• Structural Engineering and Mechanics
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• 제81권6호
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• pp.729-736
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• 2022

Earth is facing a serious problem of pollution and scarcity of energy sources. The synthetic fibers used in automobile and Aerospace manufacturing sectors are non-renewable and harmful to environment. International Agency such as FAA and SAE is forcing for green fuel, green materials and structures. Further exploration is much needed to understand its potential in structural applications. In the current study, hemp and Jute fibre based composites were developed and tested for assessing their suitability for possible applications in automobile and aerospace sectors. Composites were undergone tensile test, water absorption test, and fatigue analysis to understand its behavior under various loading conditions. The finite element analysis has been carried out to understand the fatigue behaviour of composites. The results revealed that the usage of hemp and jute fibre reinforced composites can improve mechanical properties and have shown a viable alternative to replace synthetic fibres such as glass fibres for specific applications. Hemp reinforced bio-composites have shown better performance as compared to Jute reinforced bio-composites while water resistance characteristics for hemp is poorer to jute fibres.

• Wind and Structures
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• 제7권4호
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• pp.265-280
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• 2004

This paper presents some fundamental results on the dynamics of the periodic Karman wake behind a circular cylinder. The wake is treated like a dynamical system. External forcing is then introduced and its effect investigated. The main result obtained is the following. Perturbation of the wake, by controlled cylinder oscillations in the flow direction at a frequency equal to the Karman vortex shedding frequency, leads to instability of the Karman vortex structure. The resulting wake structure oscillates at half the original Karman vortex shedding frequency. For higher frequency excitation the primary pattern involves symmetry breaking of the initially shed symmetric vortex pairs. The Karman shedding phenomenon can be modeled by a nonlinear oscillator. The symmetrical flow perturbations resulting from the periodic cylinder excitation can also be similarly represented by a nonlinear oscillator. The oscillators represent two flow modes. By considering these two nonlinear oscillators, one having inline shedding symmetry and the other having the Karman wake spatio-temporal symmetry, the possible symmetries of subsequent flow perturbations resulting from the modal interaction are determined. A theoretical analysis based on symmetry (group) theory is presented. The analysis confirms the occurrence of a period-doubling instability, which is responsible for the frequency halving phenomenon observed in the experiments. Finally it is remarked that the present findings have important implications for vortex shedding control. Perturbations in the inflow direction introduce 'control' of the Karman wake by inducing a bifurcation which forces the transfer of energy to a lower frequency which is far from the original Karman frequency.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.639-644
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• 2000

An experiment on the enhancement of turbulent flow with ultrasonic forcing was carried out by using PIV measurement in a coaxial circular pipe which could offer characteristics of the turbulence flow plentifully through its jet. A large transparent acryl tank and a coaxial circular pipe nozzle were made for the above research. city water of $25^{\circ}C$ was selected as an experimental liquid and the front flow field of the coaxial circular pipe was divided vertically as 3 measuring regions to observe characteristics of flow phenomena. characteristics of fluid flow such as velocity vector distribution, kinetic energy, turbulent intensity and etc. were visualized, observed, examined and considered at 5 kinds of Re No. such as $Re=1{\times}10^3,\;2{\times}10^3,\;3{\times}10^3,\;5{\times}10^3,\;1{\times}10^4$. In result it was proved that ultrasonic vibration affected the enhancement of turbulent flow.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.497-501
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• 2000

Flow over a sphere is controlled experimentally at $Re=10^5$ using electro-magnetic actuators. The electro-magnetic actuator developed in this study is composed of the permanent magnet electro-magnet membrane and slot. Eight actuators are placed inside the sphere at equally spaced intervals on a latitudinal plane and the position of the control slot is 76 from the stagnation point. Each actuator generates a periodic blowing and suction through the slot at variable frequencies of $10{\sim}140Hz$ and variable amplitudes by controlling electric signals applied to the electro-magnet. Drag on the sphere measured using a load cell is significantly reduced with control at the forcing frequencies larger than the natural shedding frequency $({\approx}14Hz\;at\;Re=10^5)$, whereas drag is slightly increased at the forcing frequency of 10Hz. It is shown from pressure measurement that the static pressure in the rear surface of the sphere is significantly increased with control, indicating that the separation is delayed due to control. Flow visualizations also show that the detaching shear layer is more attracted to the sphere center with control, the separation bubble size is significantly reduced, and motion inside the bubble is very weak, as compared to the case of uncontrolled flow.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.916-921
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• 2003

In most industrial applications, the geometrical complexity is combined with the moving boundaries. These problems considerably increase the computational difficulties since they require, respectively, regeneration and deformation of the grid. As a result, engineering flow simulation is restricted. In order to solve this kind of problems the immersed boundary method was developed. In this study, the immersed boundary method is applied to the numerical simulation of stationary, rotating and oscillating cylinders in the 2-dimensional square cavity. No-slip velocity boundary conditions are given by imposing feedback forcing term to the momentum equation. Besides, this technique is used with a second-order accurate interpolation scheme in order to improve the accuracy of flow near the immersed boundaries. The governing equations for the mass and momentum using the immersed boundary method are discretized on the non-staggered grid by using the finite volume method(FVM). This study presents the possibility of the immersed boundary method to apply to the complex flow experienced in the industrial applications.

• 한국해양학회지
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• 제24권1호
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• pp.29-38
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• 1989

동해 해수순환의 특징은 대한해협으로부터 유입된 난수가 동한난류를 형성하여 한국연안을 따라 북상한 후 다시 외해로 분리되어 나간다는 것과 북쪽에서부터 연안을 따라 남하하는 북한한류가 형성하는 반시계 방향의 순환류가 동해 북부에 존재한다는 사실일 것이다. 본 논문에서는 이러한 사실을 설명하기 위하여 간단한 역학적 모델을 수립하였다. 동해 해수순환을 기본적으로 유입-유출 형태라 한 후 여기에 해양-대기 열 교환을 적용하였으며 해수혼합에 의한 열의 소멸, 마찰에 의한 운동량의 소멸도 고려하였다. 결과에 의하면, 상기의 순환특징은 열역학적 요인에 의하여 기인됨이 밝혀졌다. 또한, 열교환의 정도에 따라 여러가지 순환형태가 나타날 수 있음도 보였다.

• Journal of Mechanical Science and Technology
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• 제20권11호
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• pp.1993-2001
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• 2006

The piezoelectric bimorph film, which, as an actuator, can generate more effective displacement than the usual PVDF film, is used to control the turbulent boundary-layer flow. The change of wall pressures inside the turbulent boundary layer is observed by using the multi-channel microphone array flush-mounted on the surface when actuation at the non-dimensional frequency $f_b^+$:=0.008 and 0.028 is applied to the turbulent boundary layer. The wall pressure characteristics by the actuation to produce local displacement are more dominantly influenced by the size of the actuator module than the actuation frequency. The movement of large-scale turbulent structures to the upper layer is found to be the main mechanism of the reduction in the wall- pressure energy spectrum when the 700$700{\nu}/u_{\tau}$-long bimorph film is periodically actuated at the non- dimensional frequency $f_b^+$:=0.008 and 0.028. The biomorph actuator is triggered with the time delay for the active forcing at a single frequency when a 1/8' pressure-type, pin-holed microphone sensor detects the large-amplitude pressure event by the turbulent spot. The wall-pressure energy in the late-transitional boundary layer is partially reduced near the convection wavenumber by the open-loop control based on the large amplitude event.

• Wind and Structures
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• 제26권3호
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• pp.147-161
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• 2018

For wind engineering applications downbursts are, presently, almost exclusively modeled, both experimentally and numerically, as transient impinging momentum jets (IJ), even though that model contains none of the physics of real events. As a result, there is no connection between the IJ-simulated downburst wind fields and the conditions of formation of the event. The cooling source (CS) model offers a significant improvement since it incorporates the negative buoyancy forcing and baroclinic vorticity generation that occurs in nature. The present work aims at using large-scale numerical simulation of downburst-producing thunderstorms to develop a simpler model that replicates some of the key physics whilst maintaining the relative simplicity of the IJ model. Using an example of such a simulated event it is found that the non-linear scaling of the velocity field, based on the peak potential temperature (and, hence, density) perturbation forcing immediately beneath the storm cloud, produces results for the radial location of the peak radial outflow wind speeds near the ground, the magnitude of that peak and the time at which the peak occurs that match well (typically within 5%) of those produced from a simple axi-symmetric constant-density dense source simulation. The evolution of the downdraft column within the simulated thunderstorm is significantly more complex than in any axi-symmetric model, with a sequence of downdraft winds that strengthen then weaken within a much longer period (>17 minutes) of consistently downwards winds over almost all heights up to at least 2,500 m.

• 한국추진공학회지
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• 제20권6호
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• pp.1-10
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• 2016

연소불안정 능동제어를 위해서는 음향 발생기나 2차 연료 분사를 통해 압력 섭동이나 열방출 섭동에 변화를 주어야 한다. 2차 연료 분사의 위치 및 시점을 결정하기 위해서는 연소불안정 시 발생하는 열방출 섭동의 분포를 알아야 한다. 본 연구에서는 탄화수소 연료, 유입 속도, 당량비, 음향가진 조건을 변화시키며 위상에 따른 열방출 섭동의 분포를 실험적으로 측정하였다. 와류 발생에 따른 열방출 섭동은 $Damk{\ddot{o}}hler$ 수에 의해 크게 영향을 받는 것을 알 수 있었다. $Damk{\ddot{o}}hler$ 수가 대략 4 - 5 보다 큰 경우는 와류의 leading edge에서 hot spot이 trailing edge에서 cold spot이 발생하였다. 이와는 반대로 $Damk{\ddot{o}}hler$ 수가 3 보다 작은 경우는 반대의 경향이 나타남을 확인할 수 있었다.

• 콘크리트학회지
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• 제6권4호
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• pp.141-152
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• 1994

산업부산물의 플라이애쉬와 실리카흄 및 국내 부존자원이 풍부한 규사분말, 생석회 및 발포용 알루미늄 분말과 취성개선을 위한 보강용 섬유를 사용한 경량 고강도의 시멘트복합체의 개발을 위하여 오토클래브 양생에 의한 열수 알카리분위기에서의 섬유 자체의 열화현상을 조사함과 아울러 배합요인별로 건재용 경량섬유보강 규산칼슘계 시멘트복합에를 제조하여 그 역학적 특성에 관한 연구를 수행하였다. 시험결과, 탄소섬유 및 내알카리성 유리섬유는 보강용 섬유로서 적합함이 확인되었으며, 경량 섬유보강 규산칼슘계 시멘트복합체의 압축, 인장, 휨강도는 플라이애쉬와 실리카흄 혼입율 및 섬유혼입율이 증가함을 따라 증가하는 경향을 나타내었고 또한 섬유혼입율 증가에 따라 현저히 휨인성이 증가하였으며, 탄소섬유보강의 경유가 유리섬유보강의 경우에 비하여 압축, 인장, 휨강도 및 휨인성은 다소 높은 경향을 나타내었다.