• 한국물환경학회지
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• 제24권1호
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• pp.54-62
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• 2008

It is often believed that a more complex water quality model is better able to simulate reality. The more complex a model, however, the more parameters are involved thus increases the cost and uncertainty of modeling processes. The objective of this study was to compare the performance of two steady-state river water quality models, QUAL2E and QUAL-NIER, that have different complexity. QUAL-NIER is recently developed by National Institute of Environmental Research aiming to enhance the simulation capability of QUAL2E for eutrophic rivers. It is a carbon based model that considers different forms, such as dissolved versus particulate and labile versus refractory, of carbon and nutrients, and the contribution of autochthonous loading due to algal metabolism. The models were simultaneously applied to Nakdong River and their performance was evaluated by statistical verification with field data. Both models showed similar performance and satisfactorily replicated the longitudinal variations of BOD, T-N, T-P, Chl.a concentrations along the river. The algal blooms occurred at the stagnant reaches of downstream were also reasonably captured by the models. Although QUAL-NIER somewhat reduced the magnitude of errors, the hypothesis tests revealed no statistical evidence to justify its better performance. The contribution of autochthonous carbon and nutrient load by algal metabolism was insignificant because the hydraulic retention time is relatively short compare to the time scale of kinetic reactions. The results imply that the kinetic processes included in QUAL-NIER are too complex for the nature and scale of the real processes involved, thus needs to be optimized for improving the modeling efficiency.

• Nuclear Engineering and Technology
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• 제51권6호
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• pp.1487-1503
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• 2019

The methods and performance of a pin-level nuclear reactor core thermal-hydraulics (T/H) code ESCOT employing the drift-flux model are presented. This code aims at providing an accurate yet fast core thermal-hydraulics solution capability to high-fidelity multiphysics core analysis systems targeting massively parallel computing platforms. The four equation drift-flux model is adopted for two-phase calculations, and numerical solutions are obtained by applying the Finite Volume Method (FVM) and the Semi-Implicit Method for Pressure-Linked Equation (SIMPLE)-like algorithm in a staggered grid system. Constitutive models involving turbulent mixing, pressure drop, and vapor generation are employed to simulate key phenomena in subchannel-scale analyses. ESCOT is parallelized by a domain decomposition scheme that involves both radial and axial decomposition to enable highly parallelized execution. The ESCOT solutions are validated through the applications to various experiments which include CNEN $4{\times}4$, Weiss et al. two assemblies, PNNL $2{\times}6$, RPI $2{\times}2$ air-water, and PSBT covering single/two-phase and unheated/heated conditions. The parameters of interest for validation include various flow characteristics such as turbulent mixing, spacer grid pressure drop, cross-flow, reverse flow, buoyancy effect, void drift, and bubble generation. For all the validation tests, ESCOT shows good agreements with measured data in the extent comparable to those of other subchannel-scale codes: COBRA-TF, MATRA and/or CUPID. The execution performance is examined with a mini-sized whole core consisting of 89 fuel assemblies and for an OPR1000 core. It turns out that it is about 1.5 times faster than a subchannel code based on the two-fluid three field model and the axial domain decomposition scheme works as well as the radial one yielding a steady-state solution for the OPR1000 core within 30 s with 104 processors.

• Wind and Structures
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• 제26권6호
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• pp.397-413
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• 2018

Excitation mechanism of interference effect between two tall buildings is investigated with wind tunnel experiments. Synchronized building surface pressure and flow field measurements by particle image velocimetry (PIV) are conducted to explore the relationship between the disturbed wind flow field and the consequent wind load modification for twin buildings in tandem. This reveals evident excitation mechanisms for the fluctuating across-wind loads on the buildings. For small distance (X/D < 3) between two buildings, the disturbed flow pattern of impaired vortex shedding is observed and the fluctuating across-wind load on the downstream building decreases. For larger distance ($X/D{\geq}3$), strong correlation between the across-wind load of the downstream building and the oscillation of the wake of the upstream building is found. By further analysis with conditional sampling and phase-averaged techniques, the coherent flow structures in the building gap are clearly observed and the wake oscillation of the upstream building is confirmed to be the reason of the magnified across-wind force on the downstream building. For efficient PIV measurement, the experiments use a square-section high-rise building model with geometry scale smaller than the usual value. Interference factors for all three components of wind loads on the building models being surrounded by another identical building with various configurations are measured and compared with those from previous studies made at large geometry scale. The results support that for interference effect between buildings with sharp corners, the length scale effect plays a minor role provided that the minimum Reynolds number requirement is met.

• Wind and Structures
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• 제34권5호
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• pp.421-435
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• 2022

To investigate the non-Gaussian properties of fluctuating wind pressures and the error margin of extreme wind loads on a long-span curved roof with matching and mismatching ratios of turbulence integral scales to depth (L_u^x/D), a series of synchronized pressure tests on the rigid model of the complex curved roof were conducted. The regions of Gaussian distribution and non-Gaussian distribution were identified by two criteria, which were based on the cumulative probabilities of higher-order statistical moments (skewness and kurtosis coefficients, S_k and K_u) and spatial correlation of fluctuating wind pressures, respectively. Then the characteristics of fluctuating wind-loads in the non-Gaussian region were analyzed in detail in order to understand the effects of turbulence integral-scale. Results showed that the fluctuating pressures with obvious negative-skewness appear in the area near the leading edge, which is categorized as the non-Gaussian region by both two identification criteria. Comparing with those in the wind field with matching L_u^x/D, the range of non-Gaussian region almost unchanged with a smaller L_u^x/D, while the non-Gaussian features become more evident, leading to higher values of S_k, Ku and peak factor. On contrary, the values of fluctuating pressures become lower in the wind field with a smaller L_u^x/D, eventually resulting in underestimation of extreme wind loads. Hence, the matching relationship of turbulence integral scale to depth should be carefully considered as estimating the extreme wind loads of long-span roof by wind tunnel tests.

• 대한토목학회논문집
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• 제29권1C호
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• pp.21-31
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• 2009

기존의 연구에서 이미 경량성토재료로서 적용된 바 있는 저회(Bottom Ash), 폐타이어-흙 혼합토의 성능을 토대로 하여, 전통적인 성토재료인 흙 대신 저회를 활용하는 새로운 경량성토재료를 제안한 바 있다. 이어서 이 연구에서는 저회-폐타이어 혼합토의 경량성토재료로서의 적용성을 평가하기 위해 현장다짐시험과 실대형 성토 모형실험을 수행하여 기존의 성토재료인 풍화토만으로 조성된 성토구조물과 저회-폐타이어 혼합토로 조성한 성토구조물에 대한 침하, 토압, 지지력, 진동저감효과 등을 평가하였다. 그 결과 토압 및 진동은 저감되는 효과를 보였으며, 정동적 하중에 따른 침하거동은 두 재료(풍화토 단면, 저회-폐타이어 혼합재료)에서 유사하게 평가되었다.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2005년도 춘계학술대회
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• pp.42-46
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• 2005

In spite of fast growing of prediction codes, there is still not negligible uncertainty in their results. This uncertainty affects on the turbine structural design and power production prediction. With the growing size of wind turbine, reducing this uncertainty is becoming one of critical issues for high performance and efficient wind turbine design. In this respect, there are international efforts to evaluate and tune prediction codes of wind turbine. As the reference data for this purpose, field test data is not appropriate because of its uncontrollable wind characteristics and its inherent uncertainty. Wind tunnel can provide controllable wind. For this reason, NREL has done the full scale test of the 10m turbine at NASA-Ames. With this reference data, a blind comparison has been done with participation of 18 organizations with 19 modeling tools. The results were not favorable. In Europe, a similar project is going on. Nine organizations from five countries are participating in the MEXICO project to do full scale wind tunnel tests and calculation with prediction codes. In this study. these two projects were reviewed in respect of wind tunnel test and its contribution. As a conclusion, it is suggested that scale model wind tunnel tests can be a complementary tool to calculation codes which were evaluated worse than expected.

• 환경영향평가
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• 제16권2호
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• pp.157-168
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• 2007

In case there is a need to run the multi-year urban scale air qulaity model, it is a difficult task due to the computational demand, requiring the statistical approach for the long time atmospheric environmental assessment. In an effort to approach toward long term urban assessment, the sixteen synoptic meteorological conditions are statistically classified from the estimated geostrophic wind speeds and directions of 850 hPa geopotential height field during 2000 ~ 2005. The geostrophic wind directions are subdivided into four even intervals (north, east, south, and west), geostrophic wind speeds into two classes(${\leq}5m/s$ and >5m/s), and daily mean cloud amount into 2 classes(${\leq}5/10$ and >5/10), which result into sixteen classes of the synoptic meteorological cases for each season. The frequency distributions for each 16 synoptic meteorological case are examined and some discussions on how these synoptic classifications can be used in the environmental assessment are presented.

• Wind and Structures
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• 제24권2호
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• pp.119-144
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• 2017

The traditional method for the simulation of high Reynolds number (Re) effects on wind loads on cooling tower models in wind tunnels focuses only on the mean wind pressure distribution. Based on observed effects of some key factors on static/dynamic flow characteristics around cooling towers, the study reported in this paper describes a comprehensive simulation method using both mean and fluctuating wind pressure distributions at high Re as simulation targets, which is indispensable for obtaining the complete full-scale wind effects in wind tunnels. After being presented in this paper using a case study, the proposed method is examined by comparing the full covariance matrices and the cross-spectral densities of the simulated cases with those of the full-scale case. Besides, the cooling tower's dynamic structural responses obtained using the simulated wind pressure fields are compared with those obtained by using the full-scale one. Through these works, the applicability and superiority of the proposed method is validated.

• 천문학회보
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• 제35권2호
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• pp.75.1-75.1
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• 2010

The nature and origin of the intergalactic magnetic field (IGMF) are an outstanding problem of cosmology, yet they are not well understood. Measuring Faraday rotation (RM) is one of a few promising methods to explore the IGMF. We have theoretically investigated RM using a model of the IGMF based on a MHD turbulence dynamo (Ryu et al. 2008; Cho et al. 2009). In the previous KAS meeting, we reported the results for the present-day local universe; for instance, the probability distribution function (PDF) of ${\mid}RM{\mid}$ follows the lognormal distribution, the root mean square (rms) value for filaments is ~1 rad m^{-2}, and the power spectrum peaks at ~1 h^{-1} Mpc scale. In this talk, we extend our study of RM; by stacking simulation data up to redshift z=5 and taking account of the redshift distribution of radio sources, we have reproduced an observable view of RM through filaments against background radio sources. Our findings are as follows. The inducement of RM is a random walk process, so that the rms of RM increases with increasing path length. The rms value of RM for filaments reaches several rad m^{-2}. The PDF still follows the lognormal distribution, and the power spectrum of RM peaks at less than degree scale. Our predictions of RM could be tested, for instance, with LOFAR, ASKAP, MEERKAT, and SKA.

• Steel and Composite Structures
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• 제43권5호
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• pp.639-660
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• 2022

The bending and buckling behaviours of FG-GRNC laminated sandwich plates are investigated by using novel five-variables quasi 3D higher order shear deformation plate theory by considering the modified continuum nonlocal strain gradient theory. To calculate the effective Young's modulus of the GRNC sandwich plate along the thickness direction, and Poisson's ratio and mass density, the modified Halpin-Tsai model and the rule of the mixture are employed. Based on a new field of displacement, governing equilibrium equations of the GRNC sandwich plate are solved using a developed approach of Galerkin method. A detailed parametric analysis is carried out to highlight the influences of length scale and material scale parameters, GPLs distribution pattern, the weight fraction of GPLs, geometry and size of GPLs, the geometry of the sandwich plate and the total number of layers on the stresses, deformation and critical buckling loads. Some details are studied exclusively for the first time, such as stresses and the nonlocality effect.