• 대한기계학회논문집B
• /
• 제21권1호
• /
• pp.153-164
• /
• 1997

Flame oscillation phenomena in a co-flow diffusion flame was experimentally studied with periodic fuel supply using a solenoid valve. The degree of excitation was controlled by changing the volume flux of fuel passing through the valve. Flame oscillation frequencies were measured utilizing a photodiode, a spectrum analyzer, video and high speed movies. Laser planar visualization was employed to study the correlation between the flame oscillation and the toroidal vortices. Observed are three regimes of flame oscillation, where the oscillation frequencies are for the multiples of excitation, the excitation itself and the flame natural oscillation. Both periods of natural oscillation and of excitation induced oscillation exist over one cycle of the excitation in the frequency multiplied regime. It is considered as an effect of balancing the influence of buoyancy driven vortex with that of excitation induced vortex near the excitation rate of 0.2. Flame shapes are become monotonous as increasing the excitation frequency to the range of over two fold of the natural oscillation. The flame oscillation can be modulated to the frequency of either multiples of excitation or excitation itself under certain conditions. This implies that the flame oscillation could be modulated to avoid the resonance frequency of the combustor, and shows the possibility of active control of the flame oscillation.

• Wind and Structures
• /
• 제9권1호
• /
• pp.1-22
• /
• 2006

The response characteristics and suppression of flow-induced vibrations of rectangular prisms with various width-to-depth ratios were experimentally investigated. The prisms were rigid and elastically mounted at both ends to enable constrained torsional vibrations only. The present study focused on torsional vibrations, one of the three types of flow-induced vibrations generated in a rectangular prism. First, the response characteristics of torsional vibrations generated in rectangular prisms were investigated by free-vibration tests. It was found that the response characteristics of torsional vibrations generated in rectangular prisms could be classified into six patterns depending on the width-to-depth ratio. Next, the response characteristics of torsional vibrations observed in the free-vibration tests were reproduced by forced-vibration tests, and the mechanisms by which the three types of flow-induced vibrations, low-speed torsional flutter, vortex excitation and high-speed torsional flutter, are generated in the rectangular prisms were elucidated on the basis of characteristics of fluid forces and visualized flow patterns. Experiments were also carried out to establish an effective method for suppressing flow-induced vibrations generated in the rectangular prisms, and it was found that low-speed torsional flutter and high-speed torsional flutter could be suppressed by placing a small normal plate upstream of the prism, which results in suppression of the alternating rolling-up of the shear layers separating from the leading edges of the prism. It was also found that vortex excitation could be suppressed by placing a splitter plate downstream of the prism, which results in suppression of the generation of wake vortices.

• 산업기술연구
• /
• 제22권A호
• /
• pp.137-144
• /
• 2002

A series of wind tunnel tests was conducted to investigate the existence of the galloping instability of inclined dry cables and also to Identify the influence of some parameters on it. These parameters are the structural damping and cable surface roughness, which may have significant impact on the vibration characteristics. The test results showed both the divergent type of galloping instability and the limited amplitude high wind speed vortex shedding excitation. Galloping instability was observed in only one case. Parametric study shows that the vortex shedding oscillation can be easily suppressed with an increase of structural damping. It was also shown that the instability criterion indicated by earlier research was too conservative compared to the results obtained from the present study.

• Wind and Structures
• /
• 제28권4호
• /
• pp.225-238
• /
• 2019

In this paper a novel and efficient computational framework to estimate the stress range versus number of cycles curves experienced by a cable due to external excitations (e.g., seismic excitations, traffic and wind-induced vibrations, among others) is proposed. This study is limited to the wind-cable interaction governed by the Vortex Shedding mechanism which mainly rules cables vibrations at low amplitudes that may lead to their failure due to bending fatigue damage. The algorithm relies on a stochastic approach to account for the uncertainties in the cable properties, initial conditions, damping, and wind excitation which are the variables that govern the wind-induced vibration phenomena in cables. These uncertainties are propagated adopting Monte Carlo simulations and the concept of importance sampling, which is used to reduce significantly the computational costs when new scenarios with different probabilistic models for the uncertainties are evaluated. A high fidelity cable model is also proposed, capturing the effect of its internal wires distribution and helix angles on the cables stress. Simulation results on a 15 mm diameter high-strength steel strand reveal that not accounting for the initial conditions uncertainties or using a coarse wind speed discretization lead to an underestimation of the stress range experienced by the cable. In addition, parametric studies illustrate the computational efficiency of the algorithm at estimating new scenarios with new probabilistic models, running 3000 times faster than the base case.

• 한국해양공학회지
• /
• 제23권1호
• /
• pp.74-80
• /
• 2009

In this study, the static and modal analyses to find the characteristic of eigenvalues for a towed cable were with a free boundary condition at the bottom end carried out with numerical study. The resulting numerical code with finite element method was used to study sample problems for a cable with towing speeds. After tracing the equilibrium state with a towing speed through the static analysis, modal analysis on the basis of static results was performed. The static top tension for a critical towing speed is nearly 50 percent of what it was for a free hanging pipe. From static analyses, it is found that towing speed has a noticeable effect on top tension of a towed pipe. At a high towing speed, differences between the first and second periods become larger. Compared to the fundamental period for a free hanging pipe, that for a towed pipe with a critical towing speed is approximately 1.4 times larger. This result is very important point in that the lock in condition and tension of the towed cable system with top excitation can be predicted. The corrected close form solution to solve natural periods for a towed cable was presented in this study. The code is validated by comparison of the results of theoretical and numerical studies. Two results were in very good agreement. This study can contribute to predicting the lock-in condition and tension for a towed cable or pipe with top excitation.

• Wind and Structures
• /
• 제6권3호
• /
• pp.197-208
• /
• 2003

Galloping instability of dry inclined cables of cable-stayed bridges has been reported by Japanese researchers. A suggested stability criterion based on some experimental studies in Japan implies that many of stay cables would be expected to suffer galloping instability, which, if valid, would cause serious difficulty in the design of cable-stayed bridges. However, this is not the case in reality. Thus, it is practically urgent and necessary to confirm the validity of this criterion and possible restriction of it. In the present study, a 2D sectional cable model was tested in the wind tunnel, and effects of various physical parameters were investigated. It is found that the stability criterion suggested by Japanese researchers is more conservative than the results obtained from the current study.

• 한국강구조학회 논문집
• /
• 제17권4호통권77호
• /
• pp.499-509
• /
• 2005

강풍으로 유발되는 고층건축물의 풍진동은 주로 와류에 의한 풍직각방향의 진동에 의하여 발생한다. 이러한 진동은 단면형상이 일정한 유연하고, 경량이며, 경감쇠인 고층건축물인 경우 가장 심하게 발생한다. 본 논문은 와류에 기인한 풍직각방향의 진동을 저감시키기 위한 공역학적인 방법을 논한 것이다. 항력 및 횡력방향의 압력을 균등화하고 또한 양방향의 공간적인 간섭을 분산시키고, 풍직각 방향으로 작용하는 풍력의 크기를 효율적으로 감소시키기 위하여 건축물의 풍방향 및 풍직각방향에 중공부를 설치하였다. 실험모형은 모두 형상비가 8:1이 되도록 하였고, 중공부의 형상은 2종류, 크기는 2종류, 위치는 6종류로 변화시킨 총 24종류의 모형을 제작하여 풍력실험을 실시한 후 각 모형에 대한 풍방향 및 풍직각방향의 변위응답특성을 조사하였다. 최종적으로 중공부를 가진 모형의 효율성을 분석하기 위하여 중공부를 가진 모형에 대한 결과를 중공부가 없는 정사각형 각주의 변위응답 특성과 비교 분석하여 중공부의 형상 변화, 크기 변화, 위치 변화에 따른 풍진동의 저감효과의 정도를 정량적으로 규명하였다.