• 대한기계학회논문집B
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• 제25권3호
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• pp.305-314
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• 2001

A modified model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%∼6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

• 대기
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• 제26권1호
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• pp.203-214
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• 2016

This study explores the 6-month lead prediction skill of stratospheric temperature and circulations in the Global Seasonal forecasting model version 5 (GloSea5) hindcast experiment over the period of 1996~2009. Both the tropical and extratropical circulations are considered by analyzing the Quasi-Biennial Oscillation (QBO) and Northern Hemisphere Polar Vortex (NHPV). Their prediction skills are quantitatively evaluated by computing the Anomaly Correlation Coefficient (ACC) and Mean Squared Skill Score (MSSS), and compared with those of El Nino-Southern Oscillation (ENSO) and Arctic Oscillation (AO). Stratospheric temperature is generally better predicted than tropospheric temperature. Such improved prediction skill, however, rapidly disappears in a month, and a reliable prediction skill is observed only in the tropics, indicating a higher prediction skill in the tropics than in the extratropics. Consistent with this finding, QBO is well predicted more than 6 months in advance. Its prediction skill is significant in all seasons although a relatively low prediction skill appears in the spring when QBO phase transition often takes place. This seasonality is qualitatively similar to the spring barrier of ENSO prediction skill. In contrast, NHPV exhibits no prediction skill beyond one month as in AO prediction skill. In terms of MSSS, both QBO and NHPV are better predicted than their counterparts in the troposphere, i.e., ENSO and AO, indicating that the GloSea5 has a higher prediction skill in the stratosphere than in the troposphere.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.354-357
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• 2008

Drag reduction is one of main concerns for commercial aircraft companies than ever because fuel price has been tripled in ten years. In this research, Natural Laminar Flow airfoil is designed and tested to reduce drag at cruise condition, $c_l$=0.3, Re=3.4${\times}$10$^6$ and M=0.6. NLF airfoil is characterized by delayed transition from laminar to turbulent flow, which comes from maintaining favorable pressure gradient to downstream. Transition is predicted by solving Boundary Layer equations in viscous boundary layer and by solving Euler Equation outside the boundary layer. Once boundary layer thickness and momentum thickness are obtained, $e^N$-method is used for transition point prediction. As results, KARI's NLF airfoil is designed and shows better characteristics than NLF-0115. The characteristics are tested and verified at low Reynolds numbers, but at high Reynolds numbers, laminar flow characteristics are not obtainable because of fully turbulent flow over airfoil surfaces. Precious experiences, however, relating NLF airfoil design, subsonic and transonic tests are acquired.

• 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.

• 한국항공우주학회지
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• 제43권11호
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• pp.952-962
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• 2015

효율적인 추진기관 개발을 위해 여러 종류의 고도 보정 노즐에 대한 연구가 진행되고 있다. 본 논문에서는 향후 듀얼 벨 노즐 연구를 위한 기초자료로서의 활용을 위해 독일 DLR에서 수행한 연구내용을 바탕으로 듀얼 벨 노즐의 주요 특성을 조사하고 핵심 변수를 도출하였다. DLR은 다양한 실험을 통해 변곡각과 천이 NPR은 비례한다는 것과 확장부 길이와 측하중은 비례하지만 천이 NPR과 천이 지속시간과는 반비례한다는 것을 밝혔다. 성능 예측 과정을 통해 노즐의 형상이 결정될 수 있고 요구 성능에 맞게 변수간의 최적화가 필요하다.

• 한국ITS학회 논문지
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• 제8권3호
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• pp.1-10
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• 2009

버스지체시간은 버스노선의 교통여건이 반영되어 나타나는 결과로서 버스도착시간을 예측하는데 있어 중요한 요소이다. 이에 본 연구에서는 다양한 변수를 사용하지 않아도 되는 마코브 체인을 이용하여 분석 정류장간 전이확률행렬표를 생성하고 이를 이용하여 버스지체시간을 예측하였다. 본 연구를 통하여 기존연구의 한계점인 정류장별 계획된 버스도착 시간이 존재하지 않은 경우에 대하여 배차시간을 이용한 버스지체시간 산출방법을 제시함으로서 기존연구의 한계점을 극복하였으며, 또한 정류장별 버스지체시간을 예측하기 위해 정의한 정류장간 버스지체의 전이는 동질하다는 귀무가설을 대웅표본 T검정을 통하여 채택함으로서 사용한 가정이 95% 신뢰수준에서 유의하다는 것을 확인하였다. 이를 통하여 향후마코브 체인을 이용하여 버스도착시간 예측이 가능할 것으로 판단된다.

• 응용통계연구
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• 제27권2호
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• pp.317-330
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• 2014

양극성 장애는 조증 삽화(manic episode)와 주요 우울삽화(major depressive episode)를 특징으로 하는 정신질환이다. 주요 우울삽화 시기에는 양극성 장애 환자들의 810%가 자살하는 것으로 알려져 있다. 그러므로 양극성 장애 환자를 치료할 때, 우울증상의 정도를 측정하는 것이 중요하다. 우울증상의 정도를 측정하기 위해 가장 많이 사용하는 검사법은 해밀턴 우울평가 척도(Hamilton depression rating scale)이다. 본 논문에서는 해밀턴 우울평가척도 점수를 이용하여 환자들의 치료 효과를 예측하기 위해 선형혼합효과모형(linear mixed effects model)과 전이모형(transition model)을 제시하였다. 예측을 위해 사용된 자료는 분당서울대학교병원을 방문하여 초진일 당시의 해밀턴 우울평가 척도 점수가 8 점 이상인 환자들의 정보를 사용하였다. 첫 조사시점부터 6개월, 12개월 후 세 차례에 걸쳐 관측된 해밀턴 우울평가 척도 점수를 선형혼합효과모형과 전이모형에 적합시켰다. 그 결과를 토대로 특정시점의 해밀턴 우울평가 척도 점수를 예측하였다. 첫 조사시점부터 6개월, 12개월 후의 해밀턴 우울평가 척도 점수를 사용해 선형혼합효과모형과 전이모형에 적합 시켰다. 이 모델들을 이용해 조사시점부터 24개월 후의 해밀턴 우울평가 척도 점수를 예측한다. 이 예측모델은 조사된 24개월 후의 점수와 예측된 24개월의 후의 점수를 비교하여 평가하였다.

• 대한기계학회논문집B
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• 제22권6호
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• pp.771-787
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• 1998

A Navier-Stokes code with a modified low Reynolds number k-.epsilon. turbulence model was used to study the unsteady transitional boundary layer flow due to rotor-stator interaction. The modification, proposed by Launder, to improve prediction of stagnation flows was incorporated to the low Reynolds number k-.epsilon. turbulence model by Fan-Lakshminarayana-Barnett. Numerical solution is shown to capture well the calmed laminar flow as well as the wake induced transitional strip due to rotor-stator interaction and shows improvement, in terms of onset of transition and its length, over previous Euler/boundary layer solution. The turbulent kinetic energy shows local maximum along the upstream rotor wake in the wake induced transitional strip and this characteristics is observed untill the end of transition. The wake induced strip also shown apparent even in the laminar sublayer as the upstream rotor wake penetrates inside the boundary layer.

• 한국신뢰성학회:학술대회논문집
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• 한국신뢰성학회 2006년도 학술발표대회 논문집
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• pp.158-164
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• 2006

The creep behavior of newly developed composite geogrids which consists of PET yarns sheathed in PP were evaluated using SIM. For the SIM procedure, three test parameters, the applied loads, temperature steps and number of ribs were investigated, The study confirmed that temperature steps of 10 and 14$^{\circ}C$ up to 80$^{\circ}C$ are applicable for composite geogrids due to the different transition temperatures between two materials. At applied loads of 40 and 50%, only primary creep state was measured, while secondary creep state appeared at the applied loads of 60%, The lifetimes of composite geogrids were estimated at each of loading level using statistical reliability analysis technique. The results show that the lifetimes longer than 100 years can be predicted within 16 hours. Therefore, SIM is very effective and economical accelerated creep test methods, especially for lifetime prediction. This gives guidelines for users to select the appropriate factor of safety against creep considering the field condition within shorter test times.

• Journal of Information Technology Applications and Management
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• 제17권2호
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• pp.133-154
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• 2010

Recently, Internet Telephony has become increasingly popular in telecommunication industry. However, previous research on Internet Telephony has focused on analyzing specific Internet Telephonysolutions, identifyingthe Internet Telephony movement itself. The research on prediction models about Internet Telephony adoption has been minimal. The main propose of this study is to develop models for predicting transition intention from using traditional telephones to using Internet Telephony. To do so, this study uses data mining methods to analyze demands in the IT communications market and to provide management strategies for Internet telephony providers. Especially this study uses discriminant analysis, logistic regression, classification tree, and neural nets to develop those prediction models toward Internet Telephony adoption. The models are compared with each other and a superior model is chosen.