• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.5
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• pp.545-550
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• 2014

This study provided the analytical finite element method estimating the friction-induced noise on the complex beam structure. The frictional contact model was theoretically constructed and applied to the analytical finite element squeal model. The numerical results showed that the beam structure was excited by the mode-coupling instability of the specific system modes. Also, the direction of friction was shown to influence on the dynamic instability of the modes. Besides, the unstable modal frequencies estimated from the numerical calculation were validated by the experiment of the actual beam structure.

• Journal of the Korean Society of Propulsion Engineers
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• v.2 no.1
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• pp.41-49
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• 1998

A nonlinear mathematical model of longitudinal combustion instability appropriate for ramjets and augmenters was developed based on modal analysis. The model was limited to a two-mode formulation. The associated differential equations were solved both analytically and numerically. The two-mode nonlinear model is capable of predicting the bootstrapping effect which characterizes nonlinear velocity-sensitive combustion response. Also, parametric studies were performed.

• Korean Journal of Optics and Photonics
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• v.29 no.4
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• pp.159-165
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• 2018

In this paper, we report on the high power amplification of a narrow-linewidth Yb-doped polarization-maintained (PM) fiber laser in a 3-stage, all-fiber master oscillator power amplifier (MOPA) system. The linearly polarized single-mode output power was 400 W with an 85% slope efficiency, with a linewidth of 2.5 GHz (full width at half maximum). Furthermore, mitigation of mode instability (MI) has been demonstrated by tightly coiling the gain fiber to a diameter of 11 cm. In addition, methods for higher power scaling are discussed.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2012.05a
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• pp.86-90
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• 2012

In this paper, the combustion instabilities which may occur in the hybrid rocket were studied. The rocket combustor where the vortexes can be generated was designed, and the experiments were performed. The investigations about characteristics on the presence of the diaphragm, the length of the fuel, the diameter of the fuel port, the diameter of the diaphragm, the diameter of the nozzle throat, and the variation of the Ox massflow rate were conducted. The main resonant frequency of the combustion pressure is regarded by the Vortex shedding mode, and it is considered that the other resonant frequency of the pressure fluctuation is hybrid low frequency, or helmholtz mode.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.809-813
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• 2010

For the purpose of developing a modeling technique for reduced-order dynamics of thermoacoustic instability, we constructed an electrically heated Rijke tube and measured the pressure oscillation inside the tube. Analysing the measured pressure data showed that the instability generated three major oscillation modes, among which the first mode frequency quite well matches the result from a rough acoustic analysis. As a continuing research, a data-based modeling technique for the thermoacoustic instability will be developed in the near future.

• Journal of the Korean Society of Propulsion Engineers
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• v.25 no.2
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• pp.66-78
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• 2021

In this study, the effect of Symmetry Breaking was compared according to the equivalent ratio condition and the number of nozzles where combustion instability occurs in an annular combustor. Generally, due to the relatively short combustor length, a longitudinal instability was less likely to occur in the annular combustor, but the combustion instability sometimes happens when ducts such as transition piece in gas turbine power station are present. In this case, due to the duct, only the longitudinal instability mode is observed. The characteristics of Symmetry Breaking were investigated according to the number of five lean nozzles and the equivalent ratio combination, and as the equivalent ratio decreased, the effect of Symmetry Breaking rapidly occurred, and the instability was dramatically disappeared and the amplitude was greatly reduced. In addition, it was confirmed that as the number of lean nozzles increased, a phenomenon such as a reduction in the equivalent ratio appeared.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.714-717
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• 2017

Among the factors considered in the design stage of a space launch vehicle using liquid propellant, research has been focused out on the pogo phenomenon, longitudinal dynamic instability. The pogo phenomenon refers to the instability that the longitudinal vibration of the launch vehicle structure causes a change in the pressure and flow rate of the fluids in propulsion system, and this change re-excites the fuselage structure. This mechanism constitutes a closed system to gradually increase the vibration of the launch vehicle. This paper specifically focuses on the dynamic analysis of pressure and flow changes in the propulsion system. Based on the example study of the space shuttle, the acoustic modal analysis of the propulsion system is performed to predict the modes of the supply line causing instability of the fuselage.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.279-282
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• 2006

Experiments in low strain rate methane-air counterflow diffusion flames diluted with $CO_2$ have been conducted to investigate the flame extinction behavior and edge flame oscillation The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. Onset conditions of the edge flame oscillation and the relevant modes are also provided with global strain rate. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Edge flame oscillations are categorized into three: a growing-, a decaying-, and a harmonic-oscillation mode.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.4
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• pp.425-431
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• 2011

Thermal energy generated because of the friction between the disc and pad is transferred to both sides and causes thermal expansion of the material, which affects the contact pressure distribution. This phenomenon, which is called thermoelastic instability (TEI), is affected by the natural mode of a disc. TEI results in the formation of a hot spot and causes hot judder vibrations. In this study, three-dimensional analysis of the hot judder of a ventilated disc for automotives was performed by using the commercial finite element analysis program, SAMCEF. The intermediate processor based on a staggered approach was used to exchange the result data of the mechanical and thermal model. The hot spot was formed on the surface of the disc, and the number of hot spots was compared with the natural mode of the disc.

• Proceedings of the Optical Society of Korea Conference
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• 1996.09a
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• pp.13-13
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• 1996