• Wind and Structures
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• 제16권2호
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• pp.193-211
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• 2013

This paper presents results from a wind tunnel study that examined the drag coefficient and wind flow over an asymmetric wave train immersed in turbulent boundary layer flow. The modeled wavy surface consisted of eight replicas of a statistically-valid hurricane-generated wave, located near the coast in the shoaling wave region. For an aerodynamically rough model surface, the air flow remained attached and a pronounced speed-up region was evident over the wave crest. A wavelength-averaged drag coefficient was determined using the wind profile method, common to both field and laboratory settings. It was found that the drag coefficient was approximately 50% higher than values obtained in deep water hurricane conditions. This study suggests that nearshore wave drag is markedly higher than over deep water waves of similar size, and provides the groundwork for assessing the impact of nearshore wave conditions on storm surge modeling and coastal wind engineering.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.373-379
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• 2001

When a train runs into a tunnel at high-speed, aerodynamic drag suddenly increases and the booming noise is generated at the exit of tunnel. The noise shape is very important to reduce the aerodynamic drag in tunnel as well as on open ground, and the micro-pressure wave that is a source of booming noise is dependent on nose shape, especially on area distribution. In this study, the nose shape has been optimized employing the response surface methodology and the axi-symmetric compressible Navier-Stokes equations. The optimal designs have been executed imposing various conditions of the aerodynamic drag and the micro-pressure wave on object functions. The results show that the multi-objective design was successful to decrease micro-pressure wave and aerodynamic drag of trains.

• 수산해양기술연구
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• 제53권4호
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• pp.309-316
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• 2017

In this paper, numerical modeling is conducted to analyze the tension of an anchor line by varying the size and drag coefficient of a buoy when the trapnet is influenced by the wave and the current simultaneously. A mass-spring model was used to analyze the behavior of trapnet underwater under the influence of waves and current. In the simulation of numerical model, wave height of 3, 4, 5 and 6 m, a period of 4.4 s, and the flow speed of 0.7 m/s were used for the wave and current condition. The drag coefficients of buoy were 0.8, 0.4 and 0.2, respectively. The size of buoy was 100, 50 and 25% based on the cylindrical buoy ($0.0311m^3$) used for swimming crab trap. The drag coefficient of the trapnet, the main model for numerical analysis, was obtained by a circular water channel experiment using a 6-component load cell. As a result of the simulation, the tension of the anchor line decreased proportional to buoy's drag coefficient and size; the higher the wave height, the greater the decrease rate of the tension. When the buoy drag coefficient and size decreased to one fourth, the tension of the anchor line decreased to a half and the tension of the anchor line was lower than the holding power of the anchor even at 6 m of wave height. Therefore, reducing the buoy drag coefficient and size appropriately reduces the trapnet load from the wave, which also reduces the possibility of trapnet loss.

• International Journal of Ocean Engineering and Technology Speciallssue:Selected Papers
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• 제5권1호
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• pp.29-39
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• 2002

A general investifation into the physical mechanism that is respinsible for drag above the sea surface has been undertaken. On the basis of a ID model of the Wave Boundary Layer(WBL), under a 2D wave field, a parameterization technique for estimation of the drag and mean characteristics of WBL is described. Special attention is paid to estimation of the simplifying assumption of the theory.

• 한국해양공학회지
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• 제10권1호
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• pp.75-80
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• 1996

The importance of the impact force on the vertical offshore circular structure member in the surf zone due to the breaking wave has been recognized recently. In this paper characteristics of breaking wave forces and the corresponding estimation procedures for them are investigated. For the characterization of the wave forces, three parts, drag force, inertia force, impact force are categorized and identified, respectively. Among them the impact force is maimly studied and the concise form of the force is proposed with the application scheme for the design of offshore circular structure member. The resulting form porposed here for impact force is well coincided with former research results by other people. Except the impact force, so called Morison equation can be employed for the common offshore structure design. The drag force and inertia force are represented as convertionally for the profile except the breaking part. In the numerical example, for thpical sea condition and the member size, the proposed procedures for the breaking wave forces calculation are demonstrated. It is found that the impact force is the most deminant one comparing with inertia and drag forces in the surf zone.

• 한국자동차공학회논문집
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• 제10권2호
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• pp.87-95
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• 2002

The aim of this article is to perform the numerical simulation far drop drag and breakup processes in air-assisted sprays using the Taylor analogy breakup (TAB) model with a modified drop drag model, in which a random method is newly used to consider the variation of the drop's frontal area. The predicted results for drop trajectory and Salter mean diameter (SMD) were compared with experimental data and the simulation results using the earlier published models such as TAH model, surface wave instability (Wave) model, and Wave model with original drop drag model. In addition, the effects of the breakup model constant, Ck, on prediction of spray behaviors were discussed. The results shows that the TAB model with the modified drop drag model is in better agreement with experimental data than the other models, indicating the present model is acceptable for predicting the drop breakup process in air-assisted sprays. At higher Weber numbers, the smaller Ck shows the best fitting to experimental data. It should be noted that more elaborated studies is required in order to determine the breakup model constant in the suggested model in the study.

• Journal of Advanced Marine Engineering and Technology
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• 제33권6호
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• pp.965-974
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• 2009

The current study reviews the formula used to calculate the stability of an artificial reef in the breaking wave zone of coastal waters. A comparison was carried out between the existing formula and a new formula that takes into account the water particle velocity in the breaking wave zone. Water particle velocity was analyzed using the Fluent (CADMAS-SURF) software program. The new formula took into various factors, including the difference in the drag coefficient due to the direction of the current and the ratio of distance between two reefs. The drag coefficient of the artificial reef due to the direction of the current was 0.84 when the distance ratio was 0.5. When the artificial reef was placed at 45 degree angle to the current, the product of the drag coefficient and the project area were 40 to 46 % greater than when the reef was placed at 90 degree angle. Our results regarding the stability of an artificial reef indicate that the new formula provides the designers of artificial reefs with a more rational and economic design rationale rather than the existing formula.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제36회 춘계학술대회논문집
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• pp.381-384
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• 2011

본 연구에서는 초음속 비행체의 조파저항을 감소시키기 위하여, 최대 주파수 80 kHz의 반복 레이저 펄스에 의해 야기된 에너지 부가법에 관한 실험적 연구가 수행된다. 기류 마하수 1.94의 흡입식 초음속 풍동의 바깥에 설치된 초점렌즈에 의하여 레이저 펄스가 실린더 모델 전단부에 집약된다. 시간변동 항력과 정체압력은 로드셀과 PCB 압력센서에 의해서 측정되며, 동시에 고속 카메라를 이용하여 가시화가 수행된다. 본 연구의 결과로부터, 레이저 펄스 에너지 부가에 의한 항력 저감량은 레이저 펄스 주파수가 증가할 때, 최대 21%까지 거의 선형적으로 증가하였다. 부가 에너지 효율은 레이저 펄스 에너지에만 의존하는 결과를 얻었으며, 최대 1000%까지 달성되었다.

• 한국전산유체공학회지
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• 제18권3호
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• pp.42-50
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• 2013

The supersonic airplane with flapped biplane, Busemann biplane equipped flap, is superior to drag and noise reduction due to wave cancelation effect between upper and lower airfoils. In this study, it is numerically calculated and analyzed the lift, drag and lift to drag ratio of flapped biplane with respect to various the length and angle of the flap. Euler solver of EDISON CFD, web based computational fluid dynamic solver for the purpose of education, is employed. Depending on the length of the flap, lift and drag increase linearly, and there exists the optimum flap angle which maximize the lift-to-drag ratio at the freestream mach 2.0 on-design condition. The predictable relational expression is driven as liner equation. As a results of comparison with drag of flapped biplane, Busemann biplane, and diamond airfoil with the same lift, the drag of flapped biplane is 88.76% lower than that of the Busemann biplane and 70.67% lower than that of the diamond airfoil. In addition, the change of pressure is compared to confirm the noise reduction effect of flapped biplane at h/c=5 of lower airfoil. The shock strength of flapped biplane is smaller than that of other airfoils.

• 대한토목학회논문집
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• 제36권5호
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• pp.831-838
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• 2016

최근 해안공학분야에서 식생을 활용하는 연안방재시스템에 관한 관심이 증가하고 있다. 이에 본 연구에서는 쓰나미-식생 비선형 상호작용에 의한 파동장과 유동장을 해석하기 위하여 식생항력에 따른 에너지소산을 직접 해석할 수 있는 3차원 수치파동수조를 기존의 3-D N-S solver (LES-WASS-3D ver. 2.0, HYMO-WASS-3D)를 토대로 개발하였다. 그리고 기존의 실험결과와 비교 및 검토를 통한 수치파동수조의 적용성을 평가하였다. 그 결과 레이놀즈수에 따라 능동적으로 추정되는 항력계수를 고려한 경우가 평균 항력계수를 적용한 경우보다 실험에서 얻어진 각 지점의 시간파형을 잘 재현하였다. 또한 식생밀도 및 입사파고에 따라 계산된 고립파의 전달률이 실험의 측정값과 높은 일치도를 나타내었다. 이로써 본 연구에서 식생에 의한 항력으로서 동적항력계수를 적용한 3차원 수치파동수조의 타당성 및 유효성을 확인하였다.