• 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.

• Wind and Structures
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• 제22권5호
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• pp.573-594
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• 2016

Simultaneous pressure measurements on 2D square prisms with various corner modifications were performed in uniform flow with low turbulence level, and the testing Reynolds numbers varied from $1.0{\times}10^5$ to $4.8{\times}10^5$. Experimental models were a square prism, three chamfered-corner square prisms (B/D=5%, 10%, and 15%, where B is the chamfered corner dimension and D is the cross-sectional dimension), and six rounded-corner square prisms (R/D =5%, 10%, 15%, 20%, 30%, and 40%, where R is the corner radius). Experimental results of drag coefficients, wind pressure distributions, power spectra of aerodynamic force coefficients, and Strouhal numbers are presented. Ten models are divided into various categories according to the variations of mean drag coefficients with Reynolds number. The mean drag coefficients of models with $B/D{\leq}15%$ and $R/D{\leq}15%$ are unaffected by the Reynolds number. On the contrary, the mean drag coefficients of models with R/D=20%, 30%, and 40% are obviously dependent on Reynolds number. Wind pressure distributions around each model are analyzed according to the categorized results.The influence mechanisms of corner modifications on the aerodynamic characteristics of the square prism are revealed from the perspective of flow around the model, which can be obtained by analyzing the local pressures acting on the model surface.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2003년도 춘계학술발표대회 논문집
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• pp.95-98
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• 2003

In this work, a closed-form analysis is performed to obtain the stiffness coefficients of thin-walled composites beams with elliptical profiles. The analytical model includes the effects of elastic couplings, shell wall thickness, torsion warping and constrained warping. Reissner's semi-complementary energy functional is used to derive the beam force-displacement relations. The theory is validated against MSC/NASTRAN results for coupled composites beams with single-cell elliptical sections. Very good correlation has been noticed for the test cases considered.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1994년도 봄 학술발표회 논문집
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• pp.33-37
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• 1994

When a bolt is tightened up to the range of plastic deformation, yielding is governed by the combined stresses due to the axial force developed in the bolt and the frictional torque developed on the thread in contact with the nut. Consideration is taken account of the fact that the unused portion of the thread has least sectional area being subject to initial yielding. Once yielding has taken place some strain hardening effect will result, Incremental stress-strain relations are used to treat the continued yielding, which is equivalent to treat continued yielding as if summing up the effects of thin walled cylinders subject to plastic deformation. M10 bolts of common and fine series thread are used for computational purposes. Variation of axial forces and frictional torques vs. the frictional coefficients tare presented together with other plots showing some characteristics of bolt under plastic deformation.

• 대한토목학회논문집
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• 제33권1호
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• pp.93-100
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• 2013

본 연구에서는 트윈박스 단면인 이순신대교의 레이놀즈수 변화에 따른 공기력의 영향을 살펴보는데 그 목적이 있다. 이를 위하여 1/30 대축척 모형을 제작하여 공군사관학교 아음속 중형 풍동에서 최대 풍속 70m/s까지 풍속을 증가시켜가면서 공기력을 측정하여, 전북대학교 소형풍동에서 수행한 저레이놀즈수 풍동실험 결과와 비교하였다. 본 연구 대상 교량 단면은 레이놀즈수의 영향을 받는 것으로 나타났으며, 고레이놀즈수 실험 결과 기존 저레이놀즈수 실험보다 항력계수는 약 23%정도 낮은 수준인걸로 나타났다. 또한 경계층 촉진장치를 효과적으로 이용하면 기존의 저레이놀즈수 풍동실험 조건에서 고레이놀즈수 모사 실험이 가능한 것으로 판단된다.

• International Journal of Aeronautical and Space Sciences
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• 제10권2호
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• pp.23-33
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• 2009

The inherent aeromechanical complexity of a rotor system necessitated the comprehensive analysis code for helicopter rotor system. In the present study, an aerodynamic analysis module has been developed as a part of rotorcraft comprehensive program. Aerodynamic analysis module is largely classified into airload calculation routine and inflow analysis routine. For airload calculation, quasi-steady analysis model is employed based on the blade element method with the correction of unsteady aerodynamic effects. In order to take unsteady effects - body motion effects and dynamic stall - into account, aerodynamic coefficients are corrected by considering Leishman-Beddoes's unsteady model. Various inflow models and vortex wake models are implemented in the aerodynamic module to consider wake induced inflow. Specifically, linear inflow, dynamic inflow, prescribed wake and free wake model are integrated into the present module. The aerodynamic characteristics of each method are compared and validated against available experimental data such as Elliot's induced inflow distribution and sectional normal force coefficients of AH-1G. In order to validate unsteady aerodynamic model, 2-D unsteady model for NACA0012 airfoil is validated against aerodynamic coefficients of McAlister's experimental data.

• Wind and Structures
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• 제15권5호
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• pp.385-407
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• 2012

Bridge hanger vibrations have been reported under icy conditions. In this paper, the results from a series of static and dynamic wind tunnel tests on a circular cylinder representing a bridge hanger with simulated thin ice accretions are presented. The experiments focus on ice accretions produced for wind perpendicular to the cylinder at velocities below 30 m/s and for temperatures between $-5^{\circ}C$ and $-1^{\circ}C$. Aerodynamic drag, lift and moment coefficients are obtained from the static tests, whilst mean and fluctuating responses are obtained from the dynamic tests. The influence of varying surface roughness is also examined. The static force coefficients are used to predict parameter regions where aerodynamic instability of the iced bridge hanger might be expected to occur, through use of an adapted theoretical 3-DOF quasi-steady galloping instability model, which accounts for sectional axial rotation. A comparison between the 3-DOF model and the instabilities found through two degree-of-freedom (2-DOF) dynamic tests is presented. It is shown that, although there is good agreement between the instabilities found through use of the quasi-steady theory and the dynamic tests, discrepancies exist-indicating the possible inability of quasi-steady theory to fully predict these vibrational instabilities.

• 한국항공우주학회지
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• 제37권3호
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• pp.224-231
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• 2009

본 논문에서는 회전익기 통합해석프로그램개발의 일환으로 공력해석코드를 개발 및 검증하였다. 기본적인 공력하중은 익형 공력테이블을 이용한 깃요소이론을 기반으로 계산하였고, 로터의 유도 유입류를 계산하기 위해 선형유입류 모델, 동적유입류 모델, 지정후류모델, 자유후류 모델 등 여러 유입류 예측기법을 사용하였다. 각 모델의 특성을 파악하기 위해 Elliott 등의 유도 유입류 실험결과와 AH-1G 실험결과의 국소 수직력계수를 비교 및 검증하였다.

• 한국지진공학회논문집
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• 제21권1호
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• pp.59-67
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• 2017

Due to a lack of the hoop action of lateral reinforcements the effective confining force in rectangular sections reduces compared to circular ones. Therefore, the stress-strain model obtained from the experimental data with circular sections overestimates the lateral confinement effect in rectangular sections, which evaluates seismic safety margin of overall structural system excessively. In this study experiments with laterally-confined square sections have been performed and the characteristic values composing stress-strain model have been analyzed. With introduction of section coefficients, in addition, the new unified stress-strain model applicable to square sections as well as circular ones has been proposed.

• 전산구조공학
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• 제7권3호
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• pp.133-141
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• 1994

강도활용을 극대화한 볼트의 소성역체결시 볼트의 항복은 볼트에 발생한 인장응력과 너트 나사산과의 접촉면에서 오는 마찰토오크에 의한 전단응력과의 조합에 의한 영향을 받는다. 볼트는 최소단면적인 나사부중 너트와 물리지 않은 여우나사부에서 먼저 항복을 일으킨다. 볼트재료는 대체로 항복과 더불어 소성경화를 보이며 그 정도는 고강도재료에서 인장강도의 10%정도이다. 본 연구에서는 소성역체결시의 거동해석을 변형도증분이론에 의거 해석하였다. 항복은 최소단명이 원통의 표면에서 시작되며 이를 두께가 얇은 원통으로 취급하였다. 항복의 전파는 이들 얇은 원통이 체결이 진행됨에 따라 순차적으로 항복에 이르는 것으로 보고 이 얇은 원통들을 항복시키는데 필요한 축력과 토오크를 합하여 체결종료시의 볼트축력과 (나사면마찰) 토오크로 하였다. M10미터 가는 나사를 계산과 실험에 사용하였다. 축력과 마찰토오크의 마찰계수에 대한 변화관계를 보여주는 그래프와 더불어 실험에서 사용한 볼트의 설계보조 선도를 제시하였다. 이 설계보조용 선도는 실험실에서 얻어진 토오크계수와 마찰계수와의 관계를 그린 것으로 윤활, 포면처리등 현장조건에 따라 달라진 마찰계수를 적절히 취급하는데 효과적으로 활용이 가능하다.