• 대한토목학회논문집
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• 제26권5A호
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• pp.887-899
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• 2006

본 연구의 목적은 교량 거더 단면의 공기역학적 특성을 나타내는 기본 자료인 공기력계수와 플러터계수가 동적응답과 어떠한 상관관계를 가지는지를 규명하는데 있다. 이를 위해 세 단계의 단면모형실험이 수행되었다. 첫 번째 단계에서는 총 7개의 거더 단면 즉, 6개의 플레이트거더 단면과 1개의 박스거더 단면이 고려되었으며 거더 단면의 기하학적 형상, 영각, 바람의 방향 그리고 기류조건이 공기력계수인 항력계수, 양력계수 그리고 모멘트계수에 미치는 영향을 정적 단면모형실험을 통해 살펴보았다. 두 번째 단계에서는 동적실험을 통해 각 단면의 공기력계수와 동적응답의 상관성을 검증하였다. 마지막으로 2자유도하의 동적 단면모형실험을 통해 세 개의 거더 단면의 플러터계수를 산출하고 이를 동적실험결과와 비교하였다. 주어진 단면형상에 대한 비정상 공기력에 의해 변화되는 시스템의 감쇠와 강성을 가장 잘 반영하는 플러터계수는 초기변위-자유진동시스템을 이용하여 추출하였다. 이를 위해 등류조건에서 풍속별로 교량단면의 수직 및 비틀림 초기변위의 시간에 따른 진폭의 감쇠를 측정하였다. 본 연구에서 제시한 교량단면의 공기력계수와 플러터계수는 공탄석해석 및 버펫팅해석을 위한 기본 자료로 유용하게 쓰일 것으로 보인다.

• Wind and Structures
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• 제5권2_3_4호
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• pp.141-150
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• 2002

Numerical simulations based on the ALE finite element method are carried out to examine the aerodynamics of an oscillating circular cylinder when the separated shear flows around the cylinder are stimulated by periodic jet excitation with a shear layer instability frequency. The excitation is applied to the flows from two points on the cylinder surface. The numerical results showed that the excitation with a shear layer instability frequency can reduce the negative damping and thereby stabilize the aerodynamics of the oscillating cylinder. The change of the lift phase seems important in stabilizing the cylinder aerodynamics. The change of lift phase is caused by the merger of the vortices induced by the periodic excitation with a shear layer instability frequency, and the vortex merging comes from the high growth rate, the rapid increase of wave number and decrease of phase velocity for the periodic excitation in the separated shear flows.

• 제어로봇시스템학회논문지
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• 제11권11호
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• pp.963-969
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• 2005

An advanced method of Relaxed Static Stability (RSS) is utilized for improving the aerodynamic performance of modem version supersonic jet fighter aircraft. The flight control system utilizes RSS criteria in both longitudinal and lateral-directional axes to achieve performance enhancements and improve stability. The T-50 advanced trainer employs the RSS concept in order to improve the aerodynamic performance and the flight control law in order to guarantee aircraft stability, The T-50 longitudinal control laws employ the dynamic inversion and proportional-plus-integral control method. This paper details the design process of developing longitudinal control laws for the RSS aircraft, utilizing the requirement of MIL-F-8785C. In addition, This paper addresses the analysis of aircraft characteristics such as damping, natural frequency, gain and phase margin about state variables for longitudinal inner loop feedback design.

• Wind and Structures
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• 제32권3호
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• pp.179-191
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• 2021

A series of wind tunnel tests, including 1:50 sectional model tests, 1:50 free-standing bridge tower tests and 1:70 full-bridge aeroelastic model tests were carried out to systematically investigate the aerodynamic performance of the Hong Kong-Zhuhai-Macao Bridge (HZMB). The test result indicates that there are three wind-resistant safety issues the HZMB encounters, including unacceptable low flutter critical wind speed, vertical vortex-induced vibration (VIV) of the main girder and galloping of the bridge tower in across-wind direction. Wind-induced vibration of HZMB can be effectively suppressed by the application of aerodynamic and mechanical measures. Acceptable flutter critical wind speed is achieved by optimizing the main girder form (before: large cantilever steel box girder, after: streamlined steel box girder) and cable type (before: central cable, after: double cable); The installations of wind fairing, guide plates and increasing structural damping are proved to be useful in suppressing the VIV of the HZMB; The galloping can be effectively suppressed by optimizing the interior angle on the windward side of the bridge tower. The present works provide scientific basis and guidance for wind resistance design of the HZMB.

• Wind and Structures
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• 제10권1호
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• pp.1-22
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• 2007

The suppression of aerodynamic response of long-span suspension bridges during erection and after completion by using single TMD and multi TMD is presented in this paper. An advanced finite-element-based aerodynamic model that can be used to analyze both flutter instability and buffeting response in the time domain is also proposed. The frequency-dependent flutter derivatives are transferred into a time-dependent rational function, through which the coupling effects of three-dimensional aerodynamic motions under gusty winds can be accurately considered. The modal damping of a structure-TMD system is analyzed by the state-space approach. The numerical examples are performed on the Akashi Kaikyo Bridge with a main span of 1990 m. The bridge is idealized by a three-dimensional finite-element model consisting of 681 nodes. The results show that when the wind velocity is low, about 20 m/s, the multi TMD type 1 (the vertical and horizontal TMD with 1% mass ratio in each direction together with the torsional TMD with ratio of 1% mass moment of inertia) can significantly reduce the buffeting response in vertical, horizontal and torsional directions by 8.6-13%. When the wind velocity increases to 40 m/s, the control efficiency of a multi TMD in reducing the torsional buffeting response increases greatly to 28%. However, its control efficiency in the vertical and horizontal directions reduces. The results also indicate that the critical wind velocity for flutter instability during erection is significantly lower than that of the completed bridge. By pylon-to-midspan configuration, the minimum critical wind velocity of 57.70 m/s occurs at stage of 85% deck completion.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 1998년도 춘계학술대회논문집; 용평리조트 타워콘도, 21-22 May 1998
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• pp.386-391
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• 1998

The effect of the changes in parameter angles(precone, droop, sweep) on the lead-lag damping was focused on. Experiment was made with hingeless 4-blade rotors and NACA 0012 airfoil. For the measurement of the rotating natural frequencies and lead-lag damping, non-rotating swash plate was oscillated at the regressing lag mode frequency and the data were acquired after the excitation was cut off. Analysis was made using a finite element formulation based on Hamilton's principle. The main blade is assumed as elastic beams. Quasi-steady strip theory is used to obtain aerodynamic forces, and non-circulatory forces are also included.

• Wind and Structures
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• 제12권3호
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• pp.259-279
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• 2009

The paper is concerned with the numerical study of the cross-wind response of the 295 m-tall six-flue industrial chimney, located in the power station of Belchatow, Poland. The response of the chimney due to turbulent wind flow is caused by the lateral turbulence component and vortex excitation with taking into account motion-induced wind forces. The cross-wind response has been estimated by means of the random vibration approach. Three power spectral density functions suggested by Kaimal, Tieleman and Solari for the evaluation of the lateral turbulence component response are taken into account. The vortex excitation response has been calculated by means of the Vickery and Basu's model including some complements. Motion-induced wind forces acting on a vibrating chimney have been modeled as a nonlinear aerodynamic damping force. The influence of three components mentioned above on the total cross-wind response of the chimney has been investigated. Moreover, the influence of damping ratios, evaluated by Multi-mode Random Decrement Technique, and number of mode shapes of the chimney have been examined. Computer programmes have been developed to obtain responses of the chimney. The numerical results and their comparison are presented.

• 항공우주기술
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• 제3권2호
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• pp.217-228
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• 2004

본 논문은 복합재 패들형 블레이드를 장착한 축소 힌지없는 로터 시스템의 정지 및 전진 비행조건에 대한 회전시험 기술과 결과에 대한 것이다. 축소 로터 시스템은 실물크기 로터 시스템의 구조 자료를 이용하여 프루드 축소화하였고, 허브 flexure는 동일한 로터의 동력학적 특성을 기준으로 금속재와 복합재 2가지를 제작하였다. 2종류의 힌지없는 허브시스템을 KARI의 GSRTS에 장착후 회전 시험을 실시하여 로터 시스템의 리드래그 감쇠비와 공력 하중을 측정하였다. 리드래그 모드의 감쇠비를 산출하기 위해 MBA(Moving Block Analysis)기법을 사용하였고, 허브와 주축 사이에 6분력 발란스를 장착하고, 블레이드에 스트레인게이지를 부착하여 공력하중을 측정하였다. 시험은 제자리 및 전진비행 조건에 따라 지상 및 풍동에서 각각 수행하였다.

• 한국항공우주학회지
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• 제34권4호
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• pp.25-32
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• 2006

현대의 고성능 전투기는 공력성능 및 조종성능의 향상을 위하여 대부분 세로축 방향으로 항공기를 불안정하게 설계하는 정안정성 완화(Relaxed Static Stability) 개념을 채택하고 있다. 비행제어법칙의 설계 작업은 불안정하게 설계된 항공기에 안정성(Stability)을 부여하고, 주어진 비행임무에 대하여 만족스런 조종성능(Controllability)을 발휘할 수 있도록 비행성능을 조작하는 일련의 과정이다. 항공기의 세로축 운동은 장주기 및 단주기 운동이라는 2개의 감쇠진동 모드가 중첩되어 있다고 간주한다. 전투기에서 세로축 제어법칙설계는 장주기 모드가 설계대상으로 고려되지 않으며, 미 군사규격인 MIL-F-8785C에 정의되어 있는 단주기 모드(Short Period Mode)의 요구조건을 이용하여 주파수와 감쇠비를 최적화한다. 이 때, 제어법칙이 포함된 고차시스템을 저차 등가시스템으로 등가 시켜 판별한다. 현재까지 항공기의 단주기 운동 특성에 관한 연구는 활발하게 진행되어 왔으나, 장주기 운동 특성에 관한 연구는 그렇지 못하였다. 본 논문에서는 T-50 훈련기의 장주기 모드 운동 특성을 분석하기 위해 선형 및 비선형해석을 수행하였으며, 받음각제한기 및 자동 피치자세각 트림 명령이 세로축 운동에 미치는 영향성을 분석하였다.

• Wind and Structures
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• 제10권5호
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• pp.463-479
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• 2007

A series of wind tunnel sectional model dynamic tests of a twin-deck bridge were conducted at the CLP Power Wind/Wave Tunnel Facility (WWTF) of The Hong Kong University of Science and Technology (HKUST) to investigate the effects of gap-width on the self-excited vibrations and the dynamic and aerodynamic characteristics of the bridge. Five 2.9 m long models with different gap-widths were fabricated and suspended in the wind tunnel to simulate a two-degrees-of-freedom (2DOF) bridge dynamic system, free to vibrate in both vertical and torsional directions. The mass, vertical frequency, and the torsional-to-vertical frequency ratio of the 2DOF systems were fixed to emphasize the effects of gap-width. A free-vibration test methodology was employed and the Eigensystem Realization Algorithm (ERA) was utilized to extract the eight flutter derivatives and the modal parameters from the coupled free-decay responses. The results of the zero gap-width configuration were in reasonable agreement with the theoretical values for an ideal thin flat plate in smooth flow and the published results of models with similar cross-sections, thus validating the experimental and analytical techniques utilized in this study. The methodology was further verified by the comparison between the measured and predicted free-decay responses. A comparison of results for different gap-widths revealed that variations of the gap-width mainly affect the torsional damping property, and that the configurations with greater gap-widths show a higher torsional damping ratio and hence stronger aerodynamic stability of the bridge.