• 한국수자원학회논문집
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• 제44권6호
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• pp.461-470
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• 2011

Abrupt and gradual levee breach analyses on the flat domain were implemented by laboratory experiments and numerical simulations. To avoid the reflective wave from the side wall the experiment was performed in a large domain surrounded by waterway. A numerical model was developed for solving the two-dimensional gradual levee breach flow. The results of the numerical simulation developed in this study showed good agreement with those of the experimental data. However, even if the numerical schemes effectively replicated the trends of the observed water depth for the first shock, there were little differences for the second shock. In addition, even though the model considered the Smagorinsky horizontal eddy viscosity, the location and height of the hydraulic jump in the numerical simulation were not fairly well agree with experimental measurements. This shows the shallow water equation solver has a limitation which does not exactly reproduce the energy dissipation from the hydraulic jump. Further study might be required, considering the energy dissipation due to the hydraulic jump or transition flow from reflective wave.

• Wind and Structures
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• 제34권1호
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• pp.151-159
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• 2022

The aerodynamics of blunt bodies with separation at the sharp corner of the leading edge and reattachment on the body side are particularly important in civil engineering applications. In recent years, a number of experimental and numerical studies have become available on the aerodynamics of a rectangular cylinder with chord-to-thickness ratio equal to 5 (BARC). Despite the interest in the topic, a widely accepted set of guidelines for grid generation about these blunt bodies is still missing. In this work a new, well resolved Direct Numerical Simulation (DNS) around the BARC body at Re=3000 is presented and its results compared to previous DNSs of the same case but with different numerical approaches and mesh. Despite the simulations use different numerical approaches, mesh and domain dimensions, the main discrepancies are ascribed to the different grid spacings employed. While a more rigorous analysis is envisaged, where the order of accuracy of the schemes are kept the same while grid spacings are varied alternately along each spatial direction, this represents a first attempt in the study of the influence of spatial resolution in the Direct Numerical Simulation of flows around elongated rectangular cylinders with sharp corners.

• 한국통신학회논문지
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• 제34권12A호
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• pp.949-961
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• 2009

본 논문에서는 하이브리드 복호 후 전달(HDF)기법을 이용하는 2홉 무선 통신 네트워크의 BER(Bit Error Rate) 성능을 분석한다. 기존의 HDF 기법은 릴레이에서 복호된 데이터의 전달유무를 수신되는 신호 대 잡음비(SNR)를 기준으로 한다. 반면 본 논문에서는 SNR 기준 방식 대신 대수 우도비(Log Likelihood Ratio, LLR) 분석 기법을 제안한다. 제안한 새로운 HDF 기법의 근사적 BER 표현식을 유도하며, Monte-Carlo 시뮬레이션으로 검증한다. 또한 HDF 기법을 위한 최적의 임계값을 계산한다. 다양한 수식으로 인한 결론은 본 논문에서 제안하는 LLR기반 HDF가 기존의 SNR기반 HDF보다 레일리 페이딩채널과 AWGN환경에서 어떠한 임계값과 어떠한 릴레이 위치에도 뛰어난 성능을 보임을 증명한다.

• IEIE Transactions on Smart Processing and Computing
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• 제3권5호
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• pp.285-297
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• 2014

This paper presents the results of an investigation into bi-directional communication in spectrum sharing-based cognitive radio (Bi-CR) systems. A Bi-CR system can increase the spectral efficiency significantly by sharing the spectrum and through the bi-directional use of spatial resources for two-way communication. On the other hand, the primary user experiences more interference from the secondary users in a Bi-CR system. Satisfying the interference constraint by simply reducing the transmission power results in performance degradation for secondary users. In addition, secondary users also experience self-interference from echo channels due to full duplexing. These imperfections may weaken the potential benefits of the Bi-CR system. Therefore, a new way to overcome these defects in the Bi-CR system is needed. To address this need, this paper proposes some novel power allocation schemes for the Bi-CR system. This contribution is based on two major analytic environments, i.e., noise-limited and interference-limited environments, for providing useful analysis. This paper first proposes an optimal power allocation (OPA) scheme in a noise-limited environment and then analyzes the achievable sum rates. This OPA scheme has an effect in the noise-limited environment. In addition, a power allocation scheme for the Bi-CR system in an interference-limited environment was also investigated. The numerical results showed that the proposed schemes can achieve the full duplexing gain available from the bi-directional use of spatial resources.

• 정보처리학회논문지C
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• 제18C권2호
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• pp.97-102
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• 2011

게임 이론은 다수의 사용자에게 공유 자원을 효율적이고 공정하게 분배할 수 있는 이론적 기반을 제공하므로 이를 대역폭 할당에 이용하려는 연구가 활발히 진행되어 왔다. 그러나 게임 이론에 의한 대역폭 할당 기법들은 적용한 게임 기법들에 따라 동일 조건에서 할당하는 대역폭이 달라진다. 그러나 현재까지 네트워크 분야에서는 게임 이론의 수학적 근거만을 차용하고 있으며 이들이 대역폭 할당 문제에 적용될 때 각 기법 별 특성에 대한 정량적 비교 분석은 이루어지고 있지 않다. 이에 따라 본 논문에서는 다수의 노드가 대역폭을 공유하는 경우 파산(Bankruptcy) 게임 이론과 협상(Bargaining) 게임 이론을 이용하여 송신 노드의 입력율에 따른 각 노드 별 대역폭 할당 기법들의 특성을 비교 분석하고 이에 따른 향후 연구 방향을 제시한다.

• 한국전자파학회논문지
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• 제17권1호
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• pp.8-16
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• 2006

본 논문에서는 관심 주파수의 파장에 비하여 매우 섬세한 구조의 해석에 있어서 기존의 FDTD보다 효율적인 해석이 가능한 ADI-FDTD의 차분방정식 계산 방법에 대하여 연구하였다. 반복 구간에서 계산되는 서로 다른 계산 방법을 유도하고 두 개의 상보적인 2차원 문제를 해석하여 경계 조건의 적용 관점에서 그 특성을 논하였다.

• Journal of Communications and Networks
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• 제17권4호
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• pp.394-405
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• 2015

This paper considers a wideband cognitive radio network which can simultaneously sense multiple narrowband channels and thus aggregate the detected available channels for transmission and proposes a novel cognitive radio system that exhibits improved sensing throughput and can save power consumption of secondary user (SU) compared to the conventional cognitive radio system studied so far. More specifically, under the proposed cognitive radio system, we study the problem of designing the optimal sensing time and power allocation strategy, in order to maximize the ergodic throughput of the proposed cognitive radio system under two different schemes, namely the wideband sensing-based spectrum sharing scheme and the wideband opportunistic spectrum access scheme. In our analysis, besides the average interference power constraint at primary user, the average transmit power constraint of SU is also considered for the two schemes and then a subgradient algorithm is developed to obtain the optimal sensing time and the corresponding power allocation strategy. Finally, numerical simulations are presented to verify the performance of the two proposed schemes.

• Journal of Communications and Networks
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• 제18권4호
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• pp.619-628
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• 2016

This paper describes a theoretical framework for rate allocation to maximize the power efficiency of overall heterogeneous wireless networks whose users are assumed to have multihoming capabilities. Therefore, the paper first presents a power consumption model considering the circuit power and radio transmission power of each wireless network. Using this model, two novel power efficient rate allocation schemes (PERAS) for multihoming services are proposed. In this paper, the convex optimization problem for maximizing the power efficiency over wireless networks is formulated and solved while guaranteeing the required quality of service (QoS). Here, both constant bit rate and variable bit rate services are considered. Furthermore, we extend our theoretical framework by considering the sleep-operation management of wireless networks. The performance results obtained from numerical analysis reveal that the two proposed schemes offer superior performance over the existing rate allocation schemes for multihoming services and guarantee the required QoS.

• 물과 미래
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• 제29권1호
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• pp.203-212
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• 1996

특성곡선이 통과하는 인접 시간준위에서의 3개 격자점을 이용한 시간준위 Lagrange 보간기법을 개발하여, 기존의 시간준위 보간기법인 Reachback 기법과 두가지 공간 보간기법, 즉 Upwind 기법과 3개의 공간 격자점을 이용한 공간 2차 보간기법과 함께 두가지 형태의 오염원 이송문제에 각각 적용 하므로써 공간 및 시간준위 보간기법에 의한 수치해의 정확도를 비교, 분석하였다. 본 연구결과에 의하면 동일한 차수의 보간기법을 비교할 경우, 공간보간 기법보다 시간 보간기법을 이용하면 수치분산 또는 수치진동이 적은 보다 정확한 수치해를 구할 수 있는 것으로 나타났다. 한편, 본 연구에서 제시한 기법은 기존 시간준위 보간기법에 의한 해의 장점을 포함하면서 정확도를 향상시킬 수 있는 것으로 나타났다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.723-728
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• 2008

We performed numerical analysis of base drag phenomenon, when a projectile with backward step flies into atmosphere at supersonic speed. We compared with other researchers. From our previous studies that were 2-dimensional simulation, we found out from sophisticated simulations that need dense mesh points to compare base pressure and velocity profile after from base with experimental data. Therefore, we focus on high order spatial disceretization over 3rd order with TVD such as MUSCL TVD 3rd, 5th, and WENO 5th order, and Limiters such as minmod, Triad. Moreover, we enforce to flux averaging schemes such as Roe, RoeM, HLLE, AUSMDV. In present, one dimensional result of Euler tests, there are Sod, Lax, Shu-Osher and interacting blast wave problems. AUSMDV as a flux averaging scheme with MUSCL TVD 5th order as spatial resolution is good agreement with exact solutions than other combinations. We are carrying out the same approaches into 3-dimensional base flow only candidate flux schemes that are Roe, AUSMDV. Additionally, turbulence models are used in 3-dimensional flow, one is Menter s SST DES model and another is Sparlat-Allmaras DES/DDES model in Navier-Stokes equations.