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Real-time prediction of urban flow and dispersion
Jaewook Nam,Changhoon Lee 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.10
We propose a CFD algorithm for real-time prediction of urban flow and dispersion based on large-eddy simulation (LES). To efficiently handle complex urban building geometry, we implement a modified immersed boundary method (IBM), which can be applied to bluff building boundary on a staggered grid. For an introduction of proper inflow condition, we apply a synthetic-eddy method to the periodic domain, which is necessary for direct solving of Poisson equation for pressure. All these implementations are conducted on GPU system for a significant reduction of calculation time for real-time prediction. For validation of our algorithm, we test our model in the prediction of flow and dispersion in urban area in Seoul against windtunnel experiment result and other simulation using fire dynamics simulator (FDS). Our model simulation results of flow and dispersion show good agreement with them. Simulation time is reasonably short to warrant real-time prediction of flow and dispersion.
An immersed boundary method for complex terrain flow analysis
Jaewook Nam,Changhoon Lee 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.4
We present modified immersed boundary method (IBM) for complex terrain flow analysis which is done on coarse grid and based on large-eddy simulation (LES). The main idea is that IBM force to make fluid velocity perpendicular to terrain surface zero and to revive tangential fluid velocity near the terrain surface is applied at Lagrangian point on terrain surface. For smoothing out the effect of IBM force on coarse grid, IBM force at Lagrangian point is distributed on collocated grid and gathered at the position of fluid velocity on staggered grid. For validation of our method, we conduct complex terrain flow analysis for mountainous terrain which is mock-up model of coastal region in Gyeongju and compare our simulation results with wind-tunnel experiment results and other simulation using ANSYS. Our simulation results show better agreement with wind-tunnel experiment results than ANSYS simulation results and properly capture various flow characteristics occuring in complex terrain.
Effect of Rheology in Film Formation Flows (Slot Coating)
Jaewook Nam(남재욱) 한국고분자학회 2021 한국고분자학회 학술대회 연구논문 초록집 Vol.46 No.1
Slot coating is a high-precision liquid coating method that effectively deposits thin liquid films onto a moving substrate. The coating layer is formed from the free-surface flows that started from the liquid bridge, called the coating bead, between the moving substrate and the stationary coating die. Therefore, the stability of the corresponding film formation flow, or so-called coating flow, is strongly affected by the force balance inside the coating bead. Here, the rheological properties of the coating liquid are important in determining desired or optimal operating conditions and die design through the force balance. In this study, we will show a systematic investigation of the impact of yield stress and shear thinning behavior on the gas-liquid interface of the film formation flow and the flow through the narrow channel, typically the size of 100 microns, in the slot coating.
남재욱(Jaewook Nam),이창훈(Changhoon Lee) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.12
In CFD, tremendous computation time for big problem like DNS having many mesh is always troublesome. To solve this problem, there are many strategy like OpenMP parallelization, MPI parallelization or OpenMP-MPI Hybrid. But, these method is restricted by CPU specification. To get big speed up, we need CPU which has hundreds of core, which is very unrealistic condition for general user. However, GPU has optimized structure for parallel processing. In case of general GPU like Geforce GTX 1080, it has thousands of core. In this research, I’d like to see the possibility of GPU parallelization for scientific computing.
Ethyleneoxy substituted methanofullerenes for acceptor materials in organic photovoltaic cells.
Nam, So Youn,Lee, Hak Sung,Jung, Jaewook,Lee, Jaemin,Shin, Won Suk,Moon, Sang-Jin,Lee, Changjin,Yoon, Sung Cheol American Scientific Publishers 2009 Journal of Nanoscience and Nanotechnology Vol.9 No.12
<P>Aiming at improving the phase behavior of the OPVCs, we have designed and synthesized a series of C60 and C70 derivatives, which showed high electron mobility, electron affinity, and good solubility. These methanofullerene acceptor materials having ethyleneoxy substituent, PCBEs, were synthesized from the reaction of fullerene C60 or C70 with each hydrizide. Device performances of OPVC using the mixture of P3HT and PCBEs as an active layer were measured under illumination of 100 mW/cm2 AM 1.5G simulated light. Pristine OPVC using P3HT:compound 3 showed 2.59% of power conversion efficiency, which is much higher than 1.65% of pristine P3HT:PCBM device. The electrochemical and photophysical properties of these methanofullerene derivatives were examined by using cyclicvoltammetry and UV-Vis spectroscopy, respectively. We revealed that ethyleneoxy substituted methanofullerenes can be used as a good candidate for acceptor materials in OPVCs.</P>
Kim, Jaewook,Nam, Y.U.,Lampert, M.,Ghim, Y.-C. IOP 2016 Nuclear fusion. Fusion nucléaire. &n.Illiga Vol.56 No.10
<P>A statistical method for the estimation of the spatial correlation lengths of Gaussian-shaped fluctuating signals with two measurement points is examined to quantitatively evaluate its reliability (variance) and accuracy (bias error). The standard deviation of the correlation value is analytically derived for randomly distributed Gaussian shaped fluctuations satisfying stationarity and homogeneity, allowing us to evaluate, as a function of fluctuation-to-noise ratios, the sizes of averaging time windows and the ratios of the distance between the two measurement points to the true correlation length, and the goodness of the two-point measurement for estimating the spatial correlation length. Analytic results are confirmed with numerically generated synthetic data and real experimental data obtained with the KSTAR beam emission spectroscopy diagnostic. Our results can be applied to Gaussian-shaped fluctuating signals where a correlation length must be measured with only two measurement points.</P>
Ha Jaewook,Oh Ye-Rim,Kang Eungu,Nam Hyo-Kyoung,Rhie Young-Jun,Lee Kee-Hyoung 대한소아내분비학회 2022 Annals of Pediatirc Endocrinology & Metabolism Vol.27 No.3
Purpose: The prevalence of adolescents with type 2 diabetes mellitus (T2DM) has rapidly increased in Korea over the past few decades with the increase in the number of obese adolescents. The single point insulin sensitivity estimator (SPISE) was recently introduced as a surrogate marker for insulin sensitivity to predict T2DM in adults. We aimed to determine risk factors for T2DM in obese adolescents, including SPISE. Methods: This retrospective study included 104 adolescents diagnosed with T2DM at Korea University Hospital between January 2010 and December 2020. We compared clinical and biochemical parameters and the SPISE of normoglycemic overweight and obese individuals with those of prediabetic and diabetic adolescents to determine risk factors for T2DM. Receiver operating characteristic analysis was performed with the Youden index to determine the cutoff point of SPISE. Results: Frequency of fatty liver and family history of T2DM were significantly higher and SPISE level was significantly lower in patients with T2DM than in normoglycemic overweight/obese and prediabetic adolescents (P<0.01). A family history of T2DM, fatty liver, and SPISE value below the cutoff point (4.49) were identified as significant risk factors for T2DM in multiple logistic regression analysis after controlling for age, sex, and body mass index standard deviation score (P<0.01). Conclusion: Family history of T2DM, fatty liver, and low SPISE (<4.49) are risk factors that can independently affect the occurrence of T2DM in obese adolescents. Among these risk factors, SPISE is a promising marker for predicting adolescent T2DM; careful monitoring of these individuals is needed to prevent progression to T2DM.