• 한국전자파학회논문지
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• 제13권7호
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• pp.667-678
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• 2002

본 논문에서는 구형 빔 패턴 형성을 위한 다층 이차원 원형 도체 배열을 갖는 새로운 방사 구조를 제안하였다. 새로운 방사 구조는 방사 원형 도파관 위에 유한하게 적층된 원형 도체 배열 소자들이 무한적, 주기적 평면 배열 구조를 갖는다. 이론적 해석은 rigid full-wave 해석 방법으로 배열 구조의 각 영역에서의 전자장에 대한 모드 전개식과 원형 도체상의 전류에 대한 모드 전개식에 바탕을 두고 상세히 수행되었으며, 함수의 직교성, 모드 정합 방법, 경계조건 그리고 Galerkin 방법을 사용하여 선형 대수 방정식 시스템을 유도하였다. 또한, Gauss 소거법을 사용하여 배열 특성 계산에 필요한 미지의 진폭 계수들을 얻었다. 제안된 알고리듬은 Ka대역에서 $\pm$20$^{\circ}$의 빔 폭을 갖는 구형 빔 패턴 형성을 위한 배열 설계에 사용되었으며, 일반적인 응용을 위해 파장으로 정규화된 최적화 설계 변수들을 제시하였다. 시뮬레이션 결과와 실험 결과들을 서로 비교하기 위해, 대칭적으로 19개의 방사 소자를 갖는 Ka 대역 실험 시제품을 제작하였다. 방사 원형 도파관 개구면 위에 적층된 원형 도체 배열 구조는 얇은 필름상에 이온-빔 증착 방법을 사용하여 구현되었다. 계산된 단위소자 패턴들과 시제품의 측정된 단위소자 패턴들은 빔 스캔 범위 내에서 거의 일치함을 보여주었으며, 사이드 로브 레벨과 그레이팅 로브 레벨에 대한 결과 분석도 이루어졌다 또한, 정 방향에서 다층 원형 도체 구조에 의해 생길 수 있는 blindness 현상에 대하여 언급하였다. 제작된 시제품의 입력 VSWR은 1.14 보다 작았으며, 29.0 GHz, 29.5 GHz 그리고 30 GHz에서 측정된 이득은 각각 10.2 dB, 10.0 dB 그리고 10.7 dB 였다. 실험 및 시뮬레이션 결과들은 제안된 다층 원형 도체 배열 구조가 효율적인 구형 빔 패턴을 형성할 수 있음을 보여 주었다.능성을 시도하였고, 그 결과는 다음과 같다. 1. Cholesterol을 제거한 cheese의 제조에서 최적조건은 균질압력 1200psi(70kg$cm^2$), 균질온도 $70^{\circ}$, $\beta$-cyclodextrin 첨가량 2%였으며, 이때 우유의 cholesterol의 제거율이 86.05%로 가장 높게 나타났다. 2. Cholesterol을 제거한 cheese들의 수율은 모두 12.53%(control 10.54%) 이상으로 균질 처리가 cheese의 수율을 18.88%이상 향상시키는 것으로 나타났다. 3. 유지방 함량 23.80%인 control 치즈의 cholesterol 함량은 81.47mg/100g이었고, 균질압력 1200psi(91kg/$cm^2$)에 $\beta$-cyclodextrin 2%를 첨가한 cheese에서는 cholesterol 함량이 20.15mg/100g으로 cholesterol 제거율이 75.27%로 가장 높게 나타났다. 4. Meltability는 균질압력 1200psi(91kg/$cm^2$)에 $\beta$-cyclodextrin 1과 2%로 처리한 치즈에서 2.25cm(control 3.34cm)로 가장 낮았으며, 균질압력이 증가할수록 meltability가 감소하여 치즈의 품질을 저하시켰다. 5. Control 치즈의 stretchability는 30cm 이상 늘어나 가장 양호한 수치인 5점을 나타낸 반면, cholesterol을 제거한 cheese에서는 5~10cm 사이를 나타내어 2점으로 stretchability가 저하된 것을 볼 수 있었다. 6. Oiling off는 균질압력 1200psi(91kg/$cm^2$)에 $\beta$-cyclodextrin 1과 2%로 처리한 치즈에서

• Journal of Communications and Networks
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• 제11권1호
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• pp.26-35
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• 2009

In this paper, we investigate how to do resource allocation to guarantee a minimum user data rate at low signaling overhead in multi-cell orthogonal frequency division multiple access (OFDMA) wireless systems. We devise dynamic resource allocation (DRA) algorithms that can minimize the QoS violation ratio (i.e., the ratio of the number of users who fail to get the requested data rate to the total number of users in the overall network). We assume an OFDMA system that allows dynamic control of frequency reuse factor (FRF) of each sub-carrier. The proposed DRA algorithms determine the FRFs of the sub-carriers and allocate them to the users adaptively based on inter-cell interference and load distribution. In order to reduce the signaling overhead, we adopt a hierarchical resource allocation architecture which divides the resource allocation decision into the inter-cell coordinator (ICC) and the base station (BS) levels. We limit the information available at the ICC only to the load of each cell, that is, the total number of sub-carriers required for supporting the data rate requirement of all the users. We then present the DRA with limited coordination (DRA-LC) algorithm where the ICC performs load-adaptive inter-cell resource allocation with the limited information while the BS performs intra-cell resource allocation with full information about its own cell. For performance comparison, we design a centralized algorithm called DRA with full coordination (DRA-FC). Simulation results reveal that the DRA-LC algorithm can perform close to the DRA-FC algorithm at very low signaling overhead. In addition, it turns out to improve the QoS performance of the cell-boundary users, and achieve a better fairness among neighboring cells under non-uniform load distribution.

• 한국추진공학회지
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• 제23권1호
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• pp.33-45
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• 2019

본 연구에서는 좌, 우로 배치된 대칭형 핀틀 노즐의 고도 변화와 핀틀 위치 변화에 따른 특성을 파악하기 위해 수치해석을 수행하였다. 핀틀 노즐 형상은 선행연구를 수행한 직선형 핀틀 노즐을 사용하였고, 연소실 경계조건은 추진제 연소특성을 고려하였다. 해석을 수행할 유동해석 프로그램으로 사각노즐, 핀틀 노즐, 고고도 조건의 검증해석을 수행하여 적절한 해석기법을 설정하였다. 핀틀 위치는 full close, half open, full open 의 3가지 서로 다른 노즐 목 크기조건을 설정하였고, 고도는 0, 5, 20 km 조건을 설정하였다. 각 조건별 추력과 핀틀의 구동하중, 정적 안정성을 비교하여 연구를 수행하였다.

• Journal of Power Electronics
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• 제17권5호
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• pp.1137-1149
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• 2017

In order to reduce the cost, improve the efficiency and simplify the complicated control of existing isolated LED drivers, an improved boundary conduction mode (BCM) Buck ac-dc light emitting diode (LED) driver with extended output voltage and low total harmonic distortion is proposed. With a coupled inductor winding and a stacked output, its output voltage can be elevated to a much higher value when compared to that of the conventional Buck ac-dc converter, without sacrificing the input harmonics and power factor. Therefore, the proposed Buck LED driver can meet the IEC61000-3-2 (Class C) limitation and has a low THD. The operating principle of the topology and the design methodology of the ac-dc LED driver are presented. A 150 W ac-dc prototype was built in the laboratory and it shows that the input current harmonics meet the lighting standard. In addition, the THD is less than 16% at a typical ac input. The peak efficiency is higher than 96.5% at a full load and a normal input.

• 대한조선학회논문집
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• 제30권2호
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• pp.66-75
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• 1993

캐비테이션 터널에서 모형선 혹은 부분모형선을 사용하여 3차원 반류분포를 재현시키고자 할 때, 유동조절체를 사용하여 터널 위벽효과(Tunnel wall effect)를 감소시키는 경우가 있다. 유동조절체가 선미후류 유동에 미치는 영향과 터널 위벽효과를 해석하기 위하여 직사각형 단면의 시험부에 설치되어 있는 모형선과 유동조절체 주위 유동을 표면양력판 이론을 사용하여 해석하였다. Sydney Express 모형선 주위 유동에 대한 계산결과에 의하면 선체 표면 압력분포에 미치는 캐비테이션 터널의 위벽효과는 막음비(Blockage)가 5% 이내인 경우에는 무시할 정도이며, 막음비가 20% 이상인 경우에는 상당히 큼을 알 수 있다. 유동조절체를 선미 부근 터널벽에 설치함으로써 선미 유동중 축방향속도가 증가되었으며, 선체표면에서의 압력구배(pressure gradient)는 선미 경계층의 두께가 덜 증가되는 방향으로 변화되었음을 알 수 있었다. 유동조절체를 설치하여 재현된 반류분포는 등속도곡선의 폭이 좁아지기 때문에 추정된 실선반류 분포를 재현하기 위하여 유동조절체를 사용하는 경우가 있다. 표면양력판에 의해 계산된 반류분포는 이상유체 가정을 토대로 계산되었기 때문에 계측된 반류분포와의 차이를 보정하기 위해서는 경계층 계산이나 점성유동계산이 필요하다.

• Structural Monitoring and Maintenance
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• 제5권2호
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• pp.231-242
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• 2018

Subway tunnel structure has been rapidly developed in many cities for its strong transport capacity. The model-based damage detection of subway tunnel structure is usually difficult due to the complex modeling of soil-structure interaction, the indetermination of boundary and so on. This paper proposes a new data-based method for the damage detection of subway tunnel structure. The root mean square acceleration and cross correlation function are used to derive a statistical pattern recognition algorithm for damage detection. A damage sensitive feature is proposed based on the root mean square deviations of the cross correlation functions. X-bar control charts are utilized to monitor the variation of the damage sensitive features before and after damage. The proposed algorithm is validated by the experiment of a full-scale two-rings subway tunnel lining, and damages are simulated by loosening the connection bolts of the rings. The results verify that root mean square deviation is sensitive to bolt loosening in the tunnel lining and X-bar control charts are feasible to be used in damage detection. The proposed data-based damage detection method is applicable to the online structural health monitoring system of subway tunnel lining.

• Smart Structures and Systems
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• 제14권6호
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• pp.1173-1196
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• 2014

Real-time dynamic substructuring tests have been conducted on a cable-deck system. The cable is representative of a full scale cable for a cable-stayed bridge and it interacts with a deck, numerically modelled as a single-degree-of-freedom system. The purpose of exciting the inclined cable at the bottom is to identify its nonlinear dynamics and to mark the stability boundary of the semi-trivial solution. The latter physically corresponds to the point at which the cable starts to have an out-of-plane response when both input and previous response were in-plane. The numerical and the physical parts of the system interact through a transfer system, which is an actuator, and the input signal generated by the numerical model is assumed to interact instantaneously with the system. However, only an ideal system manifests a perfect correspondence between the desired signal and the applied signal. In fact, the transfer system introduces into the desired input signal a delay, which considerably affects the feedback force that, in turn, is processed to generate a new input. The effectiveness of the control algorithm is measured by using the synchronization technique, while the online adaptive forward prediction algorithm is used to compensate for the delay error, which is present in the performed tests. The response of the cable interacting with the deck has been experimentally observed, both in the presence of delay and when delay is compensated for, and it has been compared with the analytical model. The effects of the interface delay in real-time dynamic substructuring tests conducted on the cable-deck system are extensively discussed.

• Wind and Structures
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• 제11권2호
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• pp.75-96
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• 2008

A better understanding of tornado-induced wind loads is needed to improve the design of typical structures to resist these winds. An accurate understanding of the loads requires knowledge of near-ground tornado winds, but observations in this region are lacking. The first goal of this study was to verify how well a CFD model, when driven by far field radar observations and laboratory measurements, could capture the flow characteristics of both full scale and laboratory-simulated tornadoes. A second goal was to use the model to examine the sensitivity of the simulations to various parameters that might affect the laboratory simulator tornado. An understanding of near-ground winds in tornadoes will require coordinated efforts in both computational and physical simulation. The sensitivity of computational simulations of a tornado to geometric parameters and surface roughness within a domain based on the Iowa State University laboratory tornado simulator was investigated. In this study, CFD simulations of the flow field in a model domain that represents a laboratory tornado simulator were conducted using Doppler radar and laboratory velocity measurements as boundary conditions. The tornado was found to be sensitive to a variety of geometric parameters used in the numerical model. Increased surface roughness was found to reduce the tangential speed in the vortex near the ground and enlarge the core radius of the vortex. The core radius was a function of the swirl ratio while the peak tangential flow was a function of the magnitude of the total inflow velocity. The CFD simulations showed that it is possible to numerically simulate the surface winds of a tornado and control certain parameters of the laboratory simulator to influence the tornado characteristics of interest to engineers and match those of the field.

• Wind and Structures
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• 제25권1호
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• pp.79-100
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• 2017

The use of wind energy resources is developing rapidly in recent decades. There is an increasing number of wind farms in high wind-velocity areas such as the Pacific Rim regions. Wind turbine towers are vulnerable to tropical cyclones and tower failures have been reported in an increasing number in these regions. Existing post-disaster failure case studies were mostly performed through forensic investigations and there are few numerical studies that address the collapse mode simulation of wind turbine towers under strong wind loads. In this paper, the wind-induced failure analysis of a conventional 65 m hub high 1.5-MW wind turbine was carried out by means of nonlinear response time-history analyses in a detailed finite element model of the structure. The wind loading was generated based on the wind field parameters adapted from the cyclone boundary layer flow. The analysis results indicate that this particular tower fails due to the formation of a full-section plastic hinge at locations that are consistent with those reported from field investigations, which suggests the validity of the proposed numerical analysis in the assessment of the performance of wind-farms under cyclonic winds. Furthermore, the numerical simulation allows to distinguish different failure stages before the dynamic collapse occurs in the proposed wind turbine tower, opening the door to future research on the control of these intermediate collapse phases.