• 한국항공우주학회지
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• 제33권7호
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• pp.18-25
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• 2005

본 연구에서는 뒷전, 전단류와 초기교란의 상호작용에 의한 불안정파의 생성 기제의 분석과 뒷전 산란현상을 고차의 전산공력음향학을 이용하여 모사하였다. 수치적 알고리즘은 Hybrid 기법에 기초하였으며, Simple Linearized Euler Equation과 Full Linearized Euler Equation의 결과를 비교를 통해 정상류 구배항이 불안정파의 생성에 중요한 역할을 함을 볼 수 있었다. 또한 Full Navier-Stokes Equation을 이용한 결과와 비교함으로써, Full Linearized Euler Equation은 뒷전의 초기 근접장에서 불안정파를 해석하는데 있어서 Full Navier-Stokes Equation 보다 효율적임을 알 수 있다.

• 한국전산유체공학회지
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• 제17권4호
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• pp.103-109
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• 2012

Sound scattering in 3D complex geometry is difficult to model with body-fitted grid. Thus Brinkman Penalization method is used to compute sound scattering in 3D complex geometry. Sound propagation of monitor/TV is studied. The sound field for monitor/TV is simulated by applying Brinkman Penalization method to Linearized Euler Equation. Solid Structure and ambient air are represented as penalty terms in Linearized Euler Equation.

• 한국추진공학회지
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• 제23권6호
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• pp.1-10
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• 2019

노즐 내 파동은 노즐목을 빠져나가는 유동의 공력음향학적인 효과로 인해 연소불안정을 감쇠시키는 주요 요소 중 하나로 알려져 있다. 이와 같은 효과는 노즐 어드미턴스라는 지표를 통해 정량적으로 평가가 가능하다. 본 연구에서는 현재까지 로켓 연소불안정 억제에 가장 효과적인 노즐감쇠(nozzle damping)와 연계된 노즐 어드미턴스를 구하는 여러 기법을 소개한다. 이중, 가장 널리 알려진 1차원 선형 오일러 방정식을 도입하여 노즐의 주 설계 변수에 따른 노즐 어드미턴스의 경향을 분석하였다. 분석 결과, 노즐 수축부 길이가 짧아질수록 축방향 노즐 어드미턴스의 값이 낮게 나타나는 주파수 영역대가 확장되므로 짧은 노즐일수록 주파수 의존성을 줄인다는 기존 이론을 검증하였다. 또한, 짧은 노즐 이론을 통한 어드미턴스 예측은 1차 접선방향 압력 섭동에는 적합하지 않음을 알 수 있었다.

• 한국진공학회지
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• 제12권S1호
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• pp.33-35
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• 2003

The electrooptic properties of the reflected light in a reflective mode, $45^{\circ}C$twisted nematic liquid crystal (TNLC) cell were investigated in the voltage regions near and away from the Freedericksz transition threshold. The measured reflectivity away from the threshold voltage ($V_th$) could not be described by the model which assurnes a constant tilt angle as well as a linearized distribution of twist angle across the cell, although the data are well fitted near $V_th$. We found that in the voltage region away from $V_th$, the model considering the distributions of the tilt angle and the twist angle should be applied for the calculation of the reflectivity. The director-axis distributions were obtained from the numerical integration of the Euler-Lagrange equation.

• International Journal of Aeronautical and Space Sciences
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• 제18권2호
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• pp.197-205
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• 2017

We investigate the slat noise generation mechanism by using large-eddy simulation (LES) and simple source modeling based on linearized Euler equations. An incompressible LES of an MD 30P30N three-element airfoil in the high-lift configuration is conducted at $Re_c=1.7{\times}10^6$. Using the total derivative of the hydrodynamic pressure (DP/Dt) acquired from the incompressible LES, representative noise sources in the slat cove region are characterized in terms of simple sources such as frequency-specific monopoles and dipoles. Acoustic radiation around the 30P30N multi-element airfoil is effectively computed using the Brinkman penalization method incorporated with the linearized Euler equation. The directivity pattern of $p^{\prime}_{rms}$ at $r=20c_{slat}$ in the multiple sources is closely compared to that obtained by the application of the LES/Ffowcs-Williams and Hawking's methods to the entire flow field. The power spectrum of p' at ${\theta}=290^{\circ}$ is in good agreement with the data reported in BANC-III, especially the broadband part of the spectrum with a decaying slope ${\propto}f^{-3}$.

• Coupled systems mechanics
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• 제13권3호
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• pp.247-275
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• 2024

The paper studies the influence of the fluid flow velocity and flow direction in the initial state on the dispersion of the axisymmetric waves propagating in the inhomogeneously pre-stressed hollow cylinder containing this fluid. The corresponding eigenvalue problem is formulated within the scope of the three-dimensional linearized theory of elastic waves in bodies with initial stresses, and with linearized Euler equations for the inviscid compressible fluid. The discrete-analytical solution method is employed, and analytical expressions of the sought values are derived from the solution to the corresponding field equations by employing the discrete-analytical method. The dispersion equation is obtained using these expressions and boundary and related compatibility conditions. Numerical results related to the action of the fluid flow velocity and flow direction on the influence of the inhomogeneous initial stresses on the dispersion curves in the zeroth and first modes are presented and discussed. As a result of the analyses of the numerical results, it is established how the fluid flow velocity and flow direction act on the magnitude of the influence of the initial inhomogeneous stresses on the wave propagation velocity in the cylinder containing the fluid.

• Coupled systems mechanics
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• 제12권1호
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• pp.41-68
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• 2023

The present work is concerned with the study of the influence of inhomogeneous initial stresses in a hollow cylinder containing a compressible inviscid fluid on the propagation of axisymmetric longitudinal waves propagating in this cylinder. The study is carried out using the so-called three-dimensional linearized theory of elastic waves in bodies with initial stresses to describe the motion of the cylinder and using the linearized Euler equations to describe the flow of the compressible inviscid fluid. It is assumed that the inhomogeneous initial stresses in the cylinder are caused by the internal pressure of the fluid. To solve the corresponding eigenvalue problem, the discrete-analytic solution method is applied and the corresponding dispersion equation is obtained, which is solved numerically, after which the corresponding dispersion curves are constructed and analyzed. To obtain these dispersion curves, parameters characterizing the magnitude of the internal pressure, the ratio of the sound velocities in the cylinder material and in the fluid, and the ratio of the material densities of the fluid and the cylinder are introduced. Based on these parameters, the influence of the inhomogeneous initial stresses in the cylinder on the dispersion of the above-mentioned waves in the considered hydro-elastic system is investigated. Moreover, based on these results, appropriate conclusions about this influence are drawn. In particular, it is found that the character of the influence depends on the wavelength. Accordingly, the inhomogeneous initial stresses before (after) a certain value of the wavelength lead to a decrease (increase) of the wave propagation velocity in the zeroth and first modes.

• 한국생산제조학회지
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• 제26권1호
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• pp.144-149
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• 2017

There have been lots of interest on service and entertainment robots. To ensure that robots work in harmony with humans, their stability and compactness are some of the key issues. Obviously, robots with fewer wheels occupy a smaller floor area compared to those with more wheels. In addition, robots with fewer wheels, whose posture stabilities are maintained by feedback control, are stable even under larger accelerations and/or higher locations of the center of mass. To facilitate controller design, it is assumed that both pitch and roll dynamics are decoupled. The dynamic equations of motion for the proposed robot are derived from the Euler-Lagrange equation. To obtain the optimal balancing control law, linear quadratic regulator control methods are applied to the linearized dynamic equations. Simulation and experimental results verify the effectiveness and performance of the proposed balancing control algorithm for a single-wheeled mobile robot.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2017년도 제48회 춘계학술대회논문집
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• pp.724-729
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• 2017

노즐을 빠져나가는 유동과 연소실 내 음향 섭동의 상호작용은 연소 불안정을 억제하는 핵심 요소로 작용하며 노즐감쇠는 노즐 어드미턴스를 이용하여 정량적으로 평가가 가능하다. 본 연구에서는 노즐의 음향 감쇠 성능을 측정할 수 있는 변형된 임피던스 튜브 실험을 수행하여 수치적 해석기법인 Crocco의 이론을 적용한 1차원 선형화 오일러 방정식으로 도출된 노즐 어드미턴스 값과 비교하였다. 그 결과, 노즐 어드미턴스의 증감하는 부분의 정성적인 경향이 유사함을 확인 가능하였고 그에 따른 노즐감쇠가 효율적인 주파수대를 예측할 수 있었다.