• 천문학회보
• /
• 제38권1호
• /
• pp.36.1-36.1
• /
• 2013

I present a simple scheme for the treatment of gravitational interactions on galactic scales. In analogy with known mechanisms of quantum field theory, I assume ad hoc that gravitation is mediated by virtual exchange particles - gravitons - with very small but non-zero masses. The scheme predicts the asymptotic flattening of galactic rotation curves, the Tully-Fisher/Faber-Jackson relations, the mass discrepancy-acceleration relation of galaxies, and the surface brightness-acceleration relation of galaxies correctly; additional (dark) mass components are not required. The well-established empirical scaling laws of Modified Newtonian Dynamics follow naturally from the model. The scheme I present is not a consistent theory of gravitation; rather, it is a toy model providing a convenient scaling law that simplifies the description of gravity on galactic scales.

• Journal of Communications and Networks
• /
• 제16권2호
• /
• pp.227-237
• /
• 2014

Throughput scaling laws for two coexisting ad hoc networks with m primary users (PUs) and n secondary users (SUs) randomly distributed in an unit area have been widely studied. Early work showed that the secondary network performs as well as stand-alone networks, namely, the per-node throughput of the secondary networks is ${\Theta}(1/\sqrt{n{\log}n})$. In this paper, we show that by exploiting directional spectrum opportunities in secondary network, the throughput of secondary network can be improved. If the beamwidth of secondary transmitter (TX)'s main lobe is ${\delta}=o(1/{\log}n)$, SUs can achieve a per-node throughput of ${\Theta}(1/\sqrt{n{\log}n})$ for directional transmission and omni reception (DTOR), which is ${\Theta}({\log}n)$ times higher than the throughput with-out directional transmission. On the contrary, if ${\delta}={\omega}(1/{\log}n)$, the throughput gain of SUs is $2{\pi}/{\delta}$ for DTOR compared with the throughput without directional antennas. Similarly, we have derived the throughput for other cases of directional transmission. The connectivity is another critical metric to evaluate the performance of random ad hoc networks. The relation between the number of SUs n and the number of PUs m is assumed to be $n=m^{\beta}$. We show that with the HDP-VDP routing scheme, which is widely employed in the analysis of throughput scaling laws of ad hoc networks, the connectivity of a single SU can be guaranteed when ${\beta}$ > 1, and the connectivity of a single secondary path can be guaranteed when ${\beta}$ > 2. While circumventing routing can improve the connectivity of cognitive radio ad hoc network, we verify that the connectivity of a single SU as well as a single secondary path can be guaranteed when ${\beta}$ > 1. Thus, to achieve the connectivity of secondary networks, the density of SUs should be (asymptotically) bigger than that of PUs.

• 한국통신학회논문지
• /
• 제36권4B호
• /
• pp.422-428
• /
• 2011

본 논문에서는 n개의 균등하게 위치한 센서 노드를 가지는 수중 애드 혹 음향 네트워크에서의 시스템 및 채널 모델링, 그리고 이를 바탕으로 유도한 용량 스케일링 법칙을 소개한다. 특히 협 대역 모델에서 캐리어 주파수가 n의 함수로 스케일 되는 것을 허용하는 상황을 가정한다. 가정하는 네트워크에서, 전송 거리 뿐 아니라 주파수 스케일링에도 의존하는 감쇠 변수의 특성을 분석한다. 이를 바탕으로 단일 노드 밀도를 가지는 확장 네트워크 하에서 용량 스케일링에 대한 cut-set 기반 상향 경계선을 유도한다. 상향 경계선은 감쇠 변수에 역으로 비례한다는 결과물로부터, 전 영역에 대해 전력 제한된 네트워크가 생성됨을 보인다. 뿐만 아니라 간단한 최 근거리 다중 홉 전송에 기반 하여 용량 달성 가능한 기술을 소개한다. 확장 네트워크에서 다중 홉 기술은 감쇠 변수의 함수로 표현되는 모든 동작 영역에서 order 측면 최적임을 보인다.

• 제어로봇시스템학회:학술대회논문집
• /
• 제어로봇시스템학회 1993년도 한국자동제어학술회의논문집(국내학술편); Seoul National University, Seoul; 20-22 Oct. 1993
• /
• pp.1063-1067
• /
• 1993

Fuzzy logic is applied to a roll autopilot for missiles. Fuzzy rules are made so that the response duplicates that of the conventional control law for some flight condition. A scaling factor of the fuzzy controller is then scheduled by the missile velocity and altitude information to cope with the variation of the roll dynamics from that flight condition. By computer simulations and calculation of the stability margin, it is shown that the fuzzy control is robuster than the conventional one over the flight envelope even though two control laws work similarly for some flight conditions.

• 한국정보통신학회논문지
• /
• 제18권2호
• /
• pp.303-310
• /
• 2014

n개의 무선 노드가 랜덤하게 위치한 초 광 대역 애드 혹 네트워크에서의 개선된 용량 스케일링 법칙을 보인다. 먼저, 수정된 계층적 협력 기술이 사용되는 경우를 고려한다. 단위 면적의 밀집 네트워크에서, 유도한 용량 스케일링은 전력 제한된 성격 때문에 특정 동작 영역에서 경로손실 지수 ${\alpha}$에 의존함을 보인다. 또한, 계층적 협력 기술은 2 < ${\alpha}$ < 3에서 우월하지만 ${\alpha}{\geq}3$에 대해서는 다중 홉 라우팅을 사용하는 것이 더 높은 용량을 취득함이 밝혀진다. 둘째로, m개의 기지국이 초 광 대역 네트워크에 균일하게 분포한 경우, 인프라 구조의 영향 및 이득을 분석한다. 이때, 모든 동작 영역에 대해 전력 제한된 성격 때문에, 유도한 용량 스케일링은 ${\alpha}$에 의존한다. 게다가, 변수 m이 특정 레벨 이상일 때 전체 용량이 m과 함께 선형적으로 스케일함을 보인다. 그러므로 계층적 협력 또는 인프라 구조의 사용은 특정 조건에서 초 광 대역 네트워크의 용량을 개선하는데 도움이 된다.

• Nuclear Engineering and Technology
• /
• 제21권1호
• /
• pp.32-39
• /
• 1989

원형 단면을 가진 축대칭형 토카막 핵융합로에 적합할 수 있는 최적의 이상적 자기유체역학 베타값과 운전조건의 결정 방법을 제시하였다. 토로이달 전류밀도 분포로 조정되는 운전조건을 변화시키면서 이상적 자기유체역학 안정성을 유지시킬 수 있는 베타값의 한계를 계산하였다. 토로이달 전류밀도 분포에는 실험적 관찰들로부터 얻은 실험식들이 사용되었고, 베타 값의 한계를 결정하기 위해 필요로 여러 식들이 이 실험식들로부터 유도되었다. 토로이달 전류밀도의 각각 다른 분포에 대해서 다양한 베타 한계값 분포들이 얻어졌다. 이상적 자기 유체 역학 불안정성들에 의해 제안받는 최대의 베타값을 토카막의 기하학적 변수와 안전인자에 의한 scaling law로 표현하였다.

• Macromolecular Research
• /
• 제17권10호
• /
• pp.807-812
• /
• 2009

In this work, we focus on the tumbling dynamics of rod-like and semi-flexible polymers in mixed flows, which vary from simple shear to pure rotation. By employing a bead-rod model, the tumbling pathways and periods are examined with a focus on the angular distribution of their orientation. Under the mixed flows, the tumbling dynamics agreed well with earlier studies and confirmed the predicted scaling laws. We found that the angular distribution deviates from that of shear flow as the flow type approaches pure rotation. Finally, we investigated the angular distribution of $\lambda$-DNA in a shear flow and found that the present numerical simulations were in quantitative agreement with the previous experimental data.

• 대한전기학회논문지:전기물성ㆍ응용부문C
• /
• 제53권3호
• /
• pp.157-161
• /
• 2004

We present a simple analytical expression of plasma density by making use of the electron density equation to study the dynamic behavior of the corona discharge plasma. It assumes that a specified voltage profile is fed through the inner conductor of the reactor chamber consisting of two coaxial conducting cylinders. The analytical description is based on the electron continuity equation with ionization and attachment by electrons. It is found that the electron density profile calculated between two coaxial cylindrical electrodes depends very sensitively on the Profile of applied voltage. The analytical expression of plasma density and its generation will provide important scaling laws in the corona discharge plasma.

• 한국고분자학회:학술대회논문집
• /
• 한국고분자학회 2006년도 IUPAC International Symposium on Advanced Polymers for Emerging Technologies
• /
• pp.116-117
• /
• 2006

Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. In addition, the dependency of $R_{g}$ on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules.

• Computers and Concrete
• /
• 제22권1호
• /
• pp.39-51
• /
• 2018

This study introduces a new algorithm to determine size independent values of fracture energy, fracture toughness, and fracture process zone length in three-point bending specimens with shallow to deep notches. By using the exact beam theory, a concept of equivalent notch length is introduced for specimens with no notches in order to predict the peak loads with acceptable precisions. Moreover, the method considers the variations of fracture process zone length and effects of higher order terms of stress field in each specimen size. In this paper, it was demonstrated that the use of some recently developed size effect laws raises some concerns due to the use of nonlinear regression analysis. By using a comprehensive fracture test data, provided by Hoover and Bazant, the algorithm has been assessed. It could be concluded that the proposed algorithm can facilitate a powerful tool for size effect study of three-point bending specimens with different notch lengths.