42,649 research outputs found
Noise measurements for various configurations of a model of a mixer nozzle externally blown flap system
Noise data were taken for variations to a large scale model of an externally blown flap lift augmentation system. The variations included two different mixer nozzles (7 and 8 lobes), two different wing models (2 and 3 flaps), and different lateral distances between the wing chord line and the nozzle centerline. When the seven lobe was used with the trailing flap in the 60 deg position, increasing the wing to nozzle distance had no effect on the sound level. When the eight lobe nozzle was used there was a decrease in sound level. With the 20 deg flap setting the noise level decreased when the distance was increased using either nozzle
Identifying "communities" within energy landscapes
Potential energy landscapes can be represented as a network of minima linked
by transition states. The community structure of such networks has been
obtained for a series of small Lennard-Jones clusters. This community structure
is compared to the concept of funnels in the potential energy landscape. Two
existing algorithms have been used to find community structure, one involving
removing edges with high betweenness, the other involving optimization of the
modularity. The definition of the modularity has been refined, making it more
appropriate for networks such as these where multiple edges and
self-connections are not included. The optimization algorithm has also been
improved, using Monte Carlo methods with simulated annealing and basin hopping,
both often used successfully in other optimization problems. In addition to the
small clusters, two examples with known heterogeneous landscapes, LJ_13 with
one labelled atom and LJ_38, were studied with this approach. The network
methods found communities that are comparable to those expected from landscape
analyses. This is particularly interesting since the network model does not
take any barrier heights or energies of minima into account. For comparison,
the network associated with a two-dimensional hexagonal lattice is also studied
and is found to have high modularity, thus raising some questions about the
interpretation of the community structure associated with such partitions.Comment: 13 pages, 11 figure
PCTL Model Checking of Markov Chains: Truth and Falsity as Winning Strategies in Games
Probabilistic model checking is a technique for verifying whether a model such as a Markov chain satisfies a probabilistic, behavioral property – e.g. “with probability at least 0.999, a device will be elected leader. ” Such properties are expressible in probabilistic temporal logics, e.g. PCTL, and efficient algorithms exist for checking whether these formulae are true or false on finite-state models. Alas, these algorithms don’t supply diagnostic information for why a probabilistic property does or does not hold in a given model. We provide here complete and rigorous foundations for such diagnostics in the setting of countable labeled Markov chains and PCTL. For each model and PCTL formula, we define a game between a Verifier and a Refuter that is won by Verifier if the formula holds in the model, and won by Refuter if it doesn’t hold. Games are won by exactly one player, through monotone strategies that encode the diagnostic information for truth and falsity (respectively). These games are infinite with Büchi type acceptance conditions where simpler fairness conditions are shown not be to sufficient. Verifier can always force finite plays for certain PCTL formulae, suggesting the existence of finite-state abstractions of models that satisfy such formulae
ROSAT HRI observations of Centaurus A
We present results from a sensitive high-resolution X-ray observation of the
nearby active galaxy Centaurus A (NGC 5128) with the ROSAT HRI. The 65~ksec
X-ray image clearly distinguishes different components of the X-ray emission
from Cen A: the nucleus and the jet, the diffuse galaxy halo, and a number of
individual sources associated with the galaxy. The luminosity of the nucleus
increased by a factor of two compared to an earlier ROSAT observation in 1990.
The high spatial resolution of the ROSAT HRI shows that most of the knots in
the jet are extended both along and perpendicular to the jet axis. We report
the detection of a new X-ray feature, at the opposite side of the X-ray jet
which is probably due to compression of hot interstellar gas by the expanding
southwestern inner radio lobe.Comment: To be published in Astrophys. Journal Letters. 4 pages, 3 plate
High Harmonic Generation in SF: Raman-excited Vibrational Quantum Beats
In a recent experiment (N. Wagner et al., PNAS v103, p13279) on SF, a
high-harmonic generating laser pulse is preceded by a pump pulse which
stimulates Raman-active modes in the molecule. Varying the time delay between
the two pulses modulates high harmonic intensity, with frequencies equal to the
vibration frequencies of the Raman-active modes. We propose an explanation of
this modulation as a quantum interference between competing pathways that occur
via adjacent vibrational states of the molecule. The Raman and high harmonic
processes act as beamsplitters, producing vibrational quantum beats among the
Raman-active vibrational modes that are excited by the first pulse. We
introduce a rigorous treatment of the electron-ion recombination process and
the effect of the ionic Coulomb field in the electron propagation outside the
molecule, improving over the widely-used three-step model.Comment: submitted to PR
Plasmonic Hot Spots in Triangular Tapered Graphene Microcrystals
Recently, plasmons in graphene have been observed experimentally using
scattering scanning near-field optical microscopy. In this paper, we develop a
simplified analytical approach to describe the behavior in triangular samples.
Replacing Coulomb interaction by a short-range one reduces the problem to a
Helmholtz equation, amenable to analytical treatment. We demonstrate that even
with our simplifications, the system still exhibits the key features seen in
the experiment.Comment: 4 pages, 3 figure
Probability Models for Degree Distributions of Protein Interaction Networks
The degree distribution of many biological and technological networks has
been described as a power-law distribution. While the degree distribution does
not capture all aspects of a network, it has often been suggested that its
functional form contains important clues as to underlying evolutionary
processes that have shaped the network. Generally, the functional form for the
degree distribution has been determined in an ad-hoc fashion, with clear
power-law like behaviour often only extending over a limited range of
connectivities. Here we apply formal model selection techniques to decide which
probability distribution best describes the degree distributions of protein
interaction networks. Contrary to previous studies this well defined approach
suggests that the degree distribution of many molecular networks is often
better described by distributions other than the popular power-law
distribution. This, in turn, suggests that simple, if elegant, models may not
necessarily help in the quantitative understanding of complex biological
processes.
Simulation of associative learning with the replaced elements model
Associative learning theories can be categorised according to whether they treat the representation of stimulus compounds in an elemental or configural manner. Since it is clear that a simple elemental approach to stimulus representation is inadequate there have been several attempts to produce more elaborate elemental models. One recent approach, the Replaced Elements Model (Wagner, 2003), reproduces many results that have until recently been uniquely predicted by Pearce’s Configural Theory (Pearce, 1994). Although it is possible to simulate the Replaced Elements Model using “standard” simulation programs the generation of the correct stimulus representation is complex. The current paper describes a method for simulation of the Replaced Elements Model and presents the results of two example simulations that show differential predictions of Replaced Elements and Pearce’s Configural Theor
Cluster Analysis of Thermal Icequakes Using the Seismometer to Investigate Ice and Ocean Structure (SIIOS): Implications for Ocean World Seismology
Ocean Worlds are of high interest to the planetary community due to the potential habitability of their subsurface oceans. Over the next few decades several missions will be sent to ocean worlds including the Europa Clipper, Dragonfly, and possibly a Europa lander. The Dragonfly and Europa lander missions will carry seismic payloads tasked with detecting and locating seismic sources. The Seismometer to Investigate Ice and Ocean Structure (SIIOS) is a NASA PSTAR funded project that investigates ocean world seismology using terrestrial analogs. The goals of the SIIOS experiment include quantitatively comparing flight-candidate seismometers to traditional instruments, comparing single-station approaches to a small-aperture array, and characterizing the local seismic environment of our field sites. Here we present an analysis of detected local events at our field sites at Gulkana Glacier in Alaska and in Northwest Greenland approximately 80 km North of Qaanaaq, Greenland. Both field sites passively recorded data for about two weeks. We deployed our experiment on Gulkana Glacier in September 2017 and in Greenland in June 2018. At Gulkana there was a nearby USGS weather station which recorded wind data. Temperature data was collected using the MERRA satellite. In Greenland we deployed our own weather station to collect temperature and wind data. Gulkana represents a noisier and more active environment. Temperatures fluctuated around 0C, allowing for surface runoff to occur during the day. The glacier had several moulins, and during deployment we heard several rockfalls from nearby mountains. In addition to the local environment, Gulkana is located close to an active plate boundary (relative to Greenland). This meant that there were more regional events recorded over two weeks, than in Greenland. Greenlands local environment was also quieter, and less active. Temperatures remained below freezing. The Greenland ice was much thicker than Gulkana (~850 m versus ~100 m) and our stations were above a subglacial lake. Both conditions can reduce event detections from basal motion. Lastly, we encased our Greenland array in an aluminum vault and buried it beneath the surface unlike our array in Gulkana where the instruments were at the surface and covered with plastic bins. The vault further insulated the array from thermal and atmospheric events
- …