276,737 research outputs found
Empirical exploration of air traffic and human dynamics in terminal airspaces
Air traffic is widely known as a complex, task-critical techno-social system,
with numerous interactions between airspace, procedures, aircraft and air
traffic controllers. In order to develop and deploy high-level operational
concepts and automation systems scientifically and effectively, it is essential
to conduct an in-depth investigation on the intrinsic traffic-human dynamics
and characteristics, which is not widely seen in the literature. To fill this
gap, we propose a multi-layer network to model and analyze air traffic systems.
A Route-based Airspace Network (RAN) and Flight Trajectory Network (FTN)
encapsulate critical physical and operational characteristics; an Integrated
Flow-Driven Network (IFDN) and Interrelated Conflict-Communication Network
(ICCN) are formulated to represent air traffic flow transmissions and
intervention from air traffic controllers, respectively. Furthermore, a set of
analytical metrics including network variables, complex network attributes,
controllers' cognitive complexity, and chaotic metrics are introduced and
applied in a case study of Guangzhou terminal airspace. Empirical results show
the existence of fundamental diagram and macroscopic fundamental diagram at the
route, sector and terminal levels. Moreover, the dynamics and underlying
mechanisms of "ATCOs-flow" interactions are revealed and interpreted by
adaptive meta-cognition strategies based on network analysis of the ICCN.
Finally, at the system level, chaos is identified in conflict system and human
behavioral system when traffic switch to the semi-stable or congested phase.
This study offers analytical tools for understanding the complex human-flow
interactions at potentially a broad range of air traffic systems, and underpins
future developments and automation of intelligent air traffic management
systems.Comment: 30 pages, 28 figures, currently under revie
On Routing Disjoint Paths in Bounded Treewidth Graphs
We study the problem of routing on disjoint paths in bounded treewidth graphs
with both edge and node capacities. The input consists of a capacitated graph
and a collection of source-destination pairs . The goal is to maximize the number of pairs that
can be routed subject to the capacities in the graph. A routing of a subset
of the pairs is a collection of paths such that,
for each pair , there is a path in
connecting to . In the Maximum Edge Disjoint Paths (MaxEDP) problem,
the graph has capacities on the edges and a routing
is feasible if each edge is in at most of
the paths of . The Maximum Node Disjoint Paths (MaxNDP) problem is
the node-capacitated counterpart of MaxEDP.
In this paper we obtain an approximation for MaxEDP on graphs of
treewidth at most and a matching approximation for MaxNDP on graphs of
pathwidth at most . Our results build on and significantly improve the work
by Chekuri et al. [ICALP 2013] who obtained an approximation
for MaxEDP
Control of multi-terminal HVDC networks towards wind power integration: A review
© 2015 Elsevier Ltd.
More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft
On the hardness of unlabeled multi-robot motion planning
In unlabeled multi-robot motion planning several interchangeable robots
operate in a common workspace. The goal is to move the robots to a set of
target positions such that each position will be occupied by some robot. In
this paper, we study this problem for the specific case of unit-square robots
moving amidst polygonal obstacles and show that it is PSPACE-hard. We also
consider three additional variants of this problem and show that they are all
PSPACE-hard as well. To the best of our knowledge, this is the first hardness
proof for the unlabeled case. Furthermore, our proofs can be used to show that
the labeled variant (where each robot is assigned with a specific target
position), again, for unit-square robots, is PSPACE-hard as well, which sets
another precedence, as previous hardness results require the robots to be of
different shapes
Application of Grey Wolf Optimizer Algorithm for Optimal Power Flow of Two-Terminal HVDC Transmission System
This paper applies a relatively new optimization method, the Grey Wolf Optimizer (GWO) algorithm for Optimal Power Flow (OPF) of twoterminal High Voltage Direct Current (HVDC) electrical power system. The OPF problem of pure AC power systems considers the minimization of total costs under equality and inequality constraints. Hence, the OPF problem of integrated AC-DC power systems is
extended to incorporate HVDC links, while taking into consideration the power transfer control characteristics using a GWO algorithm. This algorithm is inspired by the hunting behavior and social leadership of grey wolves in nature. The proposed algorithm is applied to two different case-studies: the modified 5-bus and WSCC 9-bus test systems. The validity of the proposed algorithm is demonstrated by comparing the obtained results with those reported in literature using other optimization techniques. Analysis of the obtained results show that the proposed GWO algorithm is able to achieve shorter CPU time, as well as minimized total cost when compared with already existing optimization techniques. This conclusion proves the efficiency of the GWO algorithm
An Efficient Algorithm For Chinese Postman Walk on Bi-directed de Bruijn Graphs
Sequence assembly from short reads is an important problem in biology. It is
known that solving the sequence assembly problem exactly on a bi-directed de
Bruijn graph or a string graph is intractable. However finding a Shortest
Double stranded DNA string (SDDNA) containing all the k-long words in the reads
seems to be a good heuristic to get close to the original genome. This problem
is equivalent to finding a cyclic Chinese Postman (CP) walk on the underlying
un-weighted bi-directed de Bruijn graph built from the reads. The Chinese
Postman walk Problem (CPP) is solved by reducing it to a general bi-directed
flow on this graph which runs in O(|E|2 log2(|V |)) time. In this paper we show
that the cyclic CPP on bi-directed graphs can be solved without reducing it to
bi-directed flow. We present a ?(p(|V | + |E|) log(|V |) + (dmaxp)3) time
algorithm to solve the cyclic CPP on a weighted bi-directed de Bruijn graph,
where p = max{|{v|din(v) - dout(v) > 0}|, |{v|din(v) - dout(v) < 0}|} and dmax
= max{|din(v) - dout(v)}. Our algorithm performs asymptotically better than the
bidirected flow algorithm when the number of imbalanced nodes p is much less
than the nodes in the bi-directed graph. From our experimental results on
various datasets, we have noticed that the value of p/|V | lies between 0.08%
and 0.13% with 95% probability
Fault mode operation strategies for dual H-bridge current flow controller in meshed HVDC grid
Peer ReviewedPreprin
Some symmetry properties of spin currents and spin polarizations in multi-terminal mesoscopic spin-orbit coupled systems
We study theoretically some symmetry properties of spin currents and spin
polarizations in multi-terminal mesoscopic spin-orbit coupled systems. Based on
a scattering wave function approach, we show rigorously that in the equilibrium
state no finite spin polarizations can exist in a multi-terminal mesoscopic
spin-orbit coupled system (both in the leads and in the spin-orbit coupled
region) and also no finite equilibrium terminal spin currents can exist. By use
of a typical two-terminal mesoscopic spin-orbit coupled system as the example,
we show explicitly that the nonequilibrium terminal spin currents in a
multi-terminal mesoscopic spin-orbit coupled system are non-conservative in
general. This non-conservation of terminal spin currents is not caused by the
use of an improper definition of spin current but is intrinsic to
spin-dependent transports in mesoscopic spin-orbit coupled systems. We also
show that the nonequilibrium lateral edge spin accumulation induced by a
longitudinal charge current in a thin strip of \textit{finite} length of a
two-dimensional electronic system with intrinsic spin-orbit coupling may be
non-antisymmetric in general, which implies that some cautions may need to be
taken when attributing the occurrence of nonequilibrium lateral edge spin
accumulation induced by a longitudinal charge current in such a system to an
intrinsic spin Hall effect.Comment: 11 pages, 6 figure
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