• 한국전산유체공학회지
• /
• 제18권1호
• /
• pp.91-97
• /
• 2013

In the present study, the effects of contaminant accumulation and surface roughness on the aerodynamic performance of wind turbine blade sections were numerically investigated by using a flow solver based on unstructured meshes. The turbulent flow over the rough surface was modeled by a modified ${\kappa}-{\omega}$ SST turbulence model. The calculations were made for the NREL S809 airfoil with varying contaminant sizes and positions at several angles of attack. It was found that as the contaminant size increases, the degradation of the airfoil performance becomes more significant, and this trend is further amplified near the stall condition. When the contaminant is located at the upper surface near the leading edge, the loss in the aerodynamic performance of the blade section becomes more critical. It was also found that the surface roughness leads to a significant reduction of lift, in addition to increased drag.

• 한국유체기계학회 논문집
• /
• 제13권5호
• /
• pp.11-16
• /
• 2010

The development of lifting pumps that lift minerals to a mining vessel are one of the vital parts of the commercial mining process. The purpose of this study is to investigate internal flow and its effect on the performance of a mixed flow pump in order to improve the pump's performance. Numerical analysis was performed by commercial code of ANSYS CFX-11 based on flow rate and length of flexible hose. The rated rotational speed of the impeller is 1750rpm. For taking into account the turbulence, k-$\omega$ SST model was selected to guarantee more accurate prediction of flow separation. The simulated results are in good agreement with the experimental results and showed that its efficiency and the head of the pump are related mainly to the flow rate and the length of flexible hose. A lesser flow rate caused more secondary flow through the guide vane passage. The length of flexible hose and flow rate exert much more influence on the pump's performance than the shape of the flexible hose.

• 대한원격탐사학회:학술대회논문집
• /
• 대한원격탐사학회 2007년도 Proceedings of ISRS 2007
• /
• pp.33-36
• /
• 2007

We have persistently constructed gridded products of surface wind/wind stress over the world ocean using satellite scatterometer (ERS and Qscat). They are available for users as the Japanese Ocean Flux data sets with Use of Remote sensing Observation (J-OFURO) data together with heat flux components. Recently, a new version data of the Qscat/SeaWinds based on improved algorithm for rain flag and high wind-speed range have been delivered, and allowed us to reconstruct gridded product with higher spatial resolution. These products are validated by comparisons with in-situ measurement data by mooring buoys such as TAO/TRITON, NDBC and the Kuroshio Extension Observation (KEO) buoys, together with numerical weather prediction model products such as the NCEP-1 and 2. Results reveal that the new product has almost the same magnitude in mean difference as the previous version of Qscat product and much smaller than the NCEP-1 and 2. On the other hand, it is slightly larger root-mean-square (RMS) difference than the previous one and NCEPs for the comparison using the KEO buoy data. This may be due to the deficit of high wind speed data in the buoy measurement. The high resolution product, together with sea surface temperature (SST) one, is used to examine a new type of relationship between the lower atmosphere and upper ocean in the Kuroshio Extension region.

• Journal of Mechanical Science and Technology
• /
• 제19권5호
• /
• pp.1169-1181
• /
• 2005

Experimental and numerical studies on the unsteady wake field behind a square cylinder near a wall were conducted to find out how the vortex shedding mechanism is correlated with gap flow. The computations were performed by solving unsteady 2-D Incompressible Reynolds Averaged Navier-Stokes equations with a newly developed ${\epsilon}-SST$ turbulence model for more accurate prediction of large separated flows. Through spectral analysis and the smoke wire flow visualization, it was discovered that velocity profiles in a gap region have strong influences on the formation of vortex shedding behind a square cylinder near a wall. From these results, Strouhal number distributions could be found, where the transition region of the Strouhal number was at $G/D=0.5{\sim}0.7$ above the critical gap height. The primary and minor shedding frequencies measured in this region were affected by the interaction between the upper and the lower separated shear layer, and minor shedding frequency was due to the separation bubble on the wall. It was also observed that the position (y/G) and the magnitude of maximum average velocity $(u/u_{\infty})$ in the gap region affect the regular vortex shedding as the gap height increases.

• 아시아태평양코퍼스연구
• /
• 제2권1호
• /
• pp.35-45
• /
• 2021

With the wide spread of the Common European Framework of Reference (CEFR) scales, many studies attempt to apply them in routine teaching and rater training, while more evidence regarding criterial features at different CEFR levels are still urgently needed. The current study aims to explore complexity features that distinguish and predict CEFR proficiency levels in oral performance. Using a quantitative/corpus-based approach, this research analyzed lexical and syntactic complexity features over 80 transcriptions (includes A1, A2, B1 CEFR levels, and native speakers), based on an interview test, Standard Speaking Test (SST). ANOVA and correlation analysis were conducted to exclude insignificant complexity indices before the discriminant analysis. In the result, distinctive differences in complexity between CEFR speaking levels were observed, and with a combination of six major complexity features as predictors, 78.8% of the oral transcriptions were classified into the appropriate CEFR proficiency levels. It further confirms the possibility of predicting CEFR level of L2 learners based on their objective linguistic features. This study can be helpful as an empirical reference in language pedagogy, especially for L2 learners' self-assessment and teachers' prediction of students' proficiency levels. Also, it offers implications for the validation of the rating criteria, and improvement of rating system.

• 한국항공우주학회지
• /
• 제42권4호
• /
• pp.326-335
• /
• 2014

터보블로워는 상대적으로 적은 체적유량에서 높은 압력이 요구되는 곳에 사용되는 대표적인 유체기계로서 다양한 산업에 응용되어 사용된다. 본 연구에서는 고속으로 회전하는 소형 2단 터보블로워의 정압상승 메커니즘을 이해하기위해, 1단 임펠러 영역과 터보블로워 전체 영역에 대해서 상용툴인 ANSYS 14.5를 이용하여 CFD해석을 수행하였다. CFD 해석과정에는 역압력 구배에 의한 유동박리 예측에 적합한 k-${\omega}$ SST 난류 모델을 적용하였다. 터보블로워의 전산해석 결과는 KS B 6311 및 KS A 0612에 따른 성능시험방법을 통하여 해석기법이 타당함을 검증하였다. CFD 해석결과 터보블로워의 압력상승은 선형적으로 나타나지 않으며, 안내깃에서의 손실과 케이싱과 임펠러 간극에서 손실이 발생하는 것으로 분석되었다. 소형 2단 터보블로워를 공력성능을 예측하기 위해서는 전체 유동영역에 대한 전산 해석이 필요하며, 실험과 전산해석의 오차에 대해 고려된 전산해석 결과가 선정되어야 한다.

• 한국지리정보학회지
• /
• 제2권2호
• /
• pp.69-78
• /
• 1999

본 논문에서는 한반도의 지역별 강수량 예측을 위한 신경망 기법을 소개한다. 시계열 패턴 예측 문제에 적용될 수 있는 기존의 다양한 신경망 모델의 특성을 분석하고 이로부터 강수량 예측 문제에 적합한 모델 및 학습 알고리즘을 제시한다. 본 논문에서 제시하는 모델은 계층적구조의 신경망으로 각 노드의 출력값은 일정기간동안 버퍼에 저장되어 상위계층에 입력으로 작용한다. 본 연구에서는 제안된 모델에 대하여 이중연결형태의 시냅스 구조를 채택하고, 이에 대한 네트워크의 동작특성과 학습알고리즘 등을 정의한다. 이러한 이중연결구조는 기존의 다층퍼셉트론에서 바이어스 노드의 역할을 담당하며, 노드가 갖는 특징들간의 관계를 효과적으로 반영함으로써 기존의 전형적인 시계열 예측 신경망인 FIR(Finite Impulse Response) 네트워크와 비교할 때 학습의 효율을 개선시킨다. 제시된 이론은 월별 및 계절별 강수량 예측 실험에 적용하였다. 신경망 예측기의 학습자료로서 과거 수십년동안 관측된 강수량 데이터와 해수표면온도 데이터를 사용하며 예측 실험결과로부터 제시된 이론의 타당성을 고찰한다.

• 한국해양학회지:바다
• /
• 제20권3호
• /
• pp.119-130
• /
• 2015

동해 해양자료동화시스템(DA-ESROM; Kim et al., 2009)을 이용하여 Argo 관측망이 해양 분석장에 미치는 영향에 대해 살펴보았다. 본 연구에서는 2009년을 연구기간으로 하여 수온 프로파일, 해수면 온도, 그리고 해수면 고도 자료를 동화하여 분석장을 생산하고(Exp. AllDa), 이를 Argo 수온 자료를 제외한 실험(Exp. NoArgo) 결과와 비교하였다. 동해 수온 프로파일 관측자료와 두 실험결과와의 평균 제곱근 오차(Root Mean Square Error; RMSE)를 살펴본 결과, Exp. AllDa의 결과에서 Exp. NoArgo에 비해 표층 이하부터 전반적으로 낮은 RMSE가 나타났고, 특히 수심 약 100 m 부근에서 약 $0.5^{\circ}C$의 RMSE 차이(Exp. AllDa - Exp. NoArgo)를 보이는 등 아표층 부근에서 Argo 수온 자료동화의 영향이 큰 결과를 보였다. 자료동화 과정에 독립적인 표류부이 관측자료와의 비교를 통해, Argo 수온 자료의 동화로 표층해류 정확도가 전반적으로 개선되는 것을 확인하였고, 특히 동해 중남부에서 상대적으로 장기 표류한 부이의 궤적을 따라 RMSE가 약 2.0~6.0 cm/s 정도 낮아졌다. 반면, 표층수온에 대해서는 Argo 수온자료의 동화효과는 약한 것으로 나타났고, 매일 동화되는 해수면 온도 자료의 영향이 지배적인 것으로 판단된다. 또한, 동해 해양자료동화시스템(DA-ESROM)은 일주일 간격으로 해수면 고도자료를 동화하지만, Argo 수온자료가 동화되지 않으면서 나타나는 해수면 고도 변화를 완전히 보정하지 못하는 것으로 나타났다. 실험결과, Argo 수온자료의 동화는 특히 야마토 분지 남서쪽의 시계방향 순환 등 동해 중남부 해역에서의 해수면 고도 재현성을 향상시키는데 큰 영향을 미치는 것으로 나타났다.

• 한국안전학회지
• /
• 제35권3호
• /
• pp.96-104
• /
• 2020

The numerical simulations were conducted to investigate the thermal-fluid phenomena occurred inside the experimental apparatus during a PCCS, used to remove heat released in accidents from a containment of light water nuclear power plant, operation. Numerical simulations of the flow and heat transfer caused by wall condensation inside the containment simulation vessel (CSV), which equipped with 18 vertical heat exchanger tubes, were conducted using the commercial computational fluid dynamics (CFD) software ANSYS-CFX. Shear stress transport (SST) and the wall condensation model were used for turbulence closure and wall condensation, respectively. The simulation using the actual size of the apparatus. However, rather than simulating the whole experimental apparatus in consideration of the experimental cases, calculation resources, and calculation time, the simulation model was prepared only in CSV. Selective simulation was conducted to verify the effects of non-condensable gas(NC gas) concentration, CSV internal pressure, and wall sub-cooling conditions. First, as a result of the internal flow of CSV, it was observed that downward flow due to condensation occurred surface of the vertical tube and upward flow occurred in the distant place. Natural convection occurred actively around the heat exchanger tube. Due to this rising and falling internal flow, natural circulation occurred actively around the heat exchanger tubes. Next, in order to check the performance of built-in condensation model using according to the non-condensable gas concentration, CSV internal flow and wall sub-cooling, the heat flux values were compared with the experimental results. On average, the results were underestimated with and error of about 25%. In addition, the influence of CSV internal pressure and wall sub-cooling was small, but when the condensate was highly generated due to the low non-condensable gas concentration, the error was large compared to the experimental values. This is considered to be due to the nature of the condensation model of the CFX code. However, in spite of the limitations of CFD, it is valid to use the built-in condensation model of CFD for PCCS performance prediction from a conservative perspective.

• 대한조선학회논문집
• /
• 제54권3호
• /
• pp.215-226
• /
• 2017

This paper presents numerical results of the performance of a marin propeller in cavitating and non-cavitating flow conditions. The geometry and experimental validation data of the propeller are provided in Potsdam Propeller Test Case(PPTC) in the framework of the second International Symposium on Marine Propulsors 2011(SMP'11) workshop. The PPTC includes open water tests, velocity field measurements and cavitation tests. The present numerical analysis was carried out by using the Reynolds averaged Navier-Stokes(RANS) method on a wall-resolved grid ensuring a y+=1, where the SST k-${\omega}$ model was mainly used for turbulence closure. The influence of the turbulence model was investigated in the prediction of the wake field under a non-cavitating flow condition. The propeller tip vortex flows in both cavitating and non-cavitating conditions were captured through adaptation of additional grids. For the cavitation flows at three operation points, Schnerr-Sauer's cavitation model was used with a Volume-Of Fluid(VOF) approach to capture the two-phase flows. The present numerical results for the propeller wake and cavitation predictions including the open water performance showed a qualitatively reasonable agreement with the model test results.