• Journal of the Korean Society of Propulsion Engineers
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• v.13 no.3
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• pp.27-33
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• 2009

A computational study has been performed out to evaluate the effect of a vertical porous barrier on the pressure oscillations in a supersonic cavity. The porous barriers with different perforations were vertically installed into a rectangular cavity at Mach numbers 1.50, 1.83 and 2.50. TVD finite difference MUSCL scheme was employed to solve the two-dimensional, unsteady, compressible Navier-Stokes equations. The present vertical porous barrier considerably altered the characteristics of the time-dependent shear layers that occur at the upstream edge of cavity and remarkably reduced the pressure oscillations inside the supersonic cavity. The present results showed that the effectiveness of passive control using the present porous vertical barrier is dependent on Mach number and the perforation of the porous barrier.

• Journal of the Korean housing association
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• v.20 no.1
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• pp.91-99
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• 2009

With the development of the internet and the spread of 'digital home', apartment houses equipped with the homepages connecting apartment houses as one cyberspace unit have increased. The virtual community with a homepage in apartment complex have improved the effect on management businesses in the side of a apartment manager and increased the importance of the space with a strong tie between residents in the side of residents. But the apartment residents didn't make full use of the virtual community because they have the low level of recognition of it. Therefore, this study attempted to present basic materials for the scheme to revitalize the virtual community. For this purpose, it attempted to find residents' recognition of and participation in the virtual community and demand for the classified contents conducted before this study. In the research method, it conducted the questionnaire research. The results are as follows. 1) First of all, It was found that the apartment residents had the low level of recognition of the virtual community. There was a significant difference according to the degree and the level of using the internet. Some easy-access programs are necessary. 2) The apartment residents took part in the virtual community about 67 percents because there was a lack of the public relation of the virtual community. 3) It was found that there was a significant positive correlation between the level of recognition and participation. 4) An attempt was made to identify the apartment residents' demand for the type of virtual community contents. It was found that most of contents were needed. 'apartment complex introduction and management information' showed the highest score among them. 5) Most respondents are affirmative to the virtual community. Therefore the residential community could be reinforced if the contents of virtual community is developed and well operated in the future.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.63.3-64
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• 2015

In our previous work (Kim et al., 2015), we suggested a new classification scheme, which categorizes the SPEs into four groups based on association with flare or CME inferred from onset timings as well as proton acceleration patterns using multienergy observations. In this study, we have tried to find whether there are any typical characteristics of associated events and acceleration sites in each group using 42 SPEs from 1997 to 2012. We find: (i) if the proton acceleration starts from a lower energy, a SPE has a higher chance to be a strong event (>5000pfu) even if the associated flare and CME are not so strong. The only difference between the SPEs associated with flare and CME is the location of the acceleration site. For the former, the sites are very low (~1Rs) and close to the western limb, while the latter has a relatively higher and wider acceleration sites. (ii) When the proton acceleration starts from the higher energy, a SPE tends to be a relatively weak event (<1000pfu), in spite of its associated CME is relatively stronger than previous group. (iii) The SPEs categorized by the simultaneous proton acceleration in whole energy range within 10 minutes, tend to show the weakest proton flux in spite of strong related eruptions. Their acceleration heights are very close to the locations of type II radio bursts. Based on those results, we suggest that the different characteristics of the four groups are mainly due to the different mechanisms governing the acceleration pattern and interval, and different condition such as the acceleration location.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.2
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• pp.317-324
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• 1994

The circulation of Lake Paldang is analysed numerically as an ultimate goal to develop a vehicle predicting the dispersion and concentration of pollutants and sediment flowed into the lake. In finite difference formulation of 2-D depth averaged governing equations. Abbott's 3-time level scheme is employed and for nonlinear terms time centering iteration technique in time and space is used. Model parameters for shear stresses and eddy diffusivities are determined through measured data in and near the lake. Predicted velocities for steady flow are shown to be close to the measured velocities and further improved by taking into account of wind effect. This indicates that the wind effect is needed for proper circulation analysis and it calls for the inclusion of the wind effect. Simulated results of unsteady flow caused by flood inflows and release through Paldang dam are found to characterize the flow features quite well as expected. This implies that the developed model can be used as a tool to analysing the circulation in the lake.

• The Journal of the Acoustical Society of Korea
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• v.14 no.1E
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• pp.5-13
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• 1995

In order to estimate the variability of the wave propagation loss (PL) du e to hourly variations of the sound speed profiles (SSPs), we conducted oceanographic measurements every hour for 39 hours in October 1993 in the Korea Strait. Currents and meteorological data were measured simultaneously to examine the causes of the temporal variations of temperatures. During the experiment, the temporal variations of temperatures in the surface layer highly depend on the water mass transport from adjacent seas. The PL for low frequency (75-300 Hz) is calculated using the parabolic equation scheme and averaged over the whole water depth. The hourly variation of the SSP may cause a PL difference of up to 10 dB over a 30-50 km range. The variability of PL, represented by standard deviation for the 39 SSPs, is as large as 3 dB over a 50 km range.

• Journal of the Korea institute for structural maintenance and inspection
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• v.23 no.7
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• pp.40-48
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• 2019

New concept of fine dust measurement method is suggested based on ultrasonic scattering. These days, fine dust has been social problem in Korea, and many researches has been conducted including the area structural maintenance. Conventional measurement system such as optical scattering and semiconductor has a limit from environmental factors like relative humidity. However, ultrasound is based on mechanical waves, which perturb mechanical properties of medium such as density and elastic constants. Using the advantage, the algorithm for fine dust measurement is derived and evaluated using 2-D finite difference method. The numerical analysis simulates ultrasonic wave propagation inside multiple scattering medium like fine dust in air. Signal processing scheme is also suggested and the results show that the error of the algorithm is around minimum of 0.7 and maximum of 24.9 in the number density unit. It is shown that cross-section of fine dust is a key parameter to improve the accuracy of algorithm.

• Journal of KIISE
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• v.44 no.9
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• pp.954-965
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• 2017

A stable power supply is very important for the maintenance and operation of the power infrastructure. Accurate power consumption prediction is therefore needed. In particular, a university campus is an institution with one of the highest power consumptions and tends to have a wide variation of electrical load depending on time and environment. For this reason, a model that can accurately predict power consumption is required for the effective operation of the power system. The disadvantage of the existing time series prediction technique is that the prediction performance is greatly degraded because the width of the prediction interval increases as the difference between the learning time and the prediction time increases. In this paper, we first classify power data with similar time series patterns considering the date, day of the week, holiday, and semester. Next, each ARIMA model is constructed based on the classified data set and a daily power consumption forecasting method of the university campus is proposed through the time series cross-validation of the predicted time. In order to evaluate the accuracy of the prediction, we confirmed the validity of the proposed method by applying performance indicators.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.5
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• pp.1304-1312
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• 1993

The buoyancy effects on mixed convection heat transfer over a flat plate surface with unheated starting length is reported. The governing equations are solved by a finite difference method using Patankar scheme and the solution was numerically obtained for various mixed convection parametr $Gr_{x}/Re_{x}^3$, and Prandtl number of 0.7 Local heat flux was measured by using Schilieren Interferometer. The local heat transfer results show that the presence of the unheated starting length can significantly accentuate the effects of buoyancy. The degree of accentuation of the buoyancy effects is strongly influenced by the magnitude of $Gr_{x}/Re_{x}^3$. When the parameter is larger than the order of $10^{-3}$, the contribution of natural convection to the heat transfer coefficients increased significantly due to the unheated starting length. In contrast, when $Gr_{x}/Re_{x}^3$ is smaller then about $10^{-5}$ , the buoyancy contribution is essentially unaffected by the unheated starting length. The shape of the velocity profile is also found to be highly responsive to the interaction between the buoyancy and the starting length.

• Journal of the Korean Society of Propulsion Engineers
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• v.13 no.1
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• pp.27-33
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• 2009

Shock tube flow measurement has been often hampered a finite opening time of diaphragm, but there is no systematic work to investigate its effect on the shock tube flow. In the present study, both the experimental and computational works have been performed on the shock tube flows at low pressure ratios. The computational analysis has been performed using the two-dimensional, unsteady, compressible Navier-Stokes equations, based upon a TVD MUSCL finite difference scheme. It is known that the present computational results reproduce the experimental data with good accuracy and simulate successfully the process of diaphragm opening as a function of time. The concept of an imaginary center is introduced to quantify the non-centered expansion wave due to a finite opening time of diaphragm. The results obtained show that the diaphragm opening time is reduced as the initial pressure ratio of shock tube increases, leading to the effect of a finite opening time of diaphragm to be more remarkable at low pressure ratios.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.39 no.8
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• pp.14-25
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• 2002

In this paper, we propose a modified AFC algorithm which is suitable for the implementation of WCDMA reverse link receiver modem. To reduce the complexity, the modified CP-FDD algorithm named 'Normalized CP-FDD' is applied to the AFC loop. The proposed FDD algorithm overcomes the conventional CP-FDD's sensitivity to the variance of input signal amplitude and increases the linear range of S -curve. Therefore, offset frequency estimation using the proposed scheme can be more stable than the conventional method. Unlike IS-95, since pilot symbol in WCDMA is not transmitted continuously, we introduce a moving average filter at the FDD input to increase the number of cross-product. So, tracking speed and stability are improved. For more rapid frequency acquisition and tracking, we adopt a multi-stage tracking mode. Using NCO having ROM table structure, the frequency offset is compensated. We applied the proposed algorithm in the implementation of WCDMA base station modem successfully.