• The KIPS Transactions:PartD
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• v.16D no.2
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• pp.273-280
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• 2009

Today, many services and products that used to be only provided on offline have been being provided on the web according to the improvement of computing environment and the activation of web usage. These web-based services and products tend to be provided to customer by customer's preferences. This paradigm that considers customer's opinions and features in selecting is called personalization. The related research field is a recommendation. And this recommendation is performed by recommender system. Generally the recommendation is made from the preferences and tastes of customers. And recommender system provides this recommendation to user. However, the recommendation techniques have a couple of problems; they do not provide suitable recommendation to new users and also are limited to computing space that they generate recommendations which is dependent on ratings of products by users. Those problems has gathered some continuous interest from the recommendation field. In the case of new users, so similar users can't be classified because in the case of new users there is no rating created by new users. The problem of the limitation of the recommendation space is not easy to access because it is related to moneywise that the cost will be increasing rapidly when there is an addition to the dimension of recommendation. Therefore, I propose the solution of the recommendation problem of new user and the usage of item quality as weight to improve the accuracy of recommendation in this paper.

• 한국해양학회지
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• v.29 no.2
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• pp.119-131
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• 1994

Vertical structures of wind-driven and tidal currents in a rectangular shaped uniform-depth basin of homogeneous water have been investigated using a mode-splitted, multi-level grid-box, hydrodynamic numerical model. The model was verified using analytical solutions for various vertical eddy viscosity profiles such as: a constant eddy viscosity, a linearly decreasing or increasing variation with depth, a quadratic variation with depth and an exponential variation with depth. Particular attention has been paid on the effects of "near-surface wall layer" on vertical shear of velocity. In numerical calculations, the whole water depth was divided into 13 levels with an unequal grid spacing. the model satisfactorily reproduces the velocity profile, but in case the eddy viscosity decreases rapidly with depth as in quadratical or exponential variation with depth, the vertical gradient of velocity near the bottom became very steep, and analytical solutions and numerical results showed some discrepancy. The vertical structures of horizontal velocity vary with both the depth-averaged value of eddy viscosity and its profiles. the velocity near the sea surface and near the bottom responded sensitively to the eddy viscosity of wall layer. For wind-driven current, the strong velocity shear was generated near the sea surface as eddy viscosity near the surface became small. For tidal current, the velocity above the sea bottom layer was almost constant regardless of the profiles of vertical eddy viscosity, but velocity in the sea bottom layer showed strong shear as eddy viscosity became small.

• Journal for History of Mathematics
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• v.25 no.4
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• pp.53-67
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• 2012

Today, it is necessary to calculate orbits with high accuracy in space flight. The key words of Poincar$\acute{e}$ in celestial mechanics are periodic solutions, invariant integrals, asymptotic solutions, characteristic exponents and the non existence of new single-valued integrals. Poincar$\acute{e}$ define an invariant integral of the system as the form which maintains a constant value at all time $t$, where the integration is taken over the arc of a curve and $Y_i$ are some functions of $x$, and extend 2 dimension and 3 dimension. Eigenvalues are classified as the form of trajectories, as corresponding to nodes, foci, saddle points and center. In periodic solutions, the stability of periodic solutions is dependent on the properties of their characteristic exponents. Poincar$\acute{e}$ called bifurcation that is the possibility of existence of chaotic orbit in planetary motion. Existence of near exceptional trajectories as Hadamard's accounts, says that there are probabilistic orbits. In this context we study the eigenvalue problem in early 20th century in three body problem by analyzing the works of Darwin, Bruns, Gyld$\acute{e}$n, Sundman, Hill, Lyapunov, Birkhoff, Painlev$\acute{e}$ and Hadamard.

• Solar Energy
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• v.14 no.2
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• pp.75-90
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• 1994

Thermal energy transport in a two-dimensional horizontal and vertical channel with an isothermal rectangular beam attached to one adiabatic wall is investigated from the numerical solution of Navier-Stokes and energy equations. The solutions have been obtained for dimensionless aspect equations. The solutions have been obtained for dimensionless aspect ratios of beam, H/B=$0.25{sim}4$, Reynolds numbers, Re=$50{\sim}500$ and Grashof numbers, Gr=$0{\sim}5{\times}10^4$. The mean Nusselt number, $\overline{Nu}$ for horizontal and vertical channels shows same value at Gr=0 and increases as Gr increases and decreases as H/B increases at Re=100. $\overline{Nu}$ of vertical channel shows higher in $0.25{\leq}H/B<1.1$ and lower in $1.1{\leq}H/B{\leq}4.0$ than that of horizontal channel at $Gr=10^4$, Re=100. $\overline{Nu}$ of vertical channel shows higher in $0.25{\leq}H/B<1.1$ and lower in $1.1{\leq}H/B=1.0$ than that of horizontal channel at Re=100, $0<Gr{\leq}5{\times}10^4$. A comparison between the experimental and numerical results shows good agreement.

• Journal of the Korean Geosynthetics Society
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• v.17 no.4
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• pp.119-128
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• 2018

Limit state design has been implemented in Korea since 2015; however, there exists no specification of lateral load determination on retaining wall due to the Korean standard traffic load on retaining wall's backfill surface. The lateral load from traffic depends on lane number, standard truck's axle loads and locations, loading distance from the inner wall. The concept of equivalent height of soil accounting for traffic loadings is typically used for design of retaining walls to quantify the traffic loads transmitted to the inner wall faces. Due to the different characteristics of the standard design trucks between Korea and US (AASHTO), the direct use of the guidelines from AASHTO LRFD leads to incorrect estimation of traffic load effects on retaining walls. This paper presents the results of evaluation of equivalent height of soil to reflect the Korean standard truck, based on the findings from analytical solutions using Bounessq's theory and numerical assessment using 2D finite element method. Consequently, it was found that the equivalent heights of soil from the Korean standard truck load were lower for lower retaining wall height.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.2
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• pp.107-120
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• 2021

Electric-powered distributed propulsion aircraft possess a complex wake flow and mutual interference with the airframe, due to the use of many propellers. Accordingly, in the early design stage, rapid aerodynamic and load analysis considering the effect of propellers for various configurations and flight conditions are required. In this study, an efficient panel-based aerodynamic analysis method that can take into account the propeller effects is developed and validated. The induced velocity field in the region of propeller wake is calculated based on Actuator Disk Theory (ADT) and is considered as the boundary condition at the vehicle's surface in the three-dimensional steady source-doublet panel method. Analyses are carried out by selecting an isolated propeller of the Korea Aerospace Research Institute (KARI)'s Quad Tilt Propeller (QTP) aircraft and the propeller-wing configuration of the former experimental study as benchmark problems. Through comparisons with the results of computational fluid dynamics (CFD) based on actuator methods, the wake velocity of propeller and the changes in the aerodynamic load distribution of the wing due to the propeller operation are validated. The method is applied to the analysis of the Optional Piloted Personal Aerial Vehicle (OPPAV) and QTP, and the practicality and validity of the method are confirmed through comparison and analysis of the computational time and results with CFD.

• Journal of the Korean Society of Marine Environment & Safety
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• v.28 no.1
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• pp.168-174
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• 2022

In a wave power generation system, the overtopping system is known as an overtopping wave energy converter (OWEC). The performance of an OWEC is affected by wave characteristics such as height and period because its power generation system is sensitive to those characteristics; these, as well as wave direction, depend on the sea. As these characteristics vary, it is hard for the OWEC to produce power in a stable manner. Therefore, it is necessary to find an appropriate shape for an OWEC, according to the characteristics of the sea it is in. This research verified the effect of the design of the OWEC ramp on the hydraulic efficiency using the smoothed particle hydrodynamics (SPH) particle method. A total of 10 models were designed and used in simulations performed by selecting the design parameters of the ramp and changing the attack angle based on those parameters. The hydraulic efficiency was calculated based on the rate of discharged water obtained from the analysis result. The effect of each variable on the overtopping performance according to the shape of the ramp was then confirmed. In this study, we present suggestions for determining the direction for an appropriately shaped OWEC ramp, based on a specific sea area.

• Journal of the Korean Geosynthetics Society
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• v.22 no.2
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• pp.1-12
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• 2023

Research on the causes of landslides and prediction of vulnerable areas is being conducted globally. This study aims to predict the effective soil depth, a critical element in analyzing and forecasting landslide disasters, using topographic information. Topographic data from various institutions were collected and assigned as attribute information to a 100 m × 100 m grid, which was then reduced through data grading. The study predicted effective soil depth for two cases: three depths (shallow, normal, deep) and five depths (very shallow, shallow, normal, deep, very deep). Three classification models, including K-Nearest Neighbor, Random Forest, and Deep Artificial Neural Network, were used, and their performance was evaluated by calculating accuracy, precision, recall, and F1-score. Results showed that the performance was in the high 50% to early 70% range, with the accuracy of the three classification criteria being about 5% higher than the five criteria. Although the grading criteria and classification model's performance presented in this study are still insufficient, the application of the classification model is possible in predicting the effective soil depth. This study suggests the possibility of predicting more reliable values than the current effective soil depth, which assumes a large area uniformly.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.2C
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• pp.71-79
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• 2009

This study is to develope the resilient modulus prediction model, which is the function of mean effective principal stress and axial strain, for three types of railroad trackbed materials such as crushed stone, weathered granite soil, and crushed-rock soil mixture. The model consists of the maximum Young's modulus and nonlinear values for higher strain, analogous to dynamic shear modulus. The maximum value is modeled by model parameters, $A_E$ and the power of mean effective principal stress, $n_E$. The nonlinear portion is represented by modified hyperbolic model, with the model parameters of reference strain, ${\varepsilon}_r$ and curvature coefficient, a. To assess the performance of the prediction models proposed herein, the elastic response of a test trackbed near PyeongTaek, Korea, was evaluated using a 3-D elastic multilayer computer program (GEOTRACK). The results were compared with measured elastic vertical displacement during the passages of freight and passenger trains at two locations, whose sub-ballasts were crushed stone and weathered granite soil, respectively. The calculated vertical displacements of the sub-ballasts are within the order of 0.6mm, and agree well with measured values. The prediction models are thus concluded to work properly in the preliminary investigation.

• Journal of Korea Water Resources Association
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• v.56 no.10
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• pp.667-678
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• 2023

In this study, quasi-3D inundation flow simulations were conducted for a simplified subway station configuration. The effects of variations in rainwater inflow locations and discharge were investigated, analyzing the resulting inundation flow patterns and flood risk. The inundation simulation results calculated the incipient velocities for slipping and toppling accidents to assess pedestrian safety. The results indicated that velocities exceeding the incipient velocity for slipping accidents mainly occurred on the flooded staircase. Meanwhile, velocities surpassing the incipient toppling accidents were observed around the staircase and the corridor near the staircase leading to B2F. This observation is consistent with the results from the specific force distribution analysis. To provide detailed flood risk assessments, the Flood Hazard Degree (FD) was applied with four levels of criteria, along with the Flood Intensity Factor (FIF). The results demonstrated that FD identified a broader area at risk of flood-induced consequences compared to FIF. When comparing the different inundation risk assessment methods, the specific force method tended to overestimate the risk area, whereas FIF tended to underestimate it. Furthermore, among all assessment methods, the influence of rainwater discharge was found to have a more dominant effect on flood risk assessment compared to the number of rainwater inflow locations. Additionally, the direction of inundation flow influenced the assessed risk, with collision-induced flow patterns leading to higher flood risk than those with identical flow directions.