• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.11a
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• pp.522-525
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• 2009

DC-shift phenomenon can be observed in Hybrid rocket combustion. This phenomenon makes performance drop which is structure problem or reduce thrust. Understanding of DC-shift phenomenon, the condition of the hybrid rocket combustion stability can be found. In this paper, the condition of Negative DC-shift was found and made by changing oxidizer flow with pre-post chamber. The Negative C-shift phenomenon and characteristic were defined from the experimental study.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.7
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• pp.79-88
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• 2006

In the present paper, the swirl flow structure and flame characteristics of turbulent premixed combustion in a model gas turbine combustor are investigated using large eddy simulation(LES). A G-equation flamelet model is employed to simulate the unsteady flame behavior. When inlet swirl number is increased, the distinct flow structures, such as the shapes of corner recirculation and center toroidal recirculation zone, are observed and the flame length is shorted gradually. Also, the phenomena of flashback are identified at strong swirl intensity. In order to get the accurate description of unsteady flame behavior, the predictive ability of the acoustic wave in a combustor is primarily evaluated. It is found that the vortex generated near the edge of step plays an important role in the flame fluctuation. Finally it is examined systematically that the flame and heat release fluctuation are coupled strongly to the vortex shedding generated by swirl flow and acoustic wave propagation from the analysis of flame-vortex interaction.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1992.10a
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• pp.113-116
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• 1992

소음의 발생 원인은 공기역학적 측면과 구조적 측면으로 나누어지는데, 실제 로는 유동장에서 발생되는 음원과 구조물에서 발생되는 진동과의 상호 간섭 에 의해 보다 복잡한 형태로 발생된다. 음장 문제를 두가지 범주로 구분하면 첫째는 음원과 구조물과의 상호교란에 의한 산란문제(Scattering)와 둘째로 구조물의 자체 진동에 의한 음의 전파현상과 구조물내부에 회전체와 같은 음원이 존재하는 경우에 음의 전파를 관측하는 방사문제(Radiation)가 있다. 실제로 산업용 터빈이나 비행기 엔진 흡입구에서 발생되는 소음, 또는 자동 차의 배기구를 통해 발생되는 소음 그리고 엔진의 진동에 의한 구조적 소음, 기타 가전제품의 회전체(Fan & Motor)에 의한 소음은 방사(Radiation)의 문 제로서 중요 관심 과제이다 수치적 기법으로 근래에 많이 사용하는 방법으 로 BEM(경계요소법), FEM(유한요소법), FDM(유한차분법)이 있는데 본 연 구에서는 유한요소법을 이용하기로 한다. 지금까지는 주로 BEM을 통해서 Far-Field의 음향장을 해석하였지만 복잡한 형상을 갖는 구조물내부에서의 음향장 변화나 구조물 내부에 음원이 존재하는 경우 또는 구조물 자체가 갖 는 물리적 특성치 변화 즉 물체표면에서의 부분 진동문제의 음향장 해석에 있어서 가장 잘 대체해 나갈 수 있는 방법이 유한 요소법이라고 여겨진다. 본 연구에서는 2차원 또는 기하학적으로 축대칭인 3차원 Duct내부에 음원이 존재하는 경우 음원전파에 따른 Near-field와 far-field에서 음의 방향성을 예측하기 위해 먼저 기본적인 유한요소법에 의한 Robin 경계조건을 사용하 여 계산된 결과와 Infinite Element를 도입하여 계산할 결과를 비교하여, Infinite Element가 보다 효율적이며 타당한 결과를 얻음을 확인해 보기로 한다.다 복합적인 측면에서 치료에 임하여야 할 것으로 사료된다. with such configuration.trap with 2.88[eV] deep of injected space charge from the chathode in the crystaline regions. The origin of ${\alpha}$$_2$ peak was regarded as the detrapping process of ions trapped with 0.9[eV] deep originated from impurity-ion remained in the specimen during production process of the material, in the crystalline regions. The origin of ${\beta}$ peak was concluded to be due to the depolarization process of "C=0"dipole with the activation energy of 0.75[eV] in the amorphous regions. The origin of ${\gamma}$ peak was responsible to the process combined with the depolarization of "CH$_3$", chain segment, with the activation energy of carriers from the shallow trap with 0.4[eV], in he amorphous regions.의 증발산율은 우기의 기상자료를 이용하여 구한 결과 0.05 - 0.10 mm/hr 의 범위로서 이로 인한 강우손실량은 큰 의미가 없음을 알았다.재발이 나타난 3례의 환자

• Journal of the Korean Society for Railway
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• v.18 no.6
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• pp.522-532
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• 2015

In the present study, a tunnel type soundproof wall with partial opening is proposed to reduce the environmental noise caused by railway vehicles traveling on bridges, which affects residents of high-rise apartment buildings; the study also attempts to minimize load due to wind and the weight of the wall. Applying the principles of computational fluid dynamics and structural mechanics, and the ray tracing method, a reduction in noise as well as of the overall weight of the soundproof walls is estimated. Analysis results show that the proposed soundproof wall with a partial opening weighs less, while reducing the wind loading by up to 30%. To prevent direct propagation of sound through openings in the wall, an acoustic louver, which is a type of silencer, could be considered for the opening. In order to achieve a similar noise effect with existing insulation material, the fluid flow and the insulation effect of the acoustic louver are analyzed. As the considered opening is in the range of 30~40% of the total length of the soundproof wall, the noise effect and wind load are reduced by 10dB and 25% respectively. Consequently, opening some part of tunnel type soundproof walls and installing louvers on the wall openings can have the effects of weight-reduction and reduced wind load. If a partial opening is applied with proper sound material application, a gain of an additional 5~10dB of noise reduction can be achieved.

• 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.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.5
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• pp.619-627
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• 2018

As ships become faster, larger and are required to meet higher standards, the importance of flow noise is highlighted. However, unlike in the aeroacoustics field for airplanes and trains (where flow noise is considered in design), acoustics are not considered in the marine field. In this study, analysis procedures for hull-induced flow noise are established to investigate the flow noise characteristics of a wave-piercing hull form that can negate the effect of wave-breaking. The principal mechanisms behind hull-induced flow noise are fluid-structure interactions between complex flows underneath the turbulent boundary layer and the hull. Noise induced by the turbulent boundary layer was calculated using wall pressure fluctuation and energy flow analysis methods. The results obtained show that noise characteristics can be distinguished by frequency range and hull region. Also, the low-frequency range is affected by hull forms such that it is correlated with ship speed.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.12
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• pp.1098-1103
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• 2011

The structural integrity of APR1400 reactor vessel internals has been being assessed referring the US Nuclear Regulatory Commission regulatory guide 1.20, comprehensive vibration assessment program. The program is composed of a vibration and stress analysis, a vibration and stress measurement, and an inspection. This paper covers the vibration and stress analysis on the reactor vessel internals by the pulsation of reactor coolant pump. 3-dimensional models to calculate the hydraulic loads and structural responses were built and the pressure distributions and the structural responses were predicted using ANSYS. This paper presents that APR1400 reactor vessel internals have enough structural integrity against the pulsation of reactor coolant pump as the peak stress of the reactor vessel internals is much lower than the acceptance limit.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.10a
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• pp.221-226
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• 2011

The structural integrity of APR1400 reactor vessel internals has been being assessed referring the US Nuclear Regulatory Commission regulatory guide 1.20 comprehensive vibration assessment program. The program is composed of a vibration and stress analysis, a limited vibration measurement, and an inspection. This paper covers the vibration and stress analysis on the reactor vessel internals due to the pulsation of reactor coolant pump. 3-dimensional models to calculate the hydraulic loads and structural responses were built and the pressure distributions and the structural responses were predicted using ANSYS. The peak stress of the reactor vessel internals is much lower than the acceptance limit.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.9
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• pp.838-844
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• 2015

This paper includes summarization and analysis of previous research results on acoustic attenuation due to particles and flow turning in rocket motors among various damping parameters. Particle damping is the most effective mechanism in suppressing high-frequency combustion instabilities occurring in rocket combustion chambers, which is dependent on the size and the mass fraction of particles. Relatively weak attenuation by flow turning compared to particle damping depends on the geometry of propellant and a combustion chamber. Pumping driving effects need to be taken into account when realizing vorticity generation on the propellant surface. However, its driving effects become cancelled out by flow turning loss when the propellant geometry is cylindrical.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.5
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• pp.603-609
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• 1999

The effect of an external acoustic excitation on the wake structure behind a circular cylinder was experimentally investigated. The sound wave was excited in the frequency range of the shear layer instability and two sound pressure levels of 114 and 120dB were used in this study. As a result, the acoustic excitation modified the wake structure by increasing the velocity fluctuation energy without changing the vortex shedding frequency. The acoustic excitation enhanced the vortex shedding process and promoted the shear layer instability. Consequently, the acoustic excitation reduced the length of the vortex formation region and decreased the base pressure. In addition, the vortex strength of vortices was increased and the width of the wake was spread out due to the acoustic excitation. When the excitation frequency was identical to the shear layer instability frequency, the effect of the external flow control on the cylinder wake was maximized. In addition, with increasing the sound pressure level, the effect of the external acoustic excitation on the wake structure increased.