• Journal of the Korean Society of Propulsion Engineers
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• v.7 no.1
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• pp.18-27
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• 2003

To perform combined tests with propellant feeding system and engine, which were developed for KSR-III launcher, vertical test stand was organized and a series of hot-fire combustion tests were carried out with engines of several injector faceplate types. In hot-fire tests in vertical installation, combustion instabilities occurred right after ignition with an engine without baffle, and such combustion instabilities did not occur at ignition add during mainstage operation for an engine with STS or composite baffle. 1.regular and temporary pressure pulsations(popping) were detected during steady operation with a baffle engine, however a development to combustion instabilities with resonant mode was highly suppressed by baffle. With a series of tests, it was confirmed that the last developed engine, which has composite baffle, was operated successfully in KSR-III flight propulsion system.

• Journal of the Korean Society of Propulsion Engineers
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• v.21 no.5
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• pp.1-9
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• 2017

The cryogenic nitrogen spray of a swirl injector has been numerically investigated using three dimensional LES turbulence model to analyze the dynamic characteristics under supercritical condition. To predict the precise nitrogen properties under supercritical condition, SRK equation of state, Chung's method for viscosity and thermal conductivity and Takahashi's correlation based on Fuller's theory for diffusion coefficient are implemented. The complex flow structures due to interaction between flow field and acoustic field are observed inside and outside the injector under supercritical condition. FFT, POD, and DMD techniques are employed to understand the coherent structures. By implementing the FFT, the dominant frequencies are identified inside and outside the injector. The coherent flow structures related to the dominant frequencies are visualized using the POD and DMD techniques. In addition, the DMD provides the damping coefficient which is related with the instability prediction.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1990.10a
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• pp.173-178
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• 1990

로터-베어링축계는 증기및 가스터빈, 터보 발전기, 압축기등 거의 모든 산업 기계류에서 동력 전달의 기본 도구로써 사용되고 있다. 즉 회전에 의한 동력 의전달은 비교적 간단히 대용량의 동력을 효율적으로 전달할 수 있다. 이에 따라 회전기계류에 대한 연구는 산업 혁명 이후 꾸준히 발전되어 온바, 특히 근래에 들어와 산업기계류의 경쟁이 치열하여짐에 따라 산업기계류의 고정 밀화, 고속화, 고신뢰화 요구가 증대하고 있는 현실을 비추어 볼때, 산업 기 계류의 근간을 이루고 있는 로터-베어링 축계의 안정성을 포함한 진동에 관 한 문제는 회전기계류 설계의 주요 기술로써 연구.개발의 필요성이 매우 높 다 하겠다. 회전축계 진동 관련 연구는 두 분야로 대별될 수 있는데 언밸런 스(Unbalance)에 의한 Synchronous진동과 여러가지 원인에 의해 계의 불안 정성을 유발시키는 Nonsynchronous진동으로 나눌 수 있다. 본 연구에서는 이들 연구의 기본이 되는 회전축-베어링계 동특성 해석 프로그램을 개발하 였다. 여러가지 방법이 있으나 여기서는 Holzer가 비틀림 진동에 적용하고, Mykiestad(2)와 Prohl(3)에 의하여 회전축의 횡 진동에 적용된 이후 Lund(4) 등에 의하여 베어링의 영향등이 첨가된 전달 매트릭스 (Transfer Matrix) 방 법을 이용하여 임계속도(Critical Speed), 모우드 형태(Mode shapes)를 예측 하고 불안정 판정(Instability Criteria)등을 할 수 있는 프로그램을 개발하였 다. 특히 Murphy(1)의 다항식 방법(Polynomial Method)에 기본을 두어 기존 의 전달 매트릭스가 가지고 있던 반복, 수렴 시간 문제와 빠뜨리는 임계속도 예측에 대한 개선을 이루었으며 기존 논문과 실험 결과와의 비교 검토를 통 하여 개발된 프로그램의 신뢰성을 검토하였다. 특히, 각종 회전 기계의 소형 화, 경량화 추세에 따라 지반이나 케이싱이 경량이거나 유연하여 회전축과 동적으로 연성된 경우 회전축-베어링-지반으로 이루어진 2중구조의 회전축 계 동특성을 해석할 수 있는 프로그램을 개발하므로서 회전 기계류의 진동 전반에 걸친 문제점에 대한 그 원인과 현상을 명확히 분석하여 국내의 전기 계류의 보다 신뢰성있는 설계 및 제작자료를 확보하는데 기여할 수 있게 하 였다.

• Wind and Structures
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• v.25 no.2
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• pp.131-156
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• 2017

This paper deals with the dynamic stability of embedded functionally graded (FG)-carbon nanotubes (CNTs)-reinforced micro cylindrical shells. The structure is subjected to harmonic non-uniform temperature distribution and 2D magnetic field. The CNT reinforcement is either uniformly distributed or FG along the thickness direction where the effective properties of nano-composite structure are estimated through Mixture low. The viscoelastic properties of structure are captured based on the Kelvin-Voigt theory. The surrounding viscoelastic medium is considered nonhomogeneous with the spring, orthotropic shear and damper constants. The material properties of cylindrical shell and the viscoelastic medium constants are assumed temperature-dependent. The first order shear deformation theory (FSDT) or Mindlin theory in conjunction with Hamilton's principle is utilized for deriving the motion equations where the size effects are considered based on Eringen's nonlocal theory. Based on differential quadrature (DQ) and Bolotin methods, the dynamic instability region (DIR) of structure is obtained for different boundary conditions. The effects of different parameters such as volume percent and distribution type of CNTs, mode number, viscoelastic medium type, temperature, boundary conditions, magnetic field, nonlocal parameter and structural damping constant are shown on the DIR of system. Numerical results indicate that the FGX distribution of CNTs is better than other considered cases. In addition, considering structural damping of system reduces the resonance frequency.

• Advances in aircraft and spacecraft science
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• v.7 no.6
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• pp.553-570
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• 2020

To address the problems caused by the strong coupling of an airbreathing hypersonic vehicle's airframe and propulsion to the integrated control system design, an integrated airframe-propulsion model is established, and the coupling characteristics between the aircraft and engine are analyzed. First, the airframe-propulsion integration model is established based on the typical nonlinear longitudinal dynamical model of an air-breathing hypersonic vehicle and the one-dimensional dual-mode scramjet model. Thrust, moment, angle of attack, altitude, and velocity are used as transfer variables between the aircraft model and the engine model. The one-dimensional scramjet model can accurately reflect the working state of the engine and provide data to support the coupling analysis. Second, owing to the static instability of the aircraft model, the linear quadratic regulator (LQR) controller of the aircraft is designed to ensure attitude stability and height tracking. Finally, the coupling relationship between the aircraft and the engine is revealed through simulation examples. The interaction between vehicle attitude and engine working condition is analyzed, and the influence of vehicle attitude on engine safety is considered. When the engine is in a critical working state, the attitude change of the aircraft will not affect the engine safety without considering coupling, whereas when coupling is considered, the attitude change of the aircraft may cause the engine unstart, which demonstrates the significance of considering coupling characteristics.

• Transactions of the Korean hydrogen and new energy society
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• v.23 no.4
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• pp.373-381
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• 2012

To determine the performance characteristics of motorcycle engine using biogas for practical use, the intake system of a 110 cc motorcycle engine is properly modified to operate with biogas as a fuel. Biogas is a potentially renewable fuel for replacing gasoline in future, but it has high percentage of $CO_2$ that could lead to slow the burning rate of biogas-air mixture and cause instability in combustion. Thus, the performance characteristics of biogas-fueled motorcycle engines could be different from those of gasoline motorcycle engines. In this paper, the important parameters of performance characteristics (such as: power output, thermal efficiency, fuel consumption, exhaust emission,${\cdots}$) of biogas-fueled motorcycle engine are studied and estimated with change of engine speed and load. The obtained results when operating with biogas are used to compare with that of gasoline fuel under the same operating conditions. Engine speed in the experimental is changed from 1500 rpm (idle-mode) up to 3500 rpm by a step of 500 rpm. Engine load is changed from zero to maximum load with the help of an exciting voltage device from generator-type dynamometer. The experimental results show that the tested engine operated with richer biogas-air mixture than that of gasoline-air mixture under the same test conditions. Biogas-fueled engine gives a higher fuel consumption and lower thermal efficiency under the same power output. Brake thermal efficiency of biogas engine is found to be about 3% lower than gasoline-fueled motorcycle engine for whole range of speed. Exhaust emission of biogas-fueled motorcycle engine (such as: CO, HC) is found to be lower than the limitation level of the emission standards of Vietnam for motorcycle engines (CO <4.5% HC <1200 ppm).

• International Area Studies Review
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• v.14 no.3
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• pp.290-311
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• 2010

This study aimed at exploring the primary motivations in the behavior of MNCs' setting up R&D Labs in China, four main subjective motivations were founded. R&D Labs consisted of the following four types: local market support Labs, local technology support Labs, global technology center, and internationally interdependent Labs.he choices of the R&D Lab type features are different because of the various R&D investment motivations. Provided that MNCs' investment motivations in China are in order to obtain Chinese markets and customers, local market support Labs are supposed to be chosen so as to support Chinese markets; provided that investment motivations are in order to gain talents and science and technology resources, global technology centers are supposed to be used to achieve the highest synergy result; provided that investment motivations win an absolute competitive advantage in China, local market support Labs are extremely necessary in preoccupying markets; last but not the least, provided that investment motivations are environmental owing to their instability, local technology support Labs are a good choice for their relatively low R&D expenses.

• Geomechanics and Engineering
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• v.19 no.1
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• pp.49-60
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• 2019

Crack instability propagation during coal and rock mass failure is the main reason for electromagnetic radiation (EMR) generation. However, original cracks on coal and rock mass are hard to study, making it complex to reveal EMR laws and mechanisms. In this paper, we prefabricated cracks of different inclinations in coal and rock samples as the analogues of the native cracks, carried out uniaxial compression experiments using these coal and rock samples, explored, the effects of the prefabricated cracks on EMR laws, and verified these laws by measuring the surface potential signals. The results show that prefabricated cracks are the main factor leading to the failure of coal and rock samples. When the inclination between the prefabricated crack and axial stress is smaller, the wing cracks occur first from the two tips of the prefabricated crack and expand to shear cracks or coplanar secondary cracks whose advance directions are coplanar or nearly coplanar with the prefabricated crack's direction. The sample failure is mainly due to the composited tensile and shear destructions of the wing cracks. When the inclination becomes bigger, the wing cracks appear at the early stage, extend to the direction of the maximum principal stress, and eventually run through both ends of the sample, resulting in the sample's tensile failure. The effect of prefabricated cracks of different inclinations on electromagnetic (EM) signals is different. For samples with prefabricated cracks of smaller inclination, EMR is mainly generated due to the variable motion of free charges generated due to crushing, friction, and slippage between the crack walls. For samples with larger inclination, EMR is generated due to friction and slippage in between the crack walls as well as the charge separation caused by tensile extension at the cracks' tips before sample failure. These conclusions are further verified by the surface potential distribution during the loading process.

• Composites Research
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• v.20 no.3
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• pp.43-49
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• 2007

Dynamic instability of rotating disks is the most significant factor to limit its rotating speed. Application of composite materials to rotating disks may enhance the dynamic stability leading to a possible design of rotating disks with lightweight and high speed. Whereas much work has been done on the effect of transverse shear and rotary inertia, called Timoshenko effect, on the dynamic behavior of plates, there is little work on the correlation between the effect and the rotation of disk, especially nothing in case of composite disks. The dynamic equations of a rotating composite disk are formulated with the Timoshenko effect and the vibrational analysis is performed by using a commercial package MSC/NASTRAN. According to the results, the Timoshenko effect goes seesaw in some modes, unlike the well-known fact that the effect decreases as the rotating speed increases. And it can be concluded, based only on the present results, that decrement of the Timoshenko effect by disk rotation grows larger as the thickness ratio decreases, the diameter ratio increases, the modulus ratio increases, and the mode number increases.

• Steel and Composite Structures
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• v.48 no.1
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• pp.27-41
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• 2023

Under the long-term effect of corrosive environment, many cold-formed steel (CFS) structures have serious corrosion problems. Corrosion leads to the change of surface morphology and the loss of section thickness, which results in the change of instability mode and failure mechanism of CFS structure. This paper mainly investigates the elastic local buckling behavior of corroded CFS columns. The surface morphology scanning test was carried out for eight CFS columns accelerated corrosion by the outdoor periodic spray test. The thin shell finite element (FE) eigen-buckling analysis was also carried out to reveal the influence of corrosion surface characteristics, corrosion depth, corrosion location and corrosion area on the elastic local buckling behaviour of the plates with four simply supported edges. The accuracy of the proposed formulas for calculating the elastic local buckling stress of the corroded plates and columns was assessed through extensive parameter studies. The results indicated that for the plates considering corrosion surface characteristics, the maximum deformation area of local buckling was located at the plates with the minimum average section area. For the plates with localized corrosion, the main buckling shape of the plates changed from one half-wave to two half-wave with the increase in corrosion area length. The elastic local buckling stress decreased gradually with the increase in corrosion area width and length. In addition, the elastic local buckling stress decreased slowly when corrosion area thickness was relatively large, and then tends to accelerate with the reduction in corrosion area thickness. The distance from the corrosion area to the transverse and longitudinal centerline of the plate had little effect on the elastic local buckling stress. Finally, the calculation formula of the elastic local buckling stress of the corroded plates and CFS columns was proposed.