• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2005년도 후기학술대회논문집
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• pp.31-32
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• 2005

It is known that an oscillating airfoil can produce a driving force through the generation of a reversed $K\acute{a}rm\acute{a}n$ vortex street, and this can be expected to be a new highly effective propulsion system. 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. We also presented the experimental results on the unsteady vortices structure of the heaving airfoil at various parameters.

• Journal of Advanced Marine Engineering and Technology
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• 제31권7호
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• pp.842-851
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• 2007

Oscillating foil propulsion, the engineering application of fish-like movement of a hydrofoil, has received in recent decades as a possible competitor for propellers. The oscillating foil produces an effective angle of attack, resulting in a normal force vector with thrust and lift components, and it can be expected to be a new highly effective propulsion system. We have explored propulsion hydrodynamics as a concept in wake flow pattern. The present study has been examined various conditions such as oscillating frequencies and amplitudes in NACA0010 profile. Flow visualizations showed that high thrust was associated with the generation of moderately strong vortices, which subsequently combine with trailing-edge vorticity leading to the formation of a reverse $K\acute{a}rm\acute{a}n$ vortex street. Vortex generation was inherent to jet production and playeda fundamental role in the wake dynamics. And it was shown that the strong thrust coefficient obtained as the Strouhal number was larger.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2001년도 Proceedings of 2001 Korea-Japan Joint Seminar on Particle Image Velocimetry
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• pp.86-99
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• 2001

Vortex lock-on or resonance in the flow behind a circular cylinder is investigated from a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K\acute{a}rm\acute{a}n$ and streamwise vortices in the wake-transition regime at the Reynolds number 360. Streamwise vortices at the lock-on and natural shedding states are observed, as well as the changes in the wake region with the change of the shedding frequency of lock-on state. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. At the lock-on state, the $K\acute{a}rm\acute{a}n$ vortices are observed to be more disordered by the increased strength and spanwise wavelength of the streamwise vortices, which leads to a strong three-dimensional motion. Recirculation and vortex formation region at the lock-on state is reduced as the oscillating frequency is increased. By comparing the Reynolds stresses at the lock-on and natural shedding states, $\bar{u'u'}\;and \;\bar{u'u'}$ at the lock-on state are concentrated on the shear layer around the cylinder. The $\bar{u'u'}\;at\;f_o/f_n=2.0$ has a large value near the centerline, compared with that of other cases. Considering the traces of maximum of u', in the wake region near the cylinder, wake width at the lock-on state is wider than that at the natural shedding state.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2002년도 추계 학술대회논문집
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• pp.35-40
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• 2002

This paper deals with a thrust generation on flapping-airfoil by dynamic stall. Dynamic stall refers to a series of complicated aerodynamic phenomena accompanied by a stall delay in unsteady motion. In most cases, once it occurs, the dynamic stall may lead to an abrupt fluctuation of aerodynamic forces. An inverse $k\acute{a}rm\acute{a}n$ vortex has been considered as a main reason for a thrust generation. In this paper, however, we have found out that a thrust is closely related to reduced frequency and leading edge vortex in addition to inverse Karman vortex. In order to certify our opinion, picking and plunging motions were calculated with the parameter of amplitude and frequency by using the unsteady, incompressible Navier-Stokes flow solver with a two-equation turbulence model. For more efficient computation, it is parallelized by MPI programming method.

• 대한조선학회논문집
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• 제51권6호
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• pp.480-488
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• 2014

Vortex shedding which is the dominant feature of body wakes and of direct relevance to practical engineering problems, has been intensively studied for flows past a circular cylinder. In contrast, vortex shedding from a hydrofoil trailing edge has been studied to much less extent despite numerous practical applications. The physics of the problem is still poorly understood. The present study deals with $K{\acute{a}}rm{\acute{a}}n$ vortex shedding from a truncated trailing-edge hydrofoil in relatively high Reynolds number flows. The objectives of this paper are twofold. First, we aim to simulate unsteady turbulent flows past a two dimensional hydrofoil through a hybrid particle-mesh method and penalization method. The vortex-in-cell (VIC) method offers a highly efficient particle-mesh algorithm that combines Lagrangian and Eulerian schemes, and the penalization method enables to enforce body boundary conditions by adding a penalty term to the momentum equation. The second purpose is to investigate shedding frequencies of vortices behind a NACA 0009 hydrofoil operating at a zero angle of attack.

• 한국수자원학회논문집
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• 제48권11호
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• pp.891-903
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• 2015

본 연구는 식재된 개수로에서 흐름특성을 모의할 수 있는 수심 적분된 2차원 수치모형을 이용하여 원형 식생역 주변의 흐름을 수치모의하였다. 식생영향을 고려하기 위해 식생항력 항을 지배방정식에 추가하였고 다양한 식생체적비율(SVF) 조건에 따른 수치모의를 수행하였다. 흐름이 원형 식생역을 통과하고 하류에 저유속 구간인 후류영역(wake region)을 형성하며 식생체적비율이 0.08 이상이면 재순환 영역이 발생하였다. 재순환 발생위치는 식생체적비율이 감소하면 식생역에서 더욱 하류로 이동하였다. 후류영역을 지나 원형 식생역 양 측면에서 유발된 전단층들의 상호작용에 의해 von $K{\acute{a}}rm{\acute{a}}n$ 와열이 발생하였다. 원형 식생역 하류에서 발생하는 와류는 식생체적비율이 0.08 이상이 되면 나타나기 시작하였고 발생위치는 난류운동에너지가 최대값을 보이는 위치와 일치하였다. 최대 난류운동에너지는 식생체적비율이 감소하면 줄어드는 것으로 나타났고 최대값의 발생위치는 점점 하류로 이동하였다.

• Coupled systems mechanics
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• 제2권2호
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• pp.191-214
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• 2013

The unsteady flow past a circular cylinder which starts rotating or rotary oscillating impulsively from rest in a viscous fluid is investigated for Reynolds numbers Re=200 and 1000, rectilinear speed ratios ${\alpha}$ between 0.5 and 5.0, and forced oscillating frequencies $f_s$ between 0.1 and 2.0. Numerical solutions of the Navier-Stokes equations are obtained by using a finite volume method on an unstructured colocated grid. The objective of the study is to examine the effect of the rotating and rotary oscillating circular cylinder on the flow patterns and dynamics loads. The numerical results reveal that the $K\acute{a}rm\acute{a}n$ vortex street vanishes entirely behind the rotating cylinder when the ratio ${\alpha}$ exceeds the critical value, and the vortex shedding behind the rotary oscillating cylinder undergoes mainly three modes named 'synchronization', 'competition' and 'natural shedding' with the increase of $f_s$. Based on the amplitude spectra analysis of the lift coefficients, the regions of the classification of flow structure modes are presented, which provide important references for the flow control in the ocean engineering.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2013년도 추계학술대회 논문집
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• pp.753-756
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• 2013

The demand for sailing yacht is increasing in consonance the improvement of people's live. These yachts can be dually propelled by wind and by diesel engine power. A singing (humming, whistling) phenomenon induced on the propeller was discovered on a 55-foot catamaran sailing yacht. As a result, an increase in the structural vibration of the stern tube room and propulsion system with abnormal noise was detected due to this flow. In this study, the cause of the phenomenon is investigated and its possible countermeasures proposed.

• Wind and Structures
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• 제30권2호
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• pp.119-131
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• 2020

A butterfly web girder is a box-shaped girder with discretely distributed side openings along the spanwise direction. Until now, there have been few studies related to the aerodynamic performance of the butterfly web bridge. The objective of the current study was to clarify the effects of the side openings on the aerodynamic performance of the girder. Two butterfly web girders with side ratios B/D = 3.24 and 5, where B is the girder width and D is the depth, were examined through a series of wind tunnel tests. A comparison of the results for butterfly web girders and conventional box girders of the same shape confirmed that the side openings stabilized the vortex-induced vibration and galloping when B/D = 3.24, whereas the vortex-induced vibration and torsional flutter were stabilized when B/D = 5. The change in the flow field due to the side openings contributed to the stabilization against the vibration. These findings not only confirmed the good aerodynamic performance of the butterfly web bridge but also provided a new method to stabilize the box girder against aerodynamic instabilities via discretely distributed side openings.

• 한국지구과학회지
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• 제28권1호
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• pp.123-135
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• 2007

동아시아에서 광역적인 대기오염 이동과 한반도 유입을 인공위성과 지상 관측을 통해 5개의 사례를 분석하였다. 이 사례들은 주로 중국 대륙의 도시와 산업지대에서 배출된 오염물질들이 황해를 지나 한반도에 상륙 및 통과하는 광역적인 대기오염 이동이다. NOAA인공위성 관측 자료의 분석은 가시영역과 적외선 영역의 3개 채널을 합성하여 영상을 만들고, 황해 위에서 대기오염의 분포와 이동을 구별해 낼 수 있었다. 또한 청원의 배경관측지점에서 대기오염의 지상 관측 자료는 인공위성 관측 자료와 비교하여 검증하는데 매우 가치가 있었다. 특히, 광역적 대기오염의 이동 사례에서는 $PM_{10}$과 $PM_{2.5}$ 농도의 차이가 적으면서, $PM_{2.5}$값이 높게 증가한다. 그러나 황사의 경우는 $PM_{10}$ 농도가 훨씬 높게 관측되어 $PM_{2.5}$와의 차이가 매우 큼이 특징적이다. 2006년 1월 27일의 사례에서는 중국 발원의 광역적 대기오염이 한반도의 중부 및 남서지역으로의 유입을 인공위성뿐 아니라 여러 지상관측소들에서 순차적으로 관측할 수 있었다. 북북서 기류가 제주도 한라산을 넘어서 풍하측 멀리까지 구름을 소산시키고 대기오염을 감소시키며, 카르만 소용돌이를 만들고 있었다.