1,721,328 research outputs found
Gravitational-wave imprints of compact and galactic-scale environments in extreme-mass-ratio binaries
Full text linkCircumambient and galactic-scale environments are intermittently present
around black holes that reside in active galactic nuclei. As supermassive black
holes impart energy on their host galaxy, so the galactic environment affects
the dynamics of solar-mass objects around black holes and the gravitational
waves emitted from non-vacuum asymmetric binaries. Only recently an exact
general-relativistic solution has been found that describes a Schwarzschild
black hole immersed in a dark matter halo of the Hernquist type. We perform an
extensive analysis of generic geodesics delving in such non-vacuum spacetimes
and compare our results with those obtained in Schwarzschild, as well as
calculate their gravitational-wave emission. Our findings indicate that the
radial and polar oscillation frequency ratios descend deeper into the strong
gravity region as the compactness of the halo increases. This translates to a
redshift of non-vacuum geodesics and their resulting waveforms with respect to
the vacuum ones. We calculate the overlap between waveforms resulting from
Schwarzschild and non-vacuum geometries and find that it decreases as the halo
compactness grows, meaning that dark matter environments should be
distinguishable by space-borne detectors. For compact environments, we find
that the apsidal precession is strongly affected due to the gravitational pull
of dark matter; the orbit's axis can rotate in the opposite direction as that
of the orbital motion, leading to a retrograde precession drift that depends on
the halo mass, as opposed to the typical prograde precession transpiring in
galactic-scale environments. Gravitational waves in retrograde-to-prograde
alterations demonstrate transient frequency phenomena around critical
non-precessing turning points, thus they may serve as `smoking guns' for the
presence of compact dark matter environments around supermassive black holes.Comment: 19 pages, 10 figures, revisions regarding detectability and addition
of new figures and sections, abstract reduced to fit arxiv limits, accepted
for publication in PR
Experiments on surface reconstruction for partially submerged marine structures
Full text linkOver the past 10 years, significant scientific effort has been dedicated to the problem of three-dimensional (3-D) surface reconstruction for structural systems. However, the critical area of marine structures remains insufficiently studied. The research presented here focuses on the problem of 3-D surface reconstruction in the marine environment. This paper summarizes our hardware, software, and experimental contributions on surface reconstruction over the past few years (2008-2011). We propose the use of off-the-shelf sensors and a robotic platform to scan marine structures both above and below the waterline, and we develop a method and software system that uses the Ball Pivoting Algorithm (BPA) and the Poisson reconstruction algorithm to reconstruct 3-D surface models of marine structures from the scanned data. We have tested our hardware and software systems extensively in Singapore waters, including operating in rough waters, where water currents are around 1-2 m/s. We present results on construction of various 3-D models of marine structures, including slowly moving structures such as floating platforms, moving boats, and stationary jetties. Furthermore, the proposed surface reconstruction algorithm makes no use of any navigation sensor such as GPS, a Doppler velocity log, or an inertial navigation system
Asymptotically optimal inspection planning using systems with differential constraints
Full text linkThis paper proposes a new inspection planning algorithm, called Random Inspection Tree Algorithm (RITA). Given a perfect model of a structure, sensor specifications, robot's dynamics, and an initial configuration of a robot, RITA computes the optimal inspection trajectory that observes all points on the structure. Many inspection planning algorithms have been proposed, most of them consist of two sequential steps. In the first step, they compute a small set of observation points such that each point on the structure is visible. In the second step, they compute the shortest trajectory to visit all observation points at least once. The robot's kinematic and dynamic constraints are taken into account only in the second step. Thus, when the robot has differential constraints and operates in cluttered environments, the observation points may be difficult or even infeasible to reach. To alleviate this difficulty, RITA computes both observation points and the trajectory to visit the observation points simultaneously. RITA uses sampling-based techniques to find admissible trajectories with decreasing cost. Simulation results for 2-D environments are promising. Furthermore, we present analysis on the probabilistic completeness and asymptotic optimality of our algorithm
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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