• 한국가시화정보학회지
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• 제8권1호
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• pp.31-38
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• 2010

An experimental study to evaluate dynamic structures of flow and turbulence characteristics in bubble-driven liquid flow in a rectangular tank with a varying flow rate of compressed air is conducted. Liquid flow fields are measured by time-resolved particle image velocimetry (PIV) with fluorescent tracer particles to eliminate diffused reflections, and by an image intensifier to acquire enhanced clean particle images. Instantaneous vector fields are investigated by using the two frame cross-correlation function and bad vectors are eliminated by magnitude difference technique. By proper orthogonal decomposition (POD) analysis, the energy distributions of spatial and temporal modes are acquired. When Reynolds number increases, bubble-induced turbulent motion becomes dominant rather than the recirculating flow near the side wall. The total kinetic energy transferred to the liquid from the rising bubbles shows a nonlinear relation regarding the energy input because of the interaction between bubbles and free surface.

• 한국구조물진단유지관리공학회 논문집
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• 제5권1호
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• pp.195-205
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• 2001

An analytical finite element approach to nonlinear behavior of reinforced concrete shear walls with opening under monotonic loading was presented in this paper. In order to achieve the objectives of present paper, the orthogonal anisotropic models for cracked reinforced concrete element based on smeared crack concept were used as the nonlinear material models of biaxial state of stress. The stiffness of cracked concrete was evaluated through the combined use of tension and compression stiffness models in and parallel directions of crack, respectively and shear transfer effect due to the aggregate interlocking at crack surface. The stress and strain of reinforcement in concrete was evaluated using the average stress and average strain relation with bond effect. based on smeared crack concept. The diagonal reinforcing bar was modeled using truss element with bond effect. A special significance of diagonal reinforcement near opening was given to the shear wall with opening and an effective distribution of diagonal reinforcement was presented in order to give an ultimate strength increment as well as a crack control.

• Structural Engineering and Mechanics
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• 제69권1호
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• pp.95-104
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• 2019

The Standard ASCE 61-14 proposes the Substitute Structure Method (SSM) as a Nonlinear Static Procedure (NSP) to estimate nonlinear displacement demands at the center of mass of piers or wharves under seismic actions. To account for bidirectional earthquake excitation according to the Standard, results from independent pushover analyses in each orthogonal direction should be combined using either a 100/30 directional approach or a procedure referred to as the Dynamic Magnification Factor, DMF. The main purpose of this paper is to present an evaluation of these NSPs in relation to four wharf model structures on soil conditions ranging from soft to medium dense clay. Results from nonlinear static analyses were compared against benchmark values of relevant Engineering Design Parameters, EDPs. The latter are defined as the geometric mean demands that are obtained from nonlinear dynamic analyses using a set of 30 two-component ground motion records. It was found that SSM provides close estimates of the benchmark displacement demands at the center of mass of the wharf structures. Furthermore, for the most critical pile connection at a landside corner of the wharf the 100/30 and DMF approaches produced displacement, curvature, and force demands that were reasonably comparable to corresponding benchmark values.

• 대한기계학회논문집A
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• 제26권5호
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• pp.939-951
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• 2002

We propose a new multiscale Galerkin method based on interpolation wavelets for two-dimensional Poisson's and plane elasticity problems. The major contributions of the present work are: 1) full multiresolution numerical analysis is carried out, 2) general boundaries are handled by a fictitious domain method without using a penalty term or the Lagrange multiplier, 3) no special integration rule is necessary unlike in the (bi-)orthogonal wavelet-based methods, and 4) an efficient adaptive scheme is easy to incorporate. Several benchmark-type problems are considered to show the effectiveness and the potentials of the present approach. is 1-2m/s and impact deformation of the electrode depends on the strain rate at that velocity, the dynamic behavior of the sinter-forged Cu-Cr is a key to investigate the impact characteristics of the electrodes. The dynamic response of the material at the high strain rate is obtained from the split Hopkinson pressure bar test using disc-type specimens. Experimental results from both quasi-static and dynamic compressive tests are Interpolated to construct the Johnson-Cook model as the constitutive relation that should be applied to simulation of the dynamic behavior of the electrodes. The impact characteristics of a vacuum interrupter are investigated with computer simulations by changing the value of five parameters such as the initial velocity of a movable electrode, the added mass of a movable electrode, the wipe spring constant, initial offset of a wipe spring and the virtual fixed spring constant.

• 한국전자파학회논문지
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• 제15권7호
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• pp.692-697
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• 2004

OFDM(orthogonal frequency division multiplexing)통신 시스템은 다중 경로 간섭에 효과적이며, 대역효율이 우수한 통신 방식이다. 하지만, OFDM 신호는 높은 PAPR(peak to average power ratio)이 나타나므로 HPA(high power amplifier)에 의해 비선형 왜곡이 발생하게 된다. 본 연구는 HPA의 비선형을 보상하기 위해 predisotertor를 사용했을 때, IBO(input back off)에 따른 비선형 특성 변화를 연구하였다. SSPA(solid state power amplifier)에서 IBO=0 ㏈일 경우인 포화점 동작에서는 오히려 predistorter를 사용하지 않는 것이 시스템 성능이 더 좋았다. 하지만, IBO=3 ㏈일 때는 predistorter를 사용하는 시스템이 좋은 성능을 보인다. 즉, 적절한 back-off를 해야만 predistorter의 성능이 발휘된다. 결과적으로 본 연구에서는 predistorter의 이득이 발생되기 시작하는 threshold IBO를 OFDM 시스템의 sub-carrier 수와 SSPA의 비선형 파라메터 p 값에 따라 구하였다.

• 한국ITS학회 논문지
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• 제4권1호
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• pp.63-71
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• 2005

본 논문에서는 고속전송을 위해 넓은 대역폭을 사용하는 OFDM (Orthogonal Frequency Division Multiflex)시스템에서 심각한 주파수 선택적 페이딩 환경에서도 강한 특성을 갖는 FE(Frequency Estimation) 채널 추정 기법을 제안하였다. 시간적으로 천천히 변화하는 채널에서 주로 사용하는 채널추정기법은 상관도가 높은 심볼간 데이터를 이용하는 DDCE(Decision Directed Channel Estimation)기법이 있다. DDCE는 주파수 비선택적인 환경에서는 높은 이득을 보이지만 시스템 대역폭이 증가함에 따라 심각한 주파수 선택적 페이딩 환경이 되어 데이터의 신뢰도가 떨어지면 에러 플로우 현상이 일어나게 된다. 제안한 방법인 FE 기법은 주파수 선택적 페이딩 채널에서 OFDM의 부반송파간의 상관관계를 이용하여 목표 주파수에서의 채널 추정감을 인접 부반송파의 채널 추정값들의 평균으로 적응시켜주어 이득을 얻는다. FE 기법은 DDCE처럼 데이터를 이용하지 않고 프리엠블만을 이용하기 때문에 데이터의 신뢰도와 관계된 전송율과 다중 경로 수에 독립적이다 본 논문에서 제안한 FE 기법은 DDCE의 에러 플로우가 발생한 넓은 대역폭을 가진 시스템에서도 이득을 얻을 수 있어 심각한 주파수 선택적 페이딩 환경에서 유용하다.

• 대기
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• 제27권1호
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• pp.55-65
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• 2017

This study investigates how Eurasian snow cover in spring (March and April) is associated with Korean temperature during summer (June-July-August). Two leading modes of Eurasian snow cover variability in spring for 1979~2015 are obtained by Empirical Orthogonal Function (EOF) analysis. The first EOF mode of Eurasian snow cover is characterized by a zonally elongated pattern over the whole Eurasian region and its principal component is more correlated with Korean temperature during June. On the other hand, the second EOF mode of Eurasian snow cover is characterized by an east-west dipole-like pattern, showing positive anomalies over eastern Eurasian region and negative anomalies over western Eurasian region. This dipole-like pattern is related with Korean temperature during August. The first leading mode of Eurasian snow cover is associated with anomalous high (low) pressure over Korea (Sea of Okhotsk) during June, which might be induced by much evaporation of soil moisture in Eurasia during March. On the other hand, the second mode of Eurasian snow cover is associated with a wave train resembling with Eurasian (EU)-like pattern in relation to the Atlantic sea surface temperature forcing, leading to the anomalous high pressure over Korea during August. Understanding these two leading modes of snow cover in Eurasian continent in spring may contribute to predict Korean summer temperature.

• 한국수산과학회지
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• 제25권6호
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• pp.443-456
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• 1992

황해에서와 같은 선형의 양향성 대륙붕 해저 지형에서 저주파의 분산관계가 단주기$\cdot$단파까지 포함하는 장주기$\cdot$장파 경우에 대해 유도되었다. 선형의 양향성 대륙붕에서 장주기$\cdot$장파일 경우 Bessel 방정식이 유도되는데 비해 일반적인 경우에는 Hummer 방정식이 유도된다. Hummer 방정식의 해로 유도되는 confluent Hypergeometric 함수는 극한 경우에 여러 형태로 바뀐다. 단일한 대륙붕에서는 해안선과 수직한 방향의 대륵붕 규모가 Rossby deformation radius에 비해 많이 작을 때는 수평흐름의 수렴$\cdot$발산효과가 무시되지만 양향성 대륙붕에서는 수평흐름의 수린$\cdot$발산효과가 해안선과 수직한 방향의 대륙붕규모가 관계없이 파동역학에 결정적으로 중요하다. 수렴$\cdot$발산효과는 Kelvin 파를 포함시키며 대륙붕파의 파속을 감소시킨다. 끝으로 양향성 대륙붕의 비마찰 eigenfunction들의 직교가 증명되었다.

• Journal of electromagnetic engineering and science
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• 제9권1호
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• pp.17-24
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• 2009

In recent years, OFDMA(orthogonal frequency division multiple access) and SC-FDMA(Single Carrier Frequency Division Multiple Access) have been widely studied for the uplink of a mobile communication system. In this paper, PAPR(Peak-to-Average Power Ratio) and BER(Bit Error Rate) performance of the OFDMA and SC-FDMA systems are studied in relation to the uplink of a mobile communication system. Three kinds of sub-carrier allocation methods in the OFDMA system and 2 kinds of sub-carrier allocation methods in SC-FDMA system are suggested to compare and improve system performance. Simulation results show that in the OFDMA system, the first sub-band allocation method has better PAPR reduction performance than the other methods. In the SC-FDMA system, the distributed allocation method offers similar P APR, compared with the sub-band allocation method. P APR can be further reduced by adding a spectrum shaping filter with an appropriate roll of factor. Furthermore, it is found that on average, SC-FDMA can reduce the PAPR by more than 5 dB compared to OFDMA, when the total sub-carrier number is 1,024 and the sub-carrier number allocated to each user changes trom 8 to 512. Because of the frequency diversity and low PAPR characteristics, SC-FDMA system of the distributed sub-carrier allocation method can achieve better BER performance than the OFDMA system.

• 한국해안·해양공학회논문집
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• 제20권1호
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• pp.25-33
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• 2008

파랑은 주로 바람에 의해서 발생하여 많은 에너지를 해안으로 전달하며 각종 수리현상을 야기하고 물질의 이송 등에 의하여 연안환경 뿐만 아니라 인간의 활동에도 큰 영향을 미친다. 또한, 해안 구조물과 파랑의 상호작용에 의한 효과를 정확히 예측하는 것은 구조물의 설계 및 거동특성 파악에 매우 중요하다. 본 논문에서는 이층 유체에서 수표면과 저층에 설치되어 있는 얇은 연직벽에 의한 표면파와 내부파의 분산을 선형파 이론을 이용하여 이차원으로 해석하였다. 반사계수를 계산하여 여러 경우에 대하여 효과를 분석한 결과 반사계수는 구조물의 형상과는 별도로 경계층의 위치와 유체간의 밀도차에 크게 영향을 받는 것으로 밝혀졌다.