• Journal of Environmental Impact Assessment
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• v.20 no.4
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• pp.575-583
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• 2011

In this study, the plausible grid size was estimated to increase for efficiency of reservoir management using 3 dimensional water quality model. To validate utilization of a real time water quality management tool, ELCOM-CAEDYM model was applied to Soyang reservoir in korea. 100m grid size can represent the real topography and take out exact analysis results. $400{\times}400m$ grid can be easily used to analysis because of data capacity. Consequently, the grid size of 200m or 300m was recommended to establish 3D model considering the required simulation time and the irrelevance between horizontal grid size and vertical distribution for temperature and turbidity analysis.

• Atmosphere
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• v.16 no.4
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• pp.359-370
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• 2006

Terrain height variance spectra for the Korean mountain region are calculated in order to determine an adequate grid size required to resolve terrain forcing on mesoscale model simulation. One-dimensional spectral analysis is applied to specifically the central-eastern part of the Korean mountain region, where topographical-scale forcing has an important effect on mesoscale atmospheric flow. It is found that the terrain height variance spectra in this mountain region has a wavelength dependence with the power law exponents of 1.5 at the wavelength near 30 km, but this dependence is steeply changed to 2.5 at the wavelength less than 30 km. For the adequate horizontal grid size selection on mesoscale simulation two-dimensional terrain height spectral analysis is also performed. There is no directionality within 50% of spectral energy region, so one-dimensional spectral analysis can be reasonably applied to the Korea Peninsula. According to the spectral analysis of terrain height variance, the finer grid size which is higher than 6 km is required to resolve a 90% of terrain variance in this region. Numerical simulation using WRF (Weather Research and Forecasting Model) was performed to evaluate the effect of different terrain resolution in accordance with the result of spectral analysis. The simulated results were quantitatively compared to observations and there was a significant improvement in the wind prediction across the mountain region as the grid space decreased from 18 km to 2 km. The results will provide useful guidance of grid size selection on mesoscale topographical simulation over the Korean mountain region.

• Atmosphere
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• v.21 no.4
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• pp.391-404
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• 2011

This study investigates the impact of cumulus parameterization scheme (CPS) with different horizontal grid sizes on the simulation of the local heavy rainfall case over the Korean Peninsula. The Weather Research and Forecasting (WRF)-based real-time forecast system of the Joint Center for High-impact Weather and Climate Research (JHWC) is used. Three CPSs are used for sensitivity experiments: the BMJ (Betts-Miller-Janjic), GD (Grell-Devenyi ensemble), and KF (Kain-Fritsch) CPSs. The heavy rainfall case selected in this study is characterized by low-level jet and low-level transport of warm and moist air. In 27-km simulations (DM1), simulated precipitation is overestimated in the experiment with BMJ scheme, and it is underestimated with GD scheme. The experiment with KF scheme shows well-developed precipitation cells in the southern and the central region of the Korean Peninsula, which are similar to the observations. All schemes show wet bias and cold bias in the lower troposphere. The simulated rainfall in 27-km horizontal resolution has influence on rainfall forecast in 9-km horizontal resolution, so the statements on 27-km horizontal resolution can be applied to 9-km horizontal resolution. In the sensitivity experiments of CPS for DM3 (3-km resolution), the experiment with BMJ scheme shows better heavy rainfall forecast than the other experiments. The experiments with CPS in 3-km horizontal resolution improve rainfall forecasts compared to the experiments without CPS, especially in rainfall distribution. The experiments with CPS show lower LCL(Lifted Condensation Level) than those without CPS at the maximum rainfall point, and weaker vertical velocity is simulated in the experiments with CPS compared to the experiments without CPS. It means that CPS suppresses convective instability and influences mainly convective rainfall. Consequently, heavy rainfall simulation with BMJ CPS is better than the other CPSs, and even in 3-km horizontal resolution, CPS should be applied to control convective instability. This conclusion can be generalized by conducting more experiments for a variety of cases over the Korean Peninsula.

• Journal of Ocean Engineering and Technology
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• v.28 no.4
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• pp.324-330
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• 2014

In this study, the numerical model developed by Hong et al.(2008) was improved to be applied to rapidly varying flows such as the inundation of dry land or flow transitions due to large gradients of the bathymetry. A numerical approximation was applied that was consistent with the conservation of momentum in flow expansions and with the Bernoulli equation in flow contractions. The approximation was second order, but the accuracy reduced to first order near extreme values by the use of a minmod limiter. The modified model was verified by acomparison with the theoretical critical depth of weir, and for sufficiently smooth conditions and a fine grid size, both approximations converged to the same solution. In terms of the grid size, it was more effective at obtaining solutions than the previous model and reproduced the inundation of dry land.

• Environmental Engineering Research
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• v.21 no.2
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• pp.171-179
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• 2016

This study analyzes guidelines to select optimum number of grids to represent behavior of a given water system appropriately. The EFDC model was chosen as a 3-D hydrodynamic and water quality model and salt was chosen as a surrogate variable of pollutant. The model is applied to an artificial canal that receives salt water from coastal area and fresh water from a river from respective gate according to previously developed gate operation rule. Grids are subdivided in vertical and horizontal (longitudinal) directions, respectively until no significant changes are found in salinity concentrations. The optimum grid size was determined by comparing errors in average salt concentrations between a test grid systems against the most complicated grid system. MSE (mean squared error) and MAE (mean absolute error) are used to compare errors. The CFL (Courant-Friedrichs-Lewy) number was used to determine the optimum number of grid systems for the study site though it can be used when explicit numerical method is applied only. This study suggests errors seem acceptable when both MSE and MAE are less than unity approximately.

• Tunnel and Underground Space
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• v.17 no.5
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• pp.350-359
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• 2007

Most large cut slopes of open pit mines, roadways, and railways are steeply inclined and composed with rocks that do not contain soils. However, these rock slopes suffer both weathering and fragmentation. In the case of steep slopes, falling rock and collapse of a slope may often occur due to surface erosion. Cast-in place concrete and rubble work are the most widely used earth structure-based pressure supports that act as restraints against the collapse of the rock slope. In order to overcome the shortcomings of conventional retaining walls, a segmental grid retaining wall is being used with connects precasted segments to construct the wall. In this study, laboratory model test was conducted to estimate deformation behavior of segmental grid retaining wall with configuration of rear strecher, height and inclination of the wall. In order to examine the behavior characteristics of a segmental grid retaining wall, this research analyzes the aspects of spacial displacement through relative displacement according to change in the inclination of the wall. Also, the walls behavior according to the formation and status of the rear stretcher which serves the role of transferring the load from the header and the stretcher which make up the wall, the displacement of backfill materials in the wall, and the location of the maximum load were surveyed and the characteristics of displacement in the segmental grid retaining wall were observed. The test results of the segmental grid retaining wall showed that there was a sudden increase in failure load according to the decrease in the wall's height and the size of the in was greatly decreased. Furthermore, it revealed that with identical inclination and height, the structure of the rear stitcher did not greatly affect the starting point or size of maximum horizontal displacement, but rather had a stronger effect on the inclination of the wall.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.1
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• pp.23-29
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• 2017

In this study, a post-correlation analysis for shaking table test of square water storage tank is presented for the use of advances in earthquake-resistant design of liquid storage tank. For this purpose, the ANSYS CFX program is selected for the CFD analysis. Sensitivity analysis for resonant sloshing motion in terms of grid size and turbulence model suggested that (1) horizontal grid size as well as vertical grid size is a key variable in the sloshing analysis, and (2) the SST turbulence model is best for the sloshing analysis. Finally, correlation analyses for a non-resonant harmonic input and scaled earthquake excitation of the El Centro (1940) NS component are carried out using the grid and turbulence model established through the post-correlation analysis for the resonant motion. As a result, sloshing time histories by the CFD analysis agreed very well with the test results.

• Atmosphere
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• v.22 no.1
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• pp.1-12
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• 2012

The Atmospheric motion vectors (AMVs) derived using infrared (IR) channel imagery of geostationary satellites have been utilized widely for real-time weather analysis and data assimilation into global numerical prediction model. As the horizontal resolution of sensors on-board satellites gets higher, it becomes possible to identify atmospheric motions induced by convective clouds ($meso-{\beta}$ and $meso-{\gamma}$ scales). The National Institute of Meteorological Research (NIMR) developed the high resolution visible (HRV) AMV algorithm to detect mesoscale atmospheric motions including ageostrophic flows. To retrieve atmospheric motions smaller than $meso-{\beta}$ scale effectively, the target size is reduced and the visible channel imagery of geostationary satellite with 1 km resolution is used. For the accurate AMVs, optimal conditions are decided by investigating sensitivity of algorithm to target selection and correction method of height assignment. The results show that the optimal conditions are target size of 32 km ${\times}$ 32 km, the grid interval as same as target size, and the optimal target selection method. The HRV AMVs derived with these conditions depict more effectively tropical cyclone OMAIS than IR AMVs and the mean speed of HRV AMVs in OMAIS is slightly faster than that of IR AMVs. Optimized mesoscale AMVs are derived for 6 months (Feb. 2010-Jun. 2010) and validated with radiosonde observations, which indicates NIMR's HRV AMV algorithm can retrieve successfully mesoscale atmospheric motions.

• Journal of Korea Water Resources Association
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• v.47 no.9
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• pp.767-776
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• 2014

Changes in climate have largely increased concentrated heavy rainfall, which in turn is causing enormous damages to humans and properties. Floods are one of the most deadly and damaging natural disasters known to mankind. The flood forecasting and warning system concentrates on reducing injuries, deaths, and property damage caused by floods. Therefore, the exact relationship and the spatial variability analysis of hydrometeorological elements and characteristic factors is critical elements to reduce the uncertainty in rainfall-runoff model. In this study, grid resolution depending on the topographic factor in rainfall-runoff models presents how to respond. semi-distribution of rainfall-runoff model using the model GRM simulated and calibrated rainfall-runoff in the Gamcheon and Naeseongcheon watershed. To run the GRM model, input grid data used rainfall (two event), DEM, landuse and soil. This study selected cell size of 500 m(basic), 1 km, 2 km, 5 km, 10 km and 12 km. According to the resolution of each grid, in order to compare simulation results, the runoff hydrograph has been made and the runoff has also been simulated. As a result, runoff volume and peak discharge which simulated cell size of DEM 500 m~12 km were continuously reduced. that results showed decrease tendency. However, input grid data except for DEM have not contributed increase or decrease runoff tendency. These results showed that the more increased cell size of DEM make the more decreased slope value because of the increased horizontal distance.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.1
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• pp.17-25
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• 2020

In this study, three-dimensional fluid flow analyses have been performed in order to investigate the performance characteristics of a horizontal axis tidal turbine (HATT) by solving three-dimensional Reynolds-averaged Navier-Stokes equations utilizing the shear-stress-transport turbulence model. The computational domain for the flow analysis has been composed of hexahedral grids, and the grid dependency test has been carried out so as to determine the optimum grid size. Performance characteristics of the HATT have been investigated in consideration of the effects of hub nose geometry, inflow angle, and the tower. It has been found that the power output can be enhanced along with an increase of the ratio of the length to the diameter of the turbine nose, and the power of HATT has been reduced by approximately 10% when the primary fluid flow had an inflow angle of 15°. The power output of downstream HATT is found to be lower than that of the upstream HATT by about 1%.