• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.5
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• pp.1158-1174
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• 1988

A numerical technique is developed for the solution of fully developed turbulent recirculating flow in the passage of variable area using the non-orthogonal coordinate transformation. In the numerical analysis, primitive pressure-velocity finite difference equations were solved by SIMPLER algorithm with 2-equation turbulence model and algebraic stress model (ASM). QUICK scheme on the differencing of convective terms which is free from the inaccuracies of numerical diffusion has been applied to the variable grids and the results compared with those from HYBRID scheme. In order to test the effect of streamline curvatures on turbulent diffusion Lee and Choi streamline curvature correction model which has been obtained by modifying the Leschziner and Rodi's model is testes. The ASM was also employed and the results are compared to those from another turbulence model. The results show that difference of convective differencing schemes and turbulence models give significant differences in the prediction of velocity fields in the expansion region and outlet region of the combustor, however show little differences in the parallel flow region.

• Journal of the Korean Institute of Intelligent Systems
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• v.19 no.5
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• pp.595-602
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• 2009

Fuzzy rules, which represent the behavior of their system, are sensitive to fuzzy clustering techniques. If the classification abilities of such clustering techniques are improved, their systems can work for the purpose more accurately because the capabilities of the fuzzy rules and parameters are enhanced by the clustering techniques. Thus, this paper proposes a new hierarchically structured clustering algorithm that can enhance the classification abilities. The proposed clustering technique consists of two clusters based on correlationship and statistical characteristics between data, which can perform classification more accurately. In addition, this paper uses difference data sets to reflect the patterns and regularities of the original data clearly, and constructs multiple fuzzy systems to consider various characteristics of the differences suitably. To verify effectiveness of the proposed techniques, this paper applies the constructed fuzzy systems to the field of time series prediction, and performs prediction for nonlinear time series examples.

• Journal of the Institute of Electronics and Information Engineers
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• v.53 no.3
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• pp.21-28
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• 2016

We propose an adaptive scheme of a differential codebook for temporally correlated channels. And the codeword entries of the propose codebook are selected among the set of M-PSK constellations - the values of M proposed in this paper are 8, 16, or 32. Firstly, we analyze mathematically how the optimal spherical cap radius of the proposed codebook is tracked. Then, we explain the practical implementation of the proposed adaptive method. Practically, some candidate differential codebooks we propose in this paper can be switched according to the temporal correlation coefficients of wireless channels in the proposed scheme. Monte-Carlo simulations demonstrate that the achievable throughput performance employing the proposed codebook is always superior to those of the differential codebooks employing M-PSK constellations and non-adaptive differential codebooks with the same amount of feedback information.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.387-387
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• 2015

최근 기후변화에 따른 국지성 집중호우 및 돌발홍수 증가로 도심지역에 많은 침수 피해가 발생하고 있다. 본 연구는 이러한 내외수에 영향을 받는 도시 중소하천의 침수 저감을 위하여 침수 예측모형에 적용 가능한 수리학적 계산모형을 개발하는 것이다. 일반적으로 자연하천은 수지형 수계가 대부분이며, 도시 우수관로와 관계배수 시스템 등은 폐합형 수계에 포함되어 수지형 계산모형으로는 도시 중소하천의 유출 모의를 할 수 없다. 폐합형 수계의 계산모형은 수지형에 비해 복잡하지만, 적용대상 수계가 폐합형이 아닌 경우에도 합류점의 유입량을 처리하기가 편리하고, 역방향의 월류 흐름이 존재하는 감조하천에서의 월류 흐름 모의가 가능한 장점을 갖고 있어 도시 내배수 시스템은 물론 자연하천에도 적용 할 수 있다. 본 모형은 절점, 수로 및 계산점으로 구성되는데 동력학적 방법인 1차원 Saint-Venant의 연속 방정식과 운동량 방정식에 수치해법을 이용하여 구하고자 하는 시간과 지점의 수위와 유량을 계산할 수 있게 구성하였다. 수치해법으로는 가장 보편적으로 사용되는 유한차분법 중 안정성과 정확성이 우수한 것으로 평가된 Preissmann의 4점 음해법으로 차분방정식에 Newton-Raphson 방법을 사용하여 유량과 수위 보정치에 Double Sweep 알고리즘을 적용하였다. 유한차분법은 안정성 문제를 수반할 수 있기에 시간 증분이 작을수록 Courant 조건을 만족할 수 있다. 모형 비교 검증을 위하여 동력학적 방법을 적용한 대표적인 폐합형 수계모형인 EPA SWMM을 지원하는 CHI사의 PC-SWMM 프로그램을 이용하여 가상의 폐합 수계를 구축하였다. 일반적으로 상류지점의 경계조건은 하류로 추적될 입력 자료로써, 상류지점과 합류지점은 유입되는 유량값을 그리고 하류지점은 유출되는 수위값을 경계조건으로 입력하였다. 운동량방정식의 에너지 경사와 마찰경사 항에 포함된 조도계수는 변화량에 중요한 물리적 요소이지만, 고정 상수값인 0.03을 적용하여 검증에 용의하도록 하였다. 구축된 모형과 PC-SWWM을 통해 산출된 계산점별 수위와 유량에 RMS 오차를 비교한 결과 만족할만한 결과를 얻을 수 있었다. 따라서 향후 내외수를 연계에 침수예측모형에 적용 가능할 것으로 사료된다.

• Journal of the Society of Naval Architects of Korea
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• v.31 no.3
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• pp.80-86
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• 1994

A numerical method for simulations of inviscid incompressible flow fields around a ship advancing on the free surface is developed. A body fitted coordinate system, generated by numerically solving elliptic type partial differential equations is used to conform the ship and free surface configurations. Three dimensional Euler equations transformed to the non-staggered body fitted coordinate system are discretised by finite difference method. Time and spatial derivatives are discretised by forward and centered differencings, respectively, and artificial dissipations are added to discretised convection terms for improvements of numerical stability. At each time steps, free surface elevations are recomputed to satisfy nonlinear free surface conditions. Poisson equations for pressure field are solved iteratively and the velocity field for next time step is extrapolated. To verify the developed numerical method, flow fields around a Wigley model are simulated(Fn=0.250-0.408) and compared with experimental data to show good agreements.

• Journal of the Society of Naval Architects of Korea
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• v.32 no.4
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• pp.38-47
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• 1995

The three-dimensional multi-layer computer simulation technology for tidal current developed in the previous study is updated to a new version. many improvements are achieved by following changes : (1) No-reflection condition is adopted instead of no-gradient condition as an open boundary condition. (2) Time marching algorithm is changed so that velocity and pressure(surface movement) might be salved in turn at different time step (3) Convection term in equation of motion is estimated by upwind differencing scheme instead of central differencing. The stability is improved considerably and the steady state is achieved within 2 tidal periods which is about 3 times shorter than that of the old version. Moreover, fluctuations in time disappeared by introducing the new time marching technique. An application to the real near shore area(near Inchon harbor) is performed by the new version. Simulated results are compared with those by the simulation total developed in the University of Tokyo. Validity and effectiveness of the two simulation technologies are chocked through the comparative research works.

• Journal of the Korea Society of Computer and Information
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• v.25 no.4
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• pp.113-121
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• 2020

With the rapid development of information and communication technology (ICT), various sensors are being embedded in wearable devices. Consequently, these devices can continuously collect data including health data from individuals. The collected health data can be used not only for healthcare services but also for analyzing an individual's lifestyle by combining with other external data. This helps in making an individual's life more convenient and healthier. However, collecting health data may lead to privacy issues since the data is personal, and can reveal sensitive insights about the individual. Thus, in this paper, we present a method to collect an individual's health data from a smart band in a privacy-preserving manner. We leverage the local differential privacy to achieve our goal. Additionally, we propose a way to find feature points from health data. This allows for an effective trade-off between the degree of privacy and accuracy. We carry out experiments to demonstrate the effectiveness of our proposed approach and the results show that, with the proposed method, the error rate can be reduced upto 77%.

• The Journal of the Korea institute of electronic communication sciences
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• v.16 no.3
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• pp.401-406
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• 2021

Recently, mean-teacher models based on convolutional recurrent neural networks are popularly used in audio event detection. The mean-teacher model is an architecture that consists of two parallel CRNNs and it is possible to train them effectively on the weakly-labelled and unlabeled audio data by using the consistency learning metric at the output of the two neural networks. In this study, we tried to improve the performance of the mean-teacher model by using additional derivative features of the log-mel spectrum. In the audio event detection experiments using the training and test data from the Task 4 of the DCASE 2018/2019 Challenges, we could obtain maximally a 8.1% relative decrease in the ER(Error Rate) in the mean-teacher model using proposed derivative features.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.29 no.3
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• pp.589-598
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• 2019

The amount of digital data a is explosively growing, and these data have large potential values. Countries and companies are creating various added values from vast amounts of data, and are making a lot of investments in data analysis techniques. The privacy problem that occurs in data analysis is a major factor that hinders data utilization. Recently, as privacy violation attacks on neural network models have been proposed. researches on artificial neural network technology that preserves privacy is required. Therefore, various privacy preserving artificial neural network technologies have been studied in the field of differential privacy that ensures strict privacy. However, there are problems that the balance between the accuracy of the neural network model and the privacy budget is not appropriate. In this paper, we study differential privacy techniques that preserve the performance of a model within a given privacy budget and is resistant to model inversion attacks. Also, we analyze the resistance of model inversion attack according to privacy preservation strength.

• Geophysics and Geophysical Exploration
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• v.22 no.1
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• pp.12-20
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• 2019

The first-arrival traveltime calculation method based on the damped wave equation overcomes the shortcomings of ray-tracing methods. Since this algorithm needs to solve the damped wave equation, numerical modeling is essential. However, it is not desirable to use the finite-difference method (FDM), which has good computational efficiency, for simulating the land seismic data because of irregular topography. Thus, the finite-element method (FEM) which requires higher computational cost than FDM has been used to correctly describe the irregular topography. In this study, we computed first-arrival traveltimes in an irregular topographic model using FDM incorporating embedded boundary method (EBM) to overcome this problem. To verify the accuracy and efficiency of the proposed algorithm, we compared our results with those of FEM. As a result, the proposed method using EBM not only provided the same accuracy as the FEM but also showed the improved computational efficiency.