 얻을 수 있었다.

• 한국항공우주학회지
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• 제35권2호
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• pp.124-129
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• 2007

동 연구에서는 전진비행하는 로터의 평균 유입류 예측기법 중 Drees 선형모델, 몇가지 변형된 형태의 Mangler & Squire 모델을 서로 비교하여 각 유입류 모델의 특성을 알아내고, KARI 자체 자유후류기법 코드의 해석결과를 함께 비교하여 각 유입류 예측기법의 특성을 파악하였다. 각 예측기법의 비교를 위해 유입류 실험치가 존재하는 로터에 대하여 전진비 0.15, 0.23 및 0.30의 3가지 비행조건을 적용하여 예측한 결과를 비교 분석하였다. Drees 모델의 경우 비교적 실험치에 근접하게 예측하나, 선형모델의 한계로 인해 유입류의 비균일성을 모델링하기에는 미흡하며, Mangler & Squire 모델은 끝단을 제외하고는 비교적 실험치에 근접하게 예측함을 알 수 있었으며, KARI의 자유후류기법은 유입류의 비균일성을 매우 잘 예측하나, 동체에 의한 올려흐름 효과, 후퇴부에서의 동적실속 효과 등의 추가 고려가 필요함을 알 수 있었다.

• Wind and Structures
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• 제26권5호
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• pp.317-329
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• 2018

Wake characteristics of the flow over a finite square prism at different incidence angles were experimentally investigated using an open-loop wind tunnel. A finite square prism with a width D = 15 mm and a height H = 7D was vertically mounted on a horizontal flat plate. The Reynolds number was varied from $6.5{\times}10^3$ to $28.5{\times}10^3$ and the incidence angle ${\alpha}$ was changed from $0^{\circ}$ to $45^{\circ}$. The ratio of boundary layer thickness to the prism height was about ${\delta}/H=7%$. The time-averaged velocity, turbulence intensity and the vortex shedding frequency were obtained through a single-component hotwire probe. Power spectrum of the streamwise velocity fluctuations revealed that the tip and base vortices shed at the same frequency as that ofspanwise vortices. Furthermore, the results showed that the critical incidence angle corresponding to the maximum Strouhal number and minimum wake width occurs at ${\alpha}_{cr}=15^{\circ}$ which is equal to that reported for an infinite prism. There is a reduction in the size of the wake region along the height of the prism when moving away from the ground plane towards the free end.

• 대한조선학회논문집
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• 제43권3호
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• pp.322-330
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• 2006

Numerical calculations are carried out for flow past a circular cylinder forced oscillating normal to the free-stream flow at a fixed Reynolds number equal to 185. The cylinder oscillation frequency ranged from 0.8 to 1.2 of the natural vortex-shedding frequency, and the oscillation amplitude extended up to 20% of the cylinder diameter. IBM (Immersed Boundary Method) with direct momentum forcing was adopted to handle both of a stationary and an oscillating cylinder Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. The instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios showed the synchronized wakes pattern in the lock-in region and vortex switching phenomenon at higher frequency ratio than the critical frequency ratio.

• Wind and Structures
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• 제29권1호
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• pp.75-84
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• 2019

A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces and vortex-induced vibration (VIV). The freestream oncoming flow velocity ($U_{\infty}$) was from 3.8 m/s to 12.8 m/s. The width of the tested cylinder d = 40 mm and aspect ratio H/d = 5, where H was the height of the cylinder. The corresponding Reynolds number was from 10,400 to 35,000. The tested suction ratio Q, defined as the ratio of suction velocity ($U_s$) at the slot over the oncoming flow velocity at which the strongest VIV occurs ($U_{\nu}$), ranged from 0 to 3. It was found that the free-end slot suction can effectively attenuate the VIV of a cantilevered square cylinder. In the experiments, the RMS value of the VIV amplitude reduced quickly with Q increasing from 0 to 1, then kept approximately constant for $Q{\geq}1$. The maximum reduction of the VIV occurs at Q = 1, with the vibration amplitude reduced by 92%, relative to the uncontrolled case. Moreover, the overall fluctuation lift of the finite-length square cylinder was also suppressed with the maximum reduction of 87%, which occurred at Q = 1. It was interesting to discover that the free-end shear flow was sensitive to the slot suction near the leading edge. The turbulent kinetic energy (TKE) of the flow over the free end was the highest at Q = 1, which may result in the strongest mixing between the high momentum free-end shear flow and the near wake.

• Ocean Systems Engineering
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• 제6권4호
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• pp.363-375
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• 2016

Large numbers of submarine pipelines are laid as the world now is attaching great importance to offshore oil exploitation. Free spanning of submarine pipelines may be caused by seabed unevenness, change of topology, artificial supports, etc. By combining Iwan's wake oscillator model with the differential equation which describes the vibration behavior of free-span submarine pipelines, the pipe-fluid coupling equation is developed and solved in order to study the effect of both internal and external fluid on the vibration behavior of free-span submarine pipelines. Through generalized integral transform technique (GITT), the governing equation describing the transverse displacement is transformed into a system of second-order ordinary differential equations (ODEs) in temporal variable, eliminating the spatial variable. The MATHEMATICA built-in function NDSolve is then used to numerically solve the transformed ODE system. The good convergence of the eigenfunction expansions proved that this method is applicable for predicting the dynamic response of free-span pipelines subjected to both internal flow and external current.