• Selected Papers of The Society of Naval Architects of Korea
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• 제1권1호
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• pp.76-90
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• 1993

A new propeller series is developed using the newly developed blade section (KH 18 section) which has better cavitation characteristics and higher lift-drag ratio at wade angle-of-attack range than a conventional section. The radial patch distribution of the new series propellers is variable stance they were designed adaptively to a typical wake distribution. Basic geometric particulars of the series propellers. such as chord length, thickness, skew and rake distributions, are determined on the basis of recent full scale propeller geometric data. The series is developed for propellers having 4 blades, and blade area ratios of 0.3, 0.45, 0.6 and 0.75. Mean pitch ratios are varied as 0.5, 0.6, 0.7, 0.95 and 1.1 for each blade area ratio. The new propeller series consists of 20 propellers and is named as the KD(KRISO-DAEWOO)-propeller series. Propeller open-water tests are performed at the towing tank, and cavitation observation tests and fluctuating pressure tests are carried out at the cavitation tunnel of KRISO. $B_{p}-\delta$ curves, which can be used to select the optimum propeller diameter at the preliminary design stage, are derived from a regression analysis of the propeller open-water test results. The KD-cavitation chart is derived from the cavitation observation test results by choosing the local maximum lift coefficient and the local cavitation number as parameters. The cavity extent predicted by the KD-cavitation chart would be more accurate compared to that by an existing cavitation charts, such as the Burrll's cavitation chart, since the former is derived from the cavitation observation test results in a typical ship's wake, while the lather is derived from the test results in a uniform flow.

• 플랜트 저널
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• 제12권3호
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• pp.24-31
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• 2016

풍진동(Wind-induced vibration)은 바람에 의해 구조물에 진동이 발생하는 현상으로써 세장비가 큰 열기기 Stack 설계시 고려해야 할 중요한 사항이다. 따라서, 국제 규격에는 풍진동에 대한 설계 인자와 각 범위에 대해 필요한 고려 사항을 정의하고 있다. 규격에 의한 설계 검증은 몇몇 인자를 이용하여 간단하게 확인이 가능하다는 장점이 있는 반면, 실제 풍진동에 의한 영향을 정량적으로 평가하지 않기 때문에 실제 필요한 것보다 과도한 설계를 요구할 수 있다는 단점이 있다. 또한 여러 제약조건으로 Code 상의 요구조건을 만족하지 못하는 경우 실제 시스템의 거동을 예측할 수 없다는 단점이 있다. 이러한 점을 보완하기 위해 CFD 와 FEM 등의 수치적 해석 방법을 통해 풍진동이 실제 Stack에 미치는 영향을 해석하여 설계 적정성을 검증하여 Code 상의 요구 조건과 비교하였다.

• 한국항해항만학회지
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• 제35권10호
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• pp.805-810
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• 2011

트랜섬 선미 형상변화에 따른 후류 유동특성을 알아보기 위해 $Re=3.5{\times}103$ 및 $Re=7.0{\times}103$에서 수면하부 유동을 계측하였다. 선미형상변화에 따른 유동장 계측을 위해 2-프레임 그레이레벨 상호상관 PIV기법을 이용하여 분석하였다. 선미형상은 실선조사 결과를 바탕으로 선저와 이루는 각을 각각 $45^{\circ}$(모델 A), $90^{\circ}$(모델 B) 및 $135^{\circ}$(모델 C)를 실험에 적용하였다. 모델의 선미 침수면의 깊이는 자유 수면으로부터 40mm를 기초하였다. 레이놀즈수가 증가함에 따라, 와의 규모가 증가하고 하류로 길게 형성되는 경향을 나타냈다.

• 한국해양공학회지
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• 제33권2호
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• pp.123-130
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• 2019

The flow around a curved riser exposed to the current in various directions was investigated at a Reynolds number of 100 using a numerical simulation. The present study found that the flow features of the curved riser were distinct from those of a straight riser as a result of its large radius of curvature. Namely, there were various wake patterns according to the flow's incident angle. As the incident angle increased from $0^{\circ}$ to $90^{\circ}$, a two-row street of vortices that developed along the centerline of the curved riser became more apparent. However, when the incident angle approached $180^{\circ}$ from $90^{\circ}$, these vortices were completely suppressed by the interaction between the wake and an axial flow induced by the curvature of the riser. To identify this feature, the sectional force coefficients were also considered, and it was found that the force coefficients could be different from those found in a sectional analysis based on the strip theory when investigating vortex-induced vibration. As a result, this kind of study would be important to realistically estimate the riser VIV (vortex-induced vibration) and fatigue life, and a new force coefficient database that includes the three-dimensional effect should be established.

• 한국전산유체공학회지
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• 제13권1호
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• pp.57-62
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• 2008

Laminar flows over a cube and a cuboid (cube extended in the streamwise direction) are numerically investigated for the Reynolds numbers between 50 and 350. First, vortical structures behind a cube and lift characteristics are scrutinized in order to understand the variation in vortex shedding characteristics with respect to the Reynolds number. As the Reynolds number increases, the flow over a cube experiences the steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric flows. Similar to the sphere wake, the planar-symmetric flow over a cube can be divided into two different regimes: single-frequency regime and multiple-frequency regime. The former has a single frequency due to regular shedding of vortices with the same strength in time, while the latter has multiple frequency components due to temporal variation in the strength of shed vortices. Second, the effect of the length-to-height ratio of the cuboid on the flow characteristics is investigated for the Reynolds number of 270, at which planar-symmetric vortex shedding takes place behind a cube. With the ratio smaller than one, the flow over the cuboid becomes unsteady asymmetric flow, whereas it becomes steady flow for the ratios greater than one. With increasing the ratio, the drag coefficient first decreases and then increases. This feature is related to the flow reattachment on the side faces of the cuboid.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2005년도 춘계학술대회논문집
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• pp.158-163
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• 2005

In this paper the aeroacoustic characteristics of a tilt-rotor system is measured by using a sealed model tilt-rotor. With a microphone array system and the do-dopplerization algorithm, the location and the characteristics of rotor noise are successfully measured. The most of high frequency noise (4kHz) is found to be located at rotor blade tips, but the low frequency tonal noise is dominant on the middle of the rotor blades. The measured tonal noise characteristics are compared to the results of theoretical calculation. At 0.5m distance from the rotor plane, measured and calculated data are relatively well matched regardless of rotating speed and collective pitch angie for the azimuthal angles between $0^{\circ}\;and\;60^{\circ}$. However, the data on the azimuthal angles between $70^{\circ}\;and\;90^{\circ}$ are not quite comparable. In addition, the compared data for far-field case (1.5m) are quite different. This is probably due to the unsteady effect which if not included in the theoretical calculation.

• 한국전산유체공학회지
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• 제6권3호
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• pp.41-50
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• 2001

Effects of rotor-stator blade count ratio on the unsteady aerodynamic characteristics of a cascade was studied by using a Navier-Stokes code. Present Navier-Stokes code is a parallel code and works on a multi-cpu machine. It is based on the SIMPLE algorithm and uses QUICK scheme for convection terms and second order back difference for all temporal derivatives. Computations were carried out for two cases : case 1 is for 3 stator cascade passages subjected to two upstream wakes while case 2 is for 2 stator cascade passages subjected to three upstream wakes. Numerical solutions show that rotor-stator blade count ratio plays a significant role in the unsteady aerodynamic characteristics of the stator cascade. Case 2 shows smaller unsteady fluctuation than case 1, even if they show the same time averaged value. The smaller fluctuation of case 2 is believed due to strong interaction between unsteady vortices. The unsteady lift variation of case 2 is shown to have many high frequency fluctuations as more unsteady vortices travel around the cascade. The unsteady turbulent kinetic energy due to the upstream wake is also shown to decay faster through the cascade passage than in the free stream.

• 대한조선학회논문집
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• 제55권1호
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• pp.75-82
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• 2018

Wedge-shaped fins are generally used to provide sufficient forces and moments to control and maneuver a supercavitating vehicle. There are four fins placed along the girth of the vehicle, near he tail: two of the fins are horizontal and the other two fins are vertical. In a fully developed supercavitating flow condition, a part of the fin is in a cavity pocket and the other is exposed to water. In this paper, experimental investigations of hydrodynamic characteristics of the wedge-shaped fin models are presented. Experiments were conducted at a cavitation tunnel of the Chungnam National University. We first closely observed the typical formation of wake cavitation and measured lift and drag forces acting on two different test models. Next, using a special device for generating natural and artificial supercavities, we investigated hydrodynamic forces at different cavitation number conditions. This work provides a basis for interpreting the cavity stability and hydrodynamic characteristics of the wedge-shaped control fin for a supercavitating vehicle.

• Wind and Structures
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• 제26권2호
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• pp.89-98
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• 2018

For most full-scale tall buildings the Reynolds number of a flow field around a circular cylinder under strong wind is usually greater than $2{\times}10^7$, which is difficult to achieve in most wind tunnel tests. To explore the wind characteristics of tall cylindrical buildings with equidirectional grooves from subcritical to transcritical flow ($6.6{\times}10^4{\leq}Re{\leq}3.3{\times}10^5$ and $9.9{\times}10^6{\leq}Re{\leq}7.2{\times}10^7$), wind tunnel tests and full-scale large eddy simulations were carried out. The results showed that the rectangular-grooves narrow the wake width due to the downstream movement of the separation point and the deeper grooves cause smaller mean and fluctuating pressure while the peak pressure is little affected. Furthermore, the grooves lead to lower frequency of vortex shedding but the Strouhal number remains at the range from 0.15 to 0.35. The drag coefficient of the cylinders with grooves was found to be 2~3 times as large as that of smooth cylinders.

• 한국항공우주학회지
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• 제30권7호
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• pp.11-19
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• 2002

낙하산 축소모형으로부터 캐노피의 전개시간과 최대 전개력을 측정하기 위하여 낙하산 캐노피의 전개특성에 관한 실험연구를 수행하였다. 시험에는 낙하산 모형을 풍동 유동장 내부로 사출하고 모형에 작용하는 항력을 측정하도록 하기 위한 장치가 사용되었다. 시간에 따른 전개력 변화특성과 최대 전개력이 획득되었으며, 전방체로 인한 후류효과를 비롯한 이들 공력특성에 대한 비교평가를 실시하였다. 링슬롯형 낙하산의 경우 기존의 시험자료들에 비해 약 10~40% 가량 빠른 전개특성을 보였으며, reefing 형상의 리본형은 기존의 결과들과 잘 일치하였다.