• Proceedings of the KSME Conference
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• 2003.11a
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• pp.916-921
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• 2003

In most industrial applications, the geometrical complexity is combined with the moving boundaries. These problems considerably increase the computational difficulties since they require, respectively, regeneration and deformation of the grid. As a result, engineering flow simulation is restricted. In order to solve this kind of problems the immersed boundary method was developed. In this study, the immersed boundary method is applied to the numerical simulation of stationary, rotating and oscillating cylinders in the 2-dimensional square cavity. No-slip velocity boundary conditions are given by imposing feedback forcing term to the momentum equation. Besides, this technique is used with a second-order accurate interpolation scheme in order to improve the accuracy of flow near the immersed boundaries. The governing equations for the mass and momentum using the immersed boundary method are discretized on the non-staggered grid by using the finite volume method(FVM). This study presents the possibility of the immersed boundary method to apply to the complex flow experienced in the industrial applications.

• The Korean Journal of Quaternary Research
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• v.24 no.2
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• pp.1-11
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• 2010

Energy Balance Model (EBM) was used to experiment the distribution of surface equilibrium temperature which responds to external forcing associated with the surface characteristics. Surface equilibrium temperature is calculated as sum of incoming solar radiation and latitudinal transport is balanced with outgoing infrared radiation. To treat incoming solar radiation, the source of the earth energy, significantly for energy balance, the experiment for surface equilibrium temperature distribution was performed considering the energy balance with the latitudinal albedo change as well as land and sea distribution. In addition, linear albedo change experiment, arctic albedo 5%, 10%, 15% change experiments and the opposite albedo change experiments between arctic and mid-latitudes were performed using incoming solar radiation as an external forcing. Moreover, with and without ice-albedo feedback experiments were performed. Increasing of arctic albedo is blocked out the incoming solar radiation so that it induces decreasing of latitudinal heat transport. It is strengthened energy transport from low latitudes by keeping arctic low energy states. Therefore the temperature change in the mid-latitudes exhibits larger response than that of arctic due to the difference of transport. The land which has lower heat capacity than sea can be reach to equilibrium temperature shortly. Also land is more sensitive to temperature change with respects to albedo. Thus it induces the thermal difference between land and sea. As a result, the equilibrium temperature exhibits differently as the difference of albedo and heat capacity which are the one of surface characteristics. Surface equilibrium temperature decreases as albedo increase and the ratio of temperature change is large as heat capacity is small. The decreasing of surface equilibrium temperature with respects to increasing of linear albedo is accelerated by ice-albedo feedback. However local change of surface equilibrium temperature decreases non-linearly.

• English Language & Literature Teaching
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• v.9 no.2
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• pp.17-33
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• 2003

College going students who are non-native speakers of English enrolled in English language programmes are not acquiring the needed academic writing skills. Many of these students do not have positive attitudes towards writing, thus forcing language instructors to look for ways of motivating students to write in order to improve writing skills. This action research project investigates the use of portfolio writing to improve writing ability among pre-university students. Research on the use of portfolio writing suggests that it is a useful way for developing interest in writing and for developing effective writing skills over a period of time. Portfolios support the best thinking in composition pedagogy in that it encourages process writing. Although the portfolio is considered a writing product, as a whole it is evidence of the students writing process. An important feature in using portfolios is that students are able to focus on their writing without constantly worrying about grades. Instructors have noticed that students make greater improvement in their writing when their focus is shifted from punitive feedback through letter grades to constructive feedback in the form of suggestions for further revision. This paper describes the use of writing portfolios as an effective means of teaching writing. The findings revealed that writing portfolios helped develop confidence in writing and decreased anxiety towards writing. (217 words)

• Journal of the Korean Solar Energy Society
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• v.27 no.2
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• pp.37-44
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• 2007

As photovoltaic(PV) power generation system becomes more common, it will be necessary to investigate islanding detection method for PV systems. Islanding of PV systems can cause a variety of problems and must be prevented. However, if the real and reactive power of the load and PV system are closely matched, islanding detection by Passive methods becomes difficult. Also, most active methods lose effectiveness when there are several PV systems feeding the same island. The active frequency drift positive feedback method(AFDPF) enables islanding detection by forcing the frequency of the voltage in the island to drift up or down. In this paper the research for the minimum value of chopping fraction gain applied digital phase-locked-loop (DPLL) to AFDPF considering output power quality and islanding prevention performance are performed by simulation and experiment according to IEEE Std 929-2000 islanding test.

• Proceedings of the KIEE Conference
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• 2005.07b
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• pp.1684-1686
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• 2005

As photovoltaic(PV) power generation systems become more common, it will be necessary to investigate islanding detection method for PV systems. Islanding of PV systems can cause a variety of problems and must be prevented. However, if the real and reactive power of load and PV system are closely matched, islanding detection by passive methods becomes difficult. Also, most active methods lose effectiveness when there are several PV systems feeding the same island. The active frequency drift positive feedback method(AFDPF) enables islanding detection by forcing the frequency of the voltage in the island to drift up or down. In this paper the research for the minimum value of chopping fraction gain applied digital phase-locked-loop(DPLL) to AFDPF considering output power quality and islanding prevention performance are performed by simulation and experiment in IEEE Std 929-2000 islanding test.

• Atmosphere
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• v.23 no.3
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• pp.283-291
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• 2013

This study investigates the changes in the atmosphere-ocean interactions over the South China Sea (SCS) by analyzing their variables in the period of 1979~2011 during the boreal summer (June-July-August). It is found that a simultaneous correlation coefficient between sea surface temperature (SST) and precipitation over SCS during summer is significantly changed before and after the late-1990s. That is, the variation of precipitation over SCS is negatively (positively) correlated with the SST variations before (after) the late-1990s. Our further correlation analysis indicates that the atmospheric forcing of the SST is dominant before the late-1990s accompanying with wind-evaporation feedback and cloud-radiation feedback. After the late-1990s, in contrast, the SST forcing of the atmosphere through the latent heat flux from the ocean to the atmosphere is dominant. It is found that the change in the relationship of atmosphere-ocean interactions over SCS are associated with the changes in the relationship with Northeast Asian summer precipitation. In particular, a simultaneous correlation coefficient between the precipitation over SCS and Northeast Asia becomes stronger during after the late-1990s than before the late-1990s. We argue that the increase of the SST forcing of the atmosphere over SCS may lead a direct relationship of precipitation variations between SCS and Northeast Asia after the late-1990s.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.7B
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• pp.944-950
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• 2001

현재 상용화된 CDMA (code division multiple access) 방식은 사용자가 증가함에 따라 다중 사용자 간섭에 의해 통화품질 향상 및 가입자 수용 용량 증대에 한계를 가지고 있다. 이러한 문제를 극복하기 위해 다양한 형태의 다중 사용자 검파 알고리즘이 제안되었다. 본 논문에서는 여러 형태의 알고리즘들 가운데, 역상관 (decorrelating) 검파기와 병렬 간섭 제거 (parallel interference cancellation, PIC) 검파기, 연속 간섭 제거 (successive interference cancellation, SIC) 검파기와 PIC 검파기, zero-forcing decision-feedback (ZF-DF) 검파기와 PIC 검파기를 하이브리드 형태로 구성하는 방법을 제시하였고, Rayleigh 페이딩 환경에서 성능분석 및 컴퓨터 시뮬레이션을 통한 성능 비교를 하였다. 컴퓨터 모의실험 결과, 본 논문에서 제안한 세 가지 하이브리드 검파기 모두 기존 검파기에 비하여 우수한 성능을 보였다.

• Proceedings of the IEEK Conference
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• 2000.09a
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• pp.963-966
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• 2000

현재 상용화된 CDMA (code division multiple access)방식은 사용자가 증가함에 따라 다중 사용자 간섭(multiuser interference)에 의해 통화품질 향상 및 가입자 수용 용량 증대에 한계를 가지고 있다. 이러한 문제를 극복하기 위해 다양한 다중 사용자 간섭 제거 알고리즘이 제안되었다 〔1-5〕. 본 논문에서는 여러 형태의 알고리즘들 가운데, 역상관 successive interference cancellation(SIC) , zero-forcing decision-feedback(ZF-DF) 검파기와 parallel interference cancellation(PIC) 검파기를 하이브리드 형태로 구성하는 방법을 제안하였고. 그에 따른 성능 분석 및 Rayleigh 페이딩 환경에서 컴퓨터 시뮬레이션을 통한 성능비교를 하였다. 컴퓨터 모의시험결과, 본 논문에서 제안한 3가지 하이브리드 검파기 모두 기존 검파기에 비하여 우수한 성능을 보였다.

• Journal of Mechanical Science and Technology
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• v.20 no.6
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• pp.849-863
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• 2006

IBM (Immersed Boundary Method) with feedback momentum forcing was applied to stationary and moving bodies. The capability of IBM to treat the obstacle surfaces, especially with moving effect has been tested for two dimensional problems. Stationary and oscillating cylinders were simulated by using IBM based on finite volume method with Cartesian coordinates. For oscillating cylinder, lateral and vertical motions are considered, respectively. Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. Also, the instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios well represented those of previous researches. More feasibility study for IBM has been carried out to two oscillating cylinders. Drag and lift coefficients are presented for two cylinders oscillating sinusoidally with phase difference of $180^{\circ}$.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.2A
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• pp.170-176
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• 2009

It has been known that multiuser MIMO systems have better performance than single-user MIMO systems. However, multiuser MIMO systems should eliminate inter-user interferences which are generated by allocating data to multiple users simultaneously There is zero-forcing beamforming (ZFBF) as scheme used widely among algorithms to eliminate inter-user interferences. But, it needs many feedback bits since BS knows quite exact channel state information to use this scheme in real systems. Random beamforming (RBF) was proposed to cope with a defect of ZFBF[1]. RBF is a multiuser scheme to send data to users who have optimal performance with predetermined codebook, each receiver feeds back a index of codeword which has optimal performance within the codebook and its CQI information. [1] assumes that the BS knows perfect CQI information of each receiver but CQI information should be quantized in the real systems. Therefore, in this paper, efficient CQI quantization scheme for RBF system is proposed.