• 천문학회보
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• 제46권2호
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• pp.51.1-51.1
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• 2021

Merger shocks with M_s < ~ 3 - 4 have been detected in galaxy clusters through radio observations of synchrotron radiations emitted from cosmic-ray (CR) electrons. The CR electrons are believed to be produced by the so-called diffusive shock acceleration (DSA) at the merger shocks. To describe the acceleration of electrons, the injection into DSA has to be understood. Recent studies have showed that electrons could be energized through stochastic shock drift acceleration (SSDA), a mechanism mediated by multi-scale plasma waves at shock transition zone. However, such preacceleration process seems to be effective only at the supercritical shocks with M_s > ~ 2.3, implying that further studies should be done to explain radio relics with weaker shocks. In this talk, we present the results obtained by fully kinetic 2D particle-in-cell (PIC) simulations, which include pre-existing suprathermal electrons possibly ejected from active galactic nuclei (AGNs) or produced by previous episodes of turbulence/shocks. The simulations indicate that the pre-existing electrons enhance the upstream plasma waves in shocks with M_s < ~ 2.3. However, the wavelength of such waves is not long enough to scatter off suprathermal electrons and energize them to the injection momentum for DSA. Hence, we conclude that preexciting suprathermal electrons alone would not solve the problem of electron acceleration at radio relic shocks.

• 한국정보처리학회논문지
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• 제2권1호
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• pp.66-74
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• 1995

이 논문에서는 고속 저전력 분야에 적용하기 위한 8비트, 15MHz A/D 변환기 설계 에 관해 기술한다. 2단 플래시 방식인 서브레인징 구조 A/D 변환기에서 칩 면적을 줄 이기 위해 저항의 수를 감소시킨 전압분할 회로를 설계하였다. 비교기는 80 dB의 이득, 50 MHz의 대역폭, 오프셋 전압이 0.5mV이고, 전압분할 회로의 최대오차는 1mV이다. 설계된 A/D변환기는 ＋5/-5V 공급 전압에 대해 전력소비가 150mW, 지연시간이 65ns 이다. A/D 변환기는 N-well공정을 이용하여 설계하고, 제작하였다. 제안된 변환기는 고속, 저전력, 소형 단일 칩 아날로그-디지탈 혼합 시스템 응용에 적합하다. 시뮬레이 션은 PSPICE를 이용하여 수행하였고, 1차 가공된 칩을 데스트 하였다.

• Ocean Systems Engineering
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• 제6권1호
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• pp.1-21
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• 2016

The global performance of the 5 MW OC4 semisubmersible floating wind turbine in random waves with or without steady/dynamic winds is numerically simulated by using the turbine-floater-mooring fully coupled dynamic analysis program FAST-CHARM3D in time domain. The numerical simulations are based on the complete second-order diffraction/radiation potential formulations along with nonlinear viscous-drag force estimations at the body's instantaneous position. The sensitivity of hull motions and mooring dynamics with varying wave-kinematics extrapolation methods above MWL(mean-water level) and column drag coefficients is investigated. The effects of steady and dynamic winds are also illustrated. When dynamic wind is added to the irregular waves, it additionally introduces low-frequency wind loading and aerodynamic damping. The numerically simulated results for the 5 MW OC4 semisubmersible floating wind turbine by FAST-CHARM3D are also extensively compared with the DeepCWind model-test results by Technip/NREL/UMaine. Those numerical-simulation results have good correlation with experimental results for all the cases considered.

• Computers and Concrete
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• 제31권1호
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• pp.71-84
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• 2023

In this paper, the effects of the thickness of cubic samples on the tensile strength of concrete blocks were studied using experimental tests in the laboratory and numerical simulation by the particle flow code in three dimensions (PFC3D). Firstly, the physical concrete blocks with dimensions of 150 mm×190 mm (width×height) were prepared. Then, three specimens for each of seven different samples with various thicknesses were built in the laboratory. Simultaneously with the experimental tests, their numerical simulations were performed with PFC3D models. The widths, heights, and thicknesses of the numerical models were the same as those of the experimental samples. These samples were tested with a new tensile testing apparatus. The loading rate was kept at 1 kg/sec during the testing operation. Based on these analyses, it is concluded that when the thickness was less than 5 cm, the tensile strength decreased by increasing the sample thickness. On the other hand, the tensile strength was nearly constant when the sample thickness was raised to more than 5 cm (which can be regarded as a threshold limit for the specimens' thickness). The numerical outputs were similar to the experimental results, demonstrating the validity of the present analyses.

• 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.

• Structural Engineering and Mechanics
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• 제38권5호
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• pp.633-649
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• 2011

The paper presents a study aimed at understanding some characteristics of an interior explosion within a room with limited or no venting. The explosion may occur in ammunition storage or result from a terrorist action or from a warhead that had penetrated into this room. The study includes numerical simulations of the problem and analytical derivations. Different types of analysis (1-D, 2-D and 3-D analysis) were performed for a room with rigid walls and the results were analyzed. For the 3D problem the effect of the charge size and its location within the room was investigated and a new insight regarding the pressure distribution on the interior wall as function of these parameters has been gained. The numerical analyses were carried out using the Eulerian multi-material approach. Further, an approximate analytical formula to predict the residual internal pressure was developed. The formula is based on the conservation law of total energy and its implementation yields very good agreement with the results obtained numerically using the complete statement of the problem for a wide range of explosive weights and room sizes that is expressed through a non-dimensional parameter. This new formula is superior to existing literature recommendations and compares considerably better with the above numerical results.

• Journal of electromagnetic engineering and science
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• 제18권2호
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• pp.108-116
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• 2018

In this paper, a method of designing a Vivaldi type phased array antenna (PAA) which operates at S-band (2.8-3.3 GHz) is presented. The presented antenna uses broadside coupled split ring resonators (BC-SRRs) for high isolation, wide field of view, and good active S-parameter characteristics. As an example, we design a $1{\times}8$ array antenna with various BC-SRR structures using theory and EM simulations. The antenna is fabricated and measured to verify the design. With the BC-SRR implemented between the two radiating elements, the isolation is shown to be enhanced by 6 dB, up to 23 dB. The scan angle is shown to be within ${\pm}53^{\circ}$ based on a -10 dB active reflection coefficient. The operation of the scan angle is possible within ${\pm}60^{\circ}$ with a little larger reflection coefficient (-7 dB to -8 dB). The proposed design with BC-SRRs is expected to be useful for PAA applications.

• Maxillofacial Plastic and Reconstructive Surgery
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• 제42권
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• pp.24.1-24.7
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• 2020

Background: In daily practice, three-dimensional patient-specific jawbone models (3D models) are a useful tool in surgical planning and simulation, resident training, patient education, and communication between the physicians in charge. The progressive improvements of the hardware and software have made it easy to obtain 3D models. Recently, in the field of oral and maxillofacial surgery, there are many reports on the benefits of 3D models. We introduced a desktop 3D printer in our department, and after a prolonged struggle, we successfully constructed an environment for the "in-house" fabrication of the previously outsourced 3D models that were initially outsourced. Through various efforts, it is now possible to supply inexpensive 3D models stably, and thus ensure safety and precision in surgeries. We report the cases in which inexpensive 3D models were used for orthodontic surgical simulation and discuss the surgical outcomes. Review: We explained the specific CT scanning considerations for 3D printing, 3D printing failures, and how to deal with them. We also used 3D models fabricated in our system to determine the contribution to the surgery. Based on the surgical outcomes of the two operators, we compared the operating time and the amount of bleeding for 25 patients who underwent surgery using a 3D model in preoperative simulations and 20 patients without using a 3D model. There was a statistically significant difference in the operating time between the two groups. Conclusions: In this article, we present, with surgical examples, our in-house practice of 3D simulation at low costs, the reality of 3D model fabrication, problems to be resolved, and some future prospects.

• 한국통신학회논문지
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• 제28권7C호
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• pp.688-695
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• 2003

본 논문에서는, 적응형 필터를 이용한 동기획득 시스템을 실제 환경에 효과적으로 사용하기 위해 두 가지 알고리즘을 제안한다. 첫째, 하나의 필터를 여러 개의 부 필터로 나누어 적응시키는 병렬 부필터 탐색 구조를 제시하여 실제 환경에서 요구되는 빠른 동기 획득 성능을 얻는다. 둘째, 적응 파라미터들을 수신 신호의 전력에 따라 조절해주는 파라미터 결정기법을 제안하여 이상적인 성능에서 큰 열화 없이 실제 환경에서 효율적으로 동작 가능하도록 한다. 제안된 시스템에 대해 통계적인 분석을 수행하고 모의 실험을 통해 평균 동기 획득 시간 성능을 검증한다. 병렬 부필터 탐색 구조를 적용한 시스템은 기존의 전체 필터 탐색 구조에 비해 16탭에서 1∼l.5dB, 64탭에서 5∼6dB의 성능 이득을 보이며, 제안된 파라미터 결정기법을 적용한 시스템은 16탭과 32탭에서 이상적인 성능에서 약 1∼2.5dB 정도의 열화만으로 실제 환경에서 효과적으로 동작함을 보여준다.

• 한국정보전자통신기술학회논문지
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• 제14권6호
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• pp.473-478
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• 2021

본 논문은 급증하는 무선통신, 자율주행자동차, 5G 무선통신 및 광대역 어플리케이션 등과 같이 다양한 방면에서 필요로 하는 광대역, 고이득 안테나 필요성에 따라 후면 주파수 선택 표면(FSS, Frequency Selective Surface)을 이용한 광대역 고이득 평면 사다리꼴 모노폴 안테나를 설계하였다. 제안된 안테나는 이중 금속층을 사용하여 기존의 주파수 선택 표면과 구조적인 차이점을 갖는다. 또한 기존의 안테나 설계에서 갖는 설계의 복잡함을 해결하기 위해 GA(Genetic Algorithms)과 HFSS(High Frequency Structure Simulator) 시뮬레이션을 사용하여 효율성을 증가시켰다. 이를 통해 3.52 GHz ~ 5.92 GHz의 넓은 대역폭과 전체 대역폭에서 10.5 dBi 이상의 이득을 유지하며, 5.1 GHz에서 11.8 dBi의 최고 이득을 갖는다. 기존 안테나와 비교하였을 경우 1.8 GHz의 36% 임피던스 대역폭이 2.4 GHz의 50% 임피던스 대역폭으로 향상되었으며, 이득의 경우 8.6 dBi 증가하는 것을 확인하였다.