• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.8
• /
• pp.829-837
• /
• 2012

Direct numerical simulations were carried out for turbulent channel flows with $Re_{\tau}$ = 180, 395 and 590 to investigate the turbulent flow structure related to the Reynolds shear stress. By examining the probability density function, the second quadrant (Q2) events with the largest contribution to the mean Reynolds shear stress were identified. The change in the inclination angle of Q2 events varies with wall units in $y^+<50$ and with the channel half height in y/h > 0.5. Conditionally averaged flow fields for the Q2 event show that the flow structures associated with Reynolds shear stress are a quasi-streamwise vortex in the buffer layer and a hairpin-shaped vortex in the outer layer. Three-dimensional visualization of the distribution of high Reynolds shear stress reveals that the organization of hairpin vortices in the outer layer having a size of 1.5~3 h is associated with large-scale motions with high Reynolds shear stress in the outer layer.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2011.05a
• /
• pp.70-70
• /
• 2011

기후변화 연구는 불확실한 미래를 전망하는 과정이므로 '불확실성'은 모든 기후변화 영향평가의 키워드임에 분명하다. 하지만 불확실성 평가를 위해 IPCC에서 제공되고 있는 수많은 GCM 시나리오를 모두 활용하기에는 많은 시간과 노력이 필요하기 때문에 이를 효율적으로 수행할 수 있는 방법이 필요하다 본 연구에서는 시나리오 저감(scenario reduction)방법을 이용하여, 수많은 GCM 시나리오 대신 몇 개의 대표적 GCM 시나리오로도 충분히 불확실성을 유지할 수 있는 시나리오 저감(scenario reduction)방법을 수립하고 제시하였다. IPCC 기후시나리오 중 20C3M과 A & B 배출시나리오를 바탕으로 생산되는 71개의 GCM 시나리오를 다운로드 받아 월평균 기온과 강수량에 대하여 한반도를 대상으로 분석하였다. 비교결과, 기온 전망은 실측과 비슷한 경향성을 보였으나 강수량은 홍수기를 모의하지 못하는 것으로 나타났다. 시나리오 저감방법은 시나리오 분류(scenario cluster)방법과 시나리오 선정(scenario selection) 방법으로 구성된다. 시나리오 분류방법에서는 k-mean방법을 이용하여 5개의 cluster로 나누었으며, 시나리오 선정방법에서는 GCM 시나리오 선정기법을 조사 분석하여 연구방향과 목적에 따라 GCM 시나리오 선정기법을 선택할 수 있는 표를 제시하고, 이 중 시나리오의 확률밀도함수를 이용하는 PDF method를 적용하였다. 본 연구에서는 불확실성 정량화를 위해 maximum entropy를 이용하였다. 또한 시나리오 저감방법이 불확실성을 유지하는지 비교하기 위해 PDF method를 이용하여 정확성이 높은 순으로 5개의 GCM 시나리오를 선정(best 시나리오)하여 불확실성을 정량화하였다. GCM 시나리오의 분산을 이용하여 maximum entropy를 산정한 결과, 20C3M 배출시나리오에서는 모든 시나리오의 entropy는 3.08, 시나리오 저감방법은 2.75, best 시나리오는 2.28이었으며, 이는 시나리오 저감방법은 모든 시나리오의 89.3%의 불확실성을 설명하고 있으나 best 시나리오는 74.0%밖에 설명하지 못한다는 것을 나타낸다. A & B 배출시나리오에서도 시나리오 저감 방법을 사용한 GCM 시나리오가 best 시나리오보다 모든 시나리오의 불확실성을 더 잘 설명하는 것으로 나타났다. 이와 같이 수많은 GCM 시나리오를 사용하는 것보다 몇 개의 대표 시나리오를 이용하여 기후 변화 불확실성을 유지하면서 미래전망을 할 수 있다면, 매우 효율적으로 기후변화 연구를 수행할 수 있을 것으로 사료된다.

• Journal of the Korean Society of Groundwater Environment
• /
• v.5 no.2
• /
• pp.80-87
• /
• 1998

Since groundwater flow paths have one of the major roles to transport the radioactive nuclides from the radioactive waste repository to the biosphere, the discrete fracture network model is used for the rock block scale flow instead of the porous continuum model. This study aims to construct a three dimensional discrete fracture network to interpret the groundwater flow system in the study site. The modeling work includes the determination of the probabilistic distribution function from the fracture geometric and hydraulic parameters, three dimensional fracture modeling and model calibration. The results of the constant pressure tests performed in a fixed interval length at boreholes indicate that the flow dimension around boreholes shows mainly radial to spherical flow pattern. The fracture transmissivity value calculated by Cubic law is 6.12${\times}$10$\^$-7/ ㎡/sec with lognormal distribution. The conductive fracture intensity estimated by FracMan code is 1.73. Based on this intensity, the total number of conductive fractures are obtained as 3,080 in the rock block of 100 m${\times}$100 m${\times}$100 m.

• Journal of Korean Society of Transportation
• /
• v.32 no.6
• /
• pp.694-702
• /
• 2014

Since Greenhouse Gas Inventory & Research Center (GIR) of Korea was founded in 2010, the annual greenhouse gas inventory reports, one of the collections of GIR's major affairs, have been published from 2012. In the reports many items related to greenhouse gas emission quantities are included, but among them uncertainty values are replaced to basic values which IPCC guideline suggests. Even though IPCC guideline suggests the equations of each Tier level in details, the guideline recommends developing nation's own methodology on uncertainty which is closely related to statistical problems such as the estimation of a probability density function or Monte carlo methods. In the road transportation sector the emissions have been calculated by Tier 1 but the uncertainties have not been reported. This study introduce a bootstrap technique and Monte carlo method to estimates annual emission quantity and uncertainty, given activity data and emission factors such as annual traveled distances, fuel efficiencies and emission coefficients.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.25 no.1B
• /
• pp.1-7
• /
• 2000

Generally, one-dimensional fiber optic code-division multiple-access(CDMA) system is encoded and decoded using optical orthogonal codes(OOC’s), where two-dimensional fiber optic CDMA system uses optical orthogonal signature pattern codes(OOSPC’s) for parallel data link process. The OOSPC’s should have good autocorrelation and cross-correlation properties. However, if timing information or synchronization of OOSPC’s can be obtained by other means, the property of autocorrelation may not be restricted and we can increase the number of pattern codes. In this paper we introduce the fiber optic CDMA system for parallel transmission of two-dimensional data and investigate methods of generation of two-dimensional pattern codes. The probability density function of interference noise is calculated in interfering OOSPC’s of the users and the corresponding bit error rate is derived.. We compare each OOSPC’s by plotting bit error rate versus threshold values and the number of simultaneous users, from the result, we propose the optimal OOSPC’s conditions for the parallel transmission of two-dimensional data.

• Journal of Environmental Impact Assessment
• /
• v.26 no.6
• /
• pp.445-456
• /
• 2017

For impact assessment of inundation in coastal area due to sea level rise (SLR), model for estimating future peak water level was constructed using observed mean sea level (MSL), storm surge level (SSL) data and calculated tide level (TL) data. Based on time series analysis and quadratic polynomial model for SLR and Monte-Carlo simulation for IC, SSL and TL, 100-year return peak water level is expected to be 2.3, 2.6, 2.8m, respectively (each corresponding to year 2050, 2080, 2100). Further analysis on future potential inundation area showed U-dong, Yongho-dong, Songjeong-dong, Jaesong-dong to be at high risk.

• Journal of the Korean Society of Clothing and Textiles
• /
• v.29 no.8 s.145
• /
• pp.1102-1113
• /
• 2005

The propose of this study is to develop the optimal sizing system of ready-to-wear f3r elementary school girls using a newly invented statistical technique. The body measurements was classified by the method that equalizes the distribution of the subjects using the probability density function, to theoretically systemize a method to determine a size range of ready-to-wear for elementary school girls between 6 to 12 years old. The statistical method were 1) The total of 11 height groups, which size interval from one another is 6 cm that is an average height gap between each age. 2) In order to determine an approximate figure (m ${\times}$ n) to establish the appropriate sizes far each height group that fit to the combinations of bust and hip girth, which based on their means and standard deviations on the probability density curve to produce the standard normal distribution. 3) m and n were aligned by 4cm -the grading increments used for patterns making- and determined the size ranges by confirming the approximate figures of m and n. 4) The representative values were determined by an area ratio calculated by dividing the area determined from the range of bust and hip girth with the representative value. Considering the characteristics of subjects' distribution, the area ratios was used. 5) Weight was calculated by seeking a growth exponent for each age and multiplying it by the number of girls that fit to each size range. As sections that show the highest weight are more likely sought by the consumers, these sections were determined as the optimal size standards. 6) This optimal sizing system consists of sizes determined by the optimal size standards and its sizes are marked with height, bust and hip girth.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.27 no.7B
• /
• pp.695-705
• /
• 2002

In this paper, we propose a simple open loop transmit diversity (TD) scheme for the wideband code division multiple access (W-CDMA) systems in Rician multipath fading channels such as rural area or satellite channels where line-of-sight (LOS) paths are in presence. The proposed scheme does not require any pre-processing of transmit data, resulting in simpler structure as compared to conventional closed loop transmit adaptive array (TxAA) and open loop space-time transmit diversity (STTD). We analytically derive the probability density function of signal-to-noise ratio at the Rake receiver output and the uncoded bit error rate performance of the proposed scheme. Extensive simulation is Performed to verify the analytical performance of the proposed scheme under typical Rician multipath fading channel environments. Moreover, comparative results with the conventional TxAA and STTD are also provided. Simulation results show that the proposed scheme shows slightly better performance than the conventional open loop STTD under the channels with very weak LOS components, however, it significantly outperforms the STTD under the channels with dominant LOS components, and achieves a close performance of ideal closed loop TxAA.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.13 no.6
• /
• pp.538-545
• /
• 2002

In the orthogonal frequency division multiplexing (OFDM) system, the nonlinear distortion in the high power amplifier(HPA) degrades the system performance because of the high peak-to-average power ratio (PAPR). In this paper, a semi-analytical method is newly proposed for the performance evaluation of the nonlinearly distorted OFDM communication system. In the proposed method, at first, the probability density function (pdf) of the PAPR is generated by computer simulation. Then, mean and variance of the non-linear distortion noise process are computed. Next, the overall BER is found by the analytical method. When the equivalent SSPA model is applied in case of the QPSK/16-QAM and AWGN channel, the BER is calculated for the variation of the IBO(input back-off) and PAPR parameter. It is shown that the results by proposed method are very similar to those of the conventional Monte-Carlo method. The computation time can be considerably reduced than the conventional method that depends on the magnitudes of BER and IBO.

• Journal of the Society of Naval Architects of Korea
• /
• v.52 no.1
• /
• pp.88-95
• /
• 2015

For the frequency-domain spectral fatigue analysis, the probability density function of stress range needs to be estimated based on the stress spectrum only, which is a frequency domain representation of the response. The probability distribution of the stress range of the narrow-band spectrum is known to follow the Rayleigh distribution, however the PDF of wide-band spectrum is difficult to define with clarity due to the complicated fluctuation pattern of spectrum. In this paper, efforts have been made to figure out the links between the probability density function of stress range to the structural response of wide-band Gaussian random process. An artificial neural network scheme, known as one of the most powerful system identification methods, was used to identify the multivariate functional relationship between the idealized wide-band spectrums and resulting probability density functions. To achieve this, the spectrums were idealized as a superposition of two triangles with arbitrary location, height and width, targeting to comprise wide-band spectrum, and the probability density functions were represented by the linear combination of equally spaced Gaussian basis functions. To train the network under supervision, varieties of different wide-band spectrums were assumed and the converged probability density function of the stress range was derived using the rainflow counting method and all these data sets were fed into the three layer perceptron model. This nonlinear least square problem was solved using Levenberg-Marquardt algorithm with regularization term included. It was proven that the network trained using the given data set could reproduce the probability density function of arbitrary wide-band spectrum of two triangles with great success.