• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2006년도 추계 학술대회논문집
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• pp.110-119
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• 2006

An accurate and cost efficient method PSE is used for the stability analysis of 2D or 3D compressible boundary layers. A highly accurate finite difference PSE code has been developed at a general curvilinear coordinate system using an implicit marching procedure to deal with a broad range of transition predictions problems. Evolution of disturbances in compressible flat plate boundary layers are studied for free-stream Mach numbers ranging from 0 to 1.5. The effect of mean-flow nonparallelism is found to be weak on two dimensional waves and strong on three dimensional waves. The maximum amplification rate increases monotonically with Mach number. The present PSE solutions are compared with previous numerical investigations and experimental results and are found to be in good agreement.

• 한국항공우주학회지
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• 제39권9호
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• pp.805-815
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• 2011

비선형 포물형 안정성 방정식(Nonlinear Parabolized Stability Equations, NPSE)은 보다 전체적인 천이 과정 연구에 효과적으로 사용될 수 있다. NPSE는 천이 과정에서 비선형 구간의 안정성을 직접 수치 모사(Direct Numerical Simulation, DNS)에 비해 적은 계산 비용을 사용하여 효율적으로 해석 할 수 있다. 본 연구에서는 일반 좌표계에서의 NPSE를 구성하고, 수치 계산을 위한 코드를 개발하였다. 코드의 검증을 위해 비압축성 및 압축성 평판 경계층에서의 벤치마크 문제들을 해석하였다. 본 연구의 NSPE 해석 기법이 비선형 안정성 연구에 효율적이고 효과적인 방법임을 확인하였다.

• 대한기계학회논문집B
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• 제23권10호
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• pp.1229-1239
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• 1999

A computational study on unsteady compressible flows has been performed by adopting a low Reynolds number $k-{\omega}$ turbulence model in conjunction with dual time stepping scheme. An explicit four-stage Runge-Kutta scheme for the Navier-Stokes equations and an approximate factorization scheme for the $k-{\omega}$ turbulence model equations are used. Computational results obtained for blade surface pressure distributions in the process of rotor-stator interaction in a turbine stage are in good agreement with extant experimental data. The effects of the wake from the stator on the boundary-layer transition over the rotor blade surface are discussed by showing that high intensity turbulence of the stator wake induces an early transition.

• 한국항공우주학회지
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• 제45권3호
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• pp.212-216
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• 2017

두께비 18% 익형(KARI-11-180)에 대한 공력특성 분석이 전산해석기법을 활용해 수행되었다. 익형주위의 격자는 벽면에서 수직으로 투영하여 경계층 격자를 형성하였고 익형 후방에는 정밀한 후류 예측을 위해 조밀한 격자를 위치하였다. 원방경계까지의 거리는 익형 코드의 100배로 정하였고 익형시험결과와의 비교를 위해 자유류 속도, 익형 코드 및 Reynolds수를 풍동시험과 동일하게 정하였다. 또한 난류 모델은 천이지점 예측이 우수한 transition SST 및 DES 모델을 사용하였다. 3차원 전산해석은 세장비가 2와 5인 익형모델에 대해 수행되었는데 양력은 풍동시험결과보다 높은 값을 항력은 낮은 값을 예측하였다.

• International Journal of Aeronautical and Space Sciences
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• 제16권1호
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• pp.8-18
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• 2015

A numerical simulation for a nonslender BWB UCAV configuration with a rounded leading edge and span of 1.0 m was performed to analyze its aerodynamic characteristics. Numerical results were compared with experimental data obtained at a free stream velocity of 50 m/s and at angles of attack from -4 to $26^{\circ}$. The Reynolds number, based on the mean chord length, is $1.25{\times}106$. 3D multi-block hexahedral grids are used to guarantee good grid quality and to efficiently resolve the boundary layer. Menter's shear stress transport model and two transition models (${\gamma}-Re_{\theta}$ model and ${\gamma}$ model) were used to assess the effect of the laminar/turbulent transition on the flow characteristics. Aerodynamic coefficients, such as drag, lift, and the pitching moment, were compared with experimental data. Drag and lift coefficients of the UCAV were predicted well while the pitching moment coefficient was underpredicted at high angles of attack and influenced strongly by the selected turbulent models. After assessing the pressure distribution, skin friction lines and velocity field around UCAV configuration, it was found that the transition effect should be considered in the prediction of aerodynamic characteristics of vortical flow fields.

• International Journal of Naval Architecture and Ocean Engineering
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• 제13권1호
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• pp.236-245
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• 2021

In this study, direct numerical and large eddy simulations of transitional flows around studs were conducted to investigate the effectiveness of turbulence stimulators at very low speeds for the minimum propulsion power condition of four knots. For simplicity, the studs were assumed to be installed on a flat plate, while the wake was observed up to 0.23 m downstream behind the second stud. For applicability to a model ship, we also studied the flow characteristics behind the first and second studs installed on a curved plate, which was designed to describe the geometry of a bulbous bow. A laminar-to-turbulent transition was observed in the wake at Re_D ≥ 921 (U_∞≥0.290 m/s), and the wall shear stress at Re_D = 1162 (U_∞ = 0.366 m/s) in the second wake was similar to that of the fully developed turbulent boundary layer after a laminar-to-turbulent transition in the first wake. At Re_D = 581 (U_∞ = 0.183 m/s), no turbulence was stimulated in the wake behind the first and second studs on the flat plate, while a cluster of vortical structures was observed in the first wake over the curved plate. However, a cluster of vortical structures was revealed to be generated by the reattachment process of the separated shear layer, which was disturbed by the first stud rather than directly initiated by the first stud. It was quite different from a typical process of transition, which was observed at relatively high Re_D that the spanwise scope of the turbulent vortical structures expanded gradually as it went downstream.

• Advances in aircraft and spacecraft science
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• 제4권5호
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• pp.555-571
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• 2017

The description of transitional flows by means of RANS equations is sometimes based on non-local approaches which require the computation of some boundary layer properties. In this work a non-local Laminar Kinetic Energy model is used to predict transitional and separated flows. Usually the non-local term of this model is evaluated along the grid lines of a structured mesh. An alternative approach, which does not rely on grid lines, is introduced in the present work. This new approach allows the use of fully unstructured meshes. Furthermore, it reduces the grid-dependence of the predicted results. The approach is employed to study the transitional flows in the T106c turbine cascade and around a NACA0021 airfoil by means of a discontinuous Galerkin method. The local nature of the discontinuous Galerkin reconstruction is exploited to implement an adaptive algorithm which automatically refines the mesh in the most significant regions.

• 한국음향학회지
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• 제22권2호
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• pp.135-144
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• 2003

본 논문에서는 문-음성 합성기에서 사용되는 대용량 데이터 베이스의 구성을 목적으로 하는 음성 신호의 자동 분할기법을 기술하였다. 주된 내용은 은닉 마코프 모델에 기반을 둔 음소 분할과 여기서 얻어진 결과를 초기 음소 경계로 사용하여 이를 자동으로 수정하는 방법으로 구성되어 있다. 다층 퍼셉트론이 음성 경계의 검출기로 사용되었으며, 음소 분할의 성능을 증가시키기 위해, 음소의 천이 패턴에 따라 다층 퍼셉트론을 개별적으로 학습시키는 방법이 제안되었다. 음소 천이 패턴은 수작업에 의해 생성된 레이블 정보를 기준 음소 경계로 사용하여, 기준 음소 경계와 추정된 음소 경계간의 전체 오차를 최소화하는 관점에서 분할되도록 하였다. 단일 화자를 대상으로 하는 실험에서 제안된 기법을 통해 생성된 음소 경계는 기준 경계와 비교하여 95%의 음소가 20 msec 이내의 경계 오차를 갖는 것으로 나타났으며, 평균 자승 제곱근 오차면에서 수정 작업을 통해 25% 향상된 결과를 나타내었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.481-486
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• 2001

The hydrodynamic instability of the three-dimensional boundary layer on a rotating disk introduces a periodic modulation of the mean flow in the form of stationary cross flow vortices. Detailed numerical values of the growth rates, neutral curves and other characteristics have been calculated for the Type II-instabilities. Presented are the neutral stability results concerning the two instability modes by solving new linear stability equations reformulated not only by considering whole convective terms but by correcting some errors in the previous stability equations. The present stability results are agree with the previously known ones within reasonable limit. The spatial amplification contours have been calculated for the moving disturbance wave, whose azimuth angle is between $\varepsilon=-10^{\circ}$ and $-20^{\circ}$. The transition flow of the moving disturbance wave will be developed at $\varepsilon=-15^{\circ}$ and Re=352 corresponding at the growth rates n = 5.8 from the spatial amplification contours.

• Journal of Welding and Joining
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• 제4권1호
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• pp.40-46
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• 1986

This investigation studied the toughness variations in the narrow gap weldment with the notch location. Specimens with the notch at the center of the weld metal showed the lowest toughness. As the location of notchmoves to fusion line, the impact properties improve reaching a maximum at the fusion boundaries. This improvement in toughness can be explained by the microstructural feature showing in the narrow gap weldment. "one pass/layer" technique performed in narrow gap welding results in the increased proportion of refined structure as approaching to fusion boundary from weld center and thus leave 100% refined structure along the fusion boundary. HAZ also shows 100% refined structure with respect to base metal structure accompanied with the significant suppression of ductile-brittle transition temperature.mperature.