• Wind and Structures
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• 제24권3호
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• pp.223-247
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• 2017

By using numerical simulation, vast and detailed information and observation of the physics of flow over a train model can be obtained. However, the accuracy of the numerical results is questionable as it is affected by grid convergence error. This paper describes a systematic method of computational grid refinement for the Unsteady Reynolds Navier-Stokes (URANS) of flow around a generic model of trains using the OpenFOAM software. The sensitivity of the computed flow field on different mesh resolutions is investigated in this paper. This involves solutions on three different grid refinements, namely fine, medium, and coarse grids to investigate the effect of grid dependency. The level of grid independence is evaluated using a form of Richardson extrapolation and Grid Convergence Index (GCI). This is done by comparing the GCI results of various parameters between different levels of mesh resolutions. In this study, monotonic convergence criteria were achieved, indicating that the grid convergence error was progressively reduced. The fine grid resolution's GCI value was less than 1%. The results from a simulation of the finest grid resolution, which includes pressure coefficient, drag coefficient and flow visualization, are presented and compared to previous available data.

• 한국항공우주학회지
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• 제34권4호
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• pp.53-62
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• 2006

초음속 주 유동으로의 수직분사에 의해 생성되는 2차원 정상상태 유동장에 대한 수치모사를 수행하였다. 난류효과를 위해서 무차원 벽면거리($y^+$)를 고려한 2방정식 k-${\omega}$ SST 모델을 사용하였다. 또한 격자계에 따른 오차범위를 나타내는 방법으로 GCI(Gird Convergence Index)를 사용하여 해의 수렴성을 측정하였다. 표면 압력분포, 박리거리, 침투높이 등에 대해 실험결과 및 다른 난류모델을 이용한 결과들과 비교하였다. k-${\omega}$ SST 난류모델은 낮은 압력비에 대해서 표면 압력분포 및 박리거리 등을 정확하게 예측하였다. 그러나 압력비가 증가함에 따라 수치적 예측이 실험결과와 차이를 보이고 있다. 상기한 모든 결과는 격자계에 따른 해의 수렴성의 오차범위 1% 이내에서 측정되었다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2005년도 추계 학술대회논문집
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• pp.287-292
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• 2005

The flowfields generated by gaseous slot injection into a supersonic flow at a Mach number of 3.75 and a Reynolds number of $2.07{\times}10^7$ are simulated numerically. Fine-scale turbulence effects are represented by a two-equation(k-w SST model) closure model which includes $y^+$ effects on the turbulence model. Grid convergence index(GCI) is also considered to provide a measure of uncertainty of the grid convergence. Comparison is made with experimental data and other turbulence model in term of surface static pressure distributions, the length of the upstream separation region, and the penetration height. Results indicate that the k-w SST model correctly predicts mean surface pressure distribution and upstream separation length. However, it is also observed that the numerical simulation over predicts the pressure spike and penetration height compared with experimental data. All these results are taken within $1\%$ error band of grid convergence.

• 한국컴퓨터정보학회논문지
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• 제24권9호
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• pp.83-95
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• 2019

Recently, a variety of IoT data is collected by attaching geosensors to many vehicles that are on the road. IoT data basically has time and space information and is composed of various data such as temperature, humidity, fine dust, Co2, etc. Although a certain sensor data can be retrieved using time, latitude and longitude, which are keys to the IoT data, advanced search engines for IoT data to handle high-level user queries are still limited. There is also a problem with searching large amounts of IoT data without generating indexes, which wastes a great deal of time through sequential scans. In this paper, we propose a unified spatial index model that handles both grid and trajectory queries using a cell-based space-filling curve method. also it presents a visualization method that helps user grasp intuitively. The Trajectory query is to aggregate the traffic of the trajectory cells passed by taxi on the road searched by the user. The grid query is to find the cells on the road searched by the user and to aggregate the fine dust. Based on the generated spatial index, the user interface quickly summarizes the trajectory and grid queries for specific road and all roads, and proposes a Web-based prototype system that can be analyzed intuitively through road and heat map visualization.

• International journal of advanced smart convergence
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• 제12권1호
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• pp.1-8
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• 2023

Smart IoT over high speed network and high performance smart devices explodes the ubiquitous services and applications. Nearest Neighbor(NN) query is one of the important type of queries that have to be supported for ubiquitous information services. In order to process efficiently NN queries in the wireless broadcast environment, it is important that the clients determine quickly the search space and filter out NN from the candidates containing the search space. In this paper, we propose a hybrid index of Voronoi and grid partition to provide quick search space decision and rapid filtering out NN from the candidates. Grid partition plays the role of helping quick search space decision and Voronoi partition providing the rapid filtering. We show the effectiveness of the proposed index by comparing the existing indexing schemes in the access time and tuning time. The evaluation shows the proposed index scheme makes the two performance parameters improved than the existing schemes.

• 한국ITS학회 논문지
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• 제19권6호
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• pp.107-117
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• 2020

최근 자율주행의 발달로 차량에 장착된 다양한 센서가 일반화 되고 그 센서에서 발생되는 빅 데이터는 교통 분야에서 활용도가 높아지고 있다. 본 연구에서는 이러한 교통 빅 데이터의 활용을 위해 실시간으로 발생되는 차량 센싱 빅 데이터와 도로 기상 등 공공데이터를 지도상에 효율적으로 맵핑하기 위한 그리드 인덱스 기법을 제안하였으며, 제안한 그리드 공간 분할 방식과 그리드 ID 부여 방식에 대하여 적용 가능성 및 효과를 분석하였다. 차량 센서에서 실시간 분석된 강수 데이터를 전국 화물차의 디지털 운행기록장치(DTG, Digital Tachograph) 데이터를 기반으로 가상 생성하여 좌표기반으로 맵핑하였으며, 제안 방식과 링크 단위 처리방식의 처리 속도를 비교하였다. 제안 방식은 링크 단위의 처리 방식 대비 약 2,400배 이상의 데이터 처리 성능 개선을 나타냈다. 추가로 그리드 맵핑의 적용 가능성 및 링크 단위 맵핑과의 차별성을 확인하고자 가상 생성한 데이터를 시각화하고 비교하였다.

• ETRI Journal
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• 제30권6호
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• pp.853-855
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• 2008

This paper presents an energy-efficient spatial join algorithm for multiple sensor networks employing a spatial semijoin strategy. For optimization of the algorithm, we propose a GR-tree index and a grid-ID-based spatial approximation method, which are unique to sensor networks. The GR-tree is a distributed spatial index over the sensor nodes, which efficiently prunes away the nodes that will not participate in a spatial join result. The grid-ID-based approximation provides great reduction in communication cost by approximating many spatial objects in simpler forms. Our experiments demonstrate that the algorithm outperforms existing methods in reducing energy consumption at the nodes.

• 전력전자학회논문지
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• 제26권2호
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• pp.135-140
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• 2021

This study introduces a new pulse width modulation (PWM) method to reduce common-mode voltages (CMVs) and then compares its performance with other reduced CMV-PWM (RCMV-PWM) methods. CMVs should be reduced to ensure the electromagnetic compatibility and safety of grid-connected inverters. RCMV-PWM methods attempt to synthesize voltage references without zero vectors, which cause high CMV peaks. In these methods, the peak-to-peak magnitude of CMVs can be reduced by one-third of the conventional space-vector PWM. The introduced method splits every reference vector into two vectors to avoid the use of zero vectors. The performances of the RCMV-PWM methods are analyzed in accordance with the modulation index through simulation and experiment.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2021년도 학술발표회
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• pp.134-134
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• 2021

Precipitation is a crucial component of water cycle and play a key role in hydrological processes. Traditionally, gauge-based precipitation is the main method to achieve high accuracy of rainfall estimation, but its distribution is sparsely in mountainous areas. Recently, satellite-based precipitation products (SPPs) provide grid-based precipitation with spatio-temporal variability, but SPPs contain a lot of uncertainty in estimated precipitation, and the spatial resolution quite coarse. To overcome these limitations, this study aims to generate new grid-based daily precipitation using Automatic weather system (AWS) in Korea and multiple SPPs(i.e. CHIRPSv2, CMORPH, GSMaP, TRMMv7) during the period of 2003-2017. And this study used a machine learning based Random Forest (RF) model for generating new merging precipitation. In addition, several statistical linear merging methods are used to compare with the results of the RF model. In order to investigate the efficiency of RF, observed data from 64 observed Automated Synoptic Observation System (ASOS) were collected to evaluate the accuracy of the products through Kling-Gupta efficiency (KGE), probability of detection (POD), false alarm rate (FAR), and critical success index (CSI). As a result, the new precipitation generated through the random forest model showed higher accuracy than each satellite rainfall product and spatio-temporal variability was better reflected than other statistical merging methods. Therefore, a random forest-based ensemble satellite precipitation product can be efficiently used for hydrological simulations in ungauged basins such as the Mekong River.

• Journal of information and communication convergence engineering
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• 제9권5호
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• pp.567-572
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• 2011

As the mobile devices have powerful CPUs and memories, it is possible to carry the large volumes of design drawings, such as DXF files, on mobile devices. However, since the format of a DXF file is the ASCII, it has the shortcoming where the size of a file is larger than a binary formatted file and it is difficult to display the entities of drawings quickly on mobile devices. In this paper, we design the DXF manager system on a smart phone. To speed up the rate of the display and panning, the DXF manager hires the spatial index, the fixed grid file. We also implement the system on the Samsung Omnia-1$^{(R)}$ to exploit the Windows Mobile$^{(R)}$ operating system.