16,940 research outputs found
Scaling properties of the Penna model
We investigate the scaling properties of the Penna model, which has become a
popular tool for the study of population dynamics and evolutionary problems in
recent years. We find that the model generates a normalised age distribution
for which a simple scaling rule is proposed, that is able to reproduce
qualitative features for all genome sizes.Comment: 4 pages, 4 figure
Sharp gene pool transition in a population affected by phenotype-based selective hunting
We use a microscopic model of population dynamics, a modified version of the
well known Penna model, to study some aspects of microevolution. This research
is motivated by recent reports on the effect of selective hunting on the gene
pool of bighorn sheep living in the Ram Mountain region, in Canada. Our model
finds a sharp transition in the structure of the gene pool as some threshold
for the number of animals hunted is reached.Comment: 5 pages, 4 figure
On the Stability of Fundamental Couplings in the Galaxy
Astrophysical tests of the stability of Nature's fundamental couplings are a
key probe of the standard paradigms in fundamental physics and cosmology. In
this report we discuss updated constraints on the stability of the
fine-structure constant and the proton-to-electron mass ratio
within the Galaxy. We revisit and improve upon the analysis by
Truppe {\it et al.} by allowing for the possibility of simultaneous variations
of both couplings and also by combining them with the recent measurements by
Levshakov {\it et al.} By considering representative unification scenarios we
find no evidence for variations of at the 0.4 ppm level, and of
at the 0.6 ppm level; if one uses the Levshakov bound on as a prior,
the bound is improved to 0.1 ppm. We also highlight how these
measurements can constrain (and discriminate among) several fundamental physics
paradigms.Comment: 7 pages, 1 figur
Transformer and LCL Filter Design for DPFCs
Flexible AC Transmission Systems (FACTS) can be used for power flow control in AC transmission grids, allowing simultaneous control of the bus voltage and line active and reactive power. However, due to high costs and reliability concerns, the application of this technology has been limited in such applications. Recently, the concept of Distributed FACTS (DFACTS) and Distributed Power Flow Controller (DPFC) has been introduced as a low cost high reliability alternative for power flow control. This paper presents the design of a coupling transformer and a LCL filter for DPFC devices. To extract the electromagnetic energy from the transmission line a transformer with a single turn primary is designed and optimized. A third-order LCL filter is used to guarantee high order harmonics filtering. Simulations results are presented and discussed
Control of distributed power flow controllers using active power from homopolar line currents
Flexible AC Transmission Systems (FACTS) devices can be used for power flow control in AC transmission grids, improving power line utilization and performance. Nowadays, Unified Power Flow Controllers (UPFC) are one of the most useful FACTS, allowing the simultaneous control of the bus voltage and line active and reactive power. However, due to high costs and reliability concerns, the utilization of this technology has been limited in such applications. The concept of Distributed FACTS (DFACTS) and Distributed Power Flow Controller (DPFC) was recently introduced as a low coast alternative for power flow control. This paper presents a distributed power flow controller that uses third-harmonic frequency currents transmitted through the line to independently control active and reactive power flow at fundamental frequency. Simulations were carried in the Matlab/Simulink environment. © 2012 IEEE
The influence of the Al stabilizer layer thickness on the normal zone propagation velocity in high current superconductors
The stability of high-current superconductors is challenging in the design of
superconducting magnets. When the stability requirements are fulfilled, the
protection against a quench must still be considered. A main factor in the
design of quench protection systems is the resistance growth rate in the magnet
following a quench. The usual method for determining the resistance growth in
impregnated coils is to calculate the longitudinal velocity with which the
normal zone propagates in the conductor along the coil windings.
Here, we present a 2D numerical model for predicting the normal zone
propagation velocity in Al stabilized Rutherford NbTi cables with large cross
section. By solving two coupled differential equations under adiabatic
conditions, the model takes into account the thermal diffusion and the current
redistribution process following a quench. Both the temperature and magnetic
field dependencies of the superconductor and the metal cladding materials
properties are included. Unlike common normal zone propagation analyses, we
study the influence of the thickness of the cladding on the propagation
velocity for varying operating current and magnetic field.
To assist in the comprehension of the numerical results, we also introduce an
analytical formula for the longitudinal normal zone propagation. The analysis
distinguishes between low-current and high-current regimes of normal zone
propagation, depending on the ratio between the characteristic times of thermal
and magnetic diffusion. We show that above a certain thickness, the cladding
acts as a heat sink with a limited contribution to the acceleration of the
propagation velocity with respect to the cladding geometry. Both numerical and
analytical results show good agreement with experimental data.Comment: To be published in Physics Procedia (ICEC 25 conference special
issue
Unified model for vortex-string network evolution
We describe and numerically test the velocity-dependent one-scale (VOS)
string evolution model, a simple analytic approach describing a string network
with the averaged correlation length and velocity. We show that it accurately
reproduces the large-scale behaviour (in particular the scaling laws) of
numerical simulations of both Goto-Nambu and field theory string networks. We
explicitly demonstrate the relation between the high-energy physics approach
and the damped and non-relativistic limits which are relevant for condensed
matter physics. We also reproduce experimental results in this context and show
that the vortex-string density is significantly reduced by loop production, an
effect not included in the usual `coarse-grained' approach.Comment: 5 pages; v2: cosmetic changes, version to appear in PR
Avaliação hídrica de um latossolo vermelho submetido a sistemas de manejo.
O conteúdo de água em sistemas de preparo do solo são essenciais para avaliar a sustentabilidade agrícola. O objetivo deste estudo foi avaliar os efeitos do teor de água em um Latossolo Vermelho distrófi co sob diferentes sistemas de manejo: 1) Crotalária (Crotalaria juncea), em sistema de semeadura direta (SDC); 2) Milheto (Pennisetum americanum sin. Tiphoydes), em sistema de semeadura direta (SDM); 3) Lablabe (Dolechus lablab), em sistema de semeadura direta (SDL); 4) Semeadura convencional após uma gradagem aradora e duas niveladoras (SSC) e 5) pousio. A propriedade física do solo foi avaliada nas camadas 0,5-0,10 m e 0,15-0,20 m . A curva de retenção de água foi mais influenciada pelo SDM na camada 0,5-0,10 m
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