27,527 research outputs found
Fractal space frames and metamaterials for high mechanical efficiency
A solid slender beam of length , made from a material of Young's modulus
and subject to a gentle compressive force , requires a volume of
material proportional to [where ] in
order to be stable against Euler buckling. By constructing a hierarchical space
frame, we are able to systematically change the scaling of required material
with so that it is proportional to , through changing
the number of hierarchical levels present in the structure. Based on simple
choices for the geometry of the space frames, we provide expressions specifying
in detail the optimal structures (in this class) for different values of the
loading parameter . These structures may then be used to create effective
materials which are elastically isotropic and have the combination of low
density and high crush strength. Such a material could be used to make
light-weight components of arbitrary shape.Comment: 6 pages, 4 figure
Empirical Study of Simulated Two-planet Microlensing Event
We undertake the first study of two-planet microlensing models recovered from
simulations of microlensing events generated by realistic multi-planet systems
in which 292 planetary events including 16 two-planet events were detected from
6690 simulated light curves. We find that when two planets are recovered, their
parameters are usually close to those of the two planets in the system most
responsible for the perturbations. However, in one of the 16 examples, the
apparent mass of both detected planets was more than doubled by the unmodeled
influence of a third, massive planet. This fraction is larger than, but
statistically consistent with, the roughly 1.5% rate of serious mass errors due
to unmodeled planetary companions for the 274 cases from the same simulation in
which a single planet is recovered. We conjecture that an analogous effect due
to unmodeled stellar companions may occur more frequently. For seven out of 23
cases in which two planets in the system would have been detected separately,
only one planet was recovered because the perturbations due to the two planets
had similar forms. This is a small fraction (7/274) of all recovered
single-planet models, but almost a third of all events that might plausibly
have led to two-planet models. Still, in these cases, the recovered planet
tends to have parameters similar to one of the two real planets most
responsible for the anomaly.Comment: 21 pages, 9 figures, 2 tables; submitted to ApJ; for a short video
introducing the key results, see https://www.youtube.com/watch?v=qhK4a6sbfO
Anomaly Detection for Science DMZs Using System Performance Data
Science DMZs are specialized networks that enable large-scale distributed scientific research, providing efficient and guaranteed performance while transferring large amounts of data at high rates. The high-speed performance of a Science DMZ is made viable via data transfer nodes (DTNs), therefore they are a critical point of failure. DTNs are usually monitored with network intrusion detection systems (NIDS). However, NIDS do not consider system performance data, such as network I/O interrupts and context switches, which can also be useful in revealing anomalous system performance potentially arising due to external network based attacks or insider attacks. In this paper, we demonstrate how system performance metrics can be applied towards securing a DTN in a Science DMZ network. Specifically, we evaluate the effectiveness of system performance data in detecting TCP-SYN flood attacks on a DTN using DBSCAN (a density-based clustering algorithm) for anomaly detection. Our results demonstrate that system interrupts and context switches can be used to successfully detect TCP-SYN floods, suggesting that system performance data could be effective in detecting a variety of attacks not easily detected through network monitoring alone
A Fast DOA Estimation Algorithm Based on Polarization MUSIC
A fast DOA estimation algorithm developed from MUSIC, which also benefits from the processing of the signals' polarization information, is presented. Besides performance enhancement in precision and resolution, the proposed algorithm can be exerted on various forms of polarization sensitive arrays, without specific requirement on the array's pattern. Depending on the continuity property of the space spectrum, a huge amount of computation incurred in the calculation of 4-D space spectrum is averted. Performance and computation complexity analysis of the proposed algorithm is discussed and the simulation results are presented. Compared with conventional MUSIC, it is indicated that the proposed algorithm has considerable advantage in aspects of precision and resolution, with a low computation complexity proportional to a conventional 2-D MUSIC
Mercury deposition in southern New Hampshire, 2006–2009
The atmospheric deposition of mercury (Hg) occurs via several mechanisms including wet, dry, and occult processes. In an effort to understand the atmospheric cycling and seasonal depositional characteristics of Hg, event-based wet deposition samples and reactive gaseous Hg (RGM) measurements were collected for approximately 3 years at Thompson Farm (TF), a near-coastal rural site in Durham, NH, part of the University of New Hampshire AIRMAP Observing Network. Total aqueous mercury exhibited seasonal patterns in Hg wet deposition at TF. The lowest Hg wet deposition was measured in the winter with an average total seasonal deposition of 1.56 μg m−2compared to the summer average of 4.71 μg m−2. Inter-annual differences in total wet deposition are generally linked with precipitation volume, with the greatest deposition occurring in the wettest year. Relationships between surface level RGM and Hg wet deposition were also investigated based on continuous RGM measurements at TF from November 2006 to September 2009. No correlations were observed between RGM mixing ratios and Hg wet deposition, however the ineffective scavenging of RGM during winter precipitation events was evidenced by the less frequent depletion of RGM below the detection level. Seasonal dry deposition of reactive gaseous Hg (RGM) was estimated using an order-of-magnitude approach. RGM mixing ratios and dry deposition estimates were greatest during the winter and spring. The seasonal ratios of Hg wet deposition to RGM dry deposition vary by up to a factor of 80
Quantum Algorithm to Solve Satisfiability Problems
A new quantum algorithm is proposed to solve Satisfiability(SAT) problems by
taking advantage of non-unitary transformation in ground state quantum
computer. The energy gap scale of the ground state quantum computer is analyzed
for 3-bit Exact Cover problems. The time cost of this algorithm on general SAT
problems is discussed.Comment: 5 pages, 3 figure
Microlensing and the Search for Extraterrestrial Life
Are microlensing searches likely to discover planets that harbor life? Given
our present state of knowledge, this is a difficult question to answer. We
therefore begin by asking a more narrowly focused question: are conditions on
planets discovered via microlensing likely to be similar to those we experience
on Earth? In this paper I link the microlensing observations to the well-known
"Goldilocks Problem" (conditions on the Earth-like planets need to be "just
right"), to find that Earth-like planets discovered via microlensing are likely
to be orbiting stars more luminous than the sun. This means that light from the
planetary system's central star may contribute a significant fraction of the
baseline flux relative to the star that is lensed. Such blending of light from
the lens with light from the lensed source can, in principle, limit our ability
to detect these events. This turns out not to be a significant problem,
however. A second consequence of blending is the opportunity to determine the
spectral type of the lensed spectral type of the lensed star. This
circumstance, plus the possibility that finite-source-size effects are
important, implies that some meaningful follow-up observations are likely to be
possible for a subset Earth-like planets discovered via microlensing. In
addition, calculations indicate that reasonable requirements on the planet's
density and surface gravity imply that the mass of Earth-like planets is likely
to be within a factor of of an Earth mass.Comment: 15 pages, 2 figures. To be published in the Astrophysical Journa
The Mason Test: A Defense Against Sybil Attacks in Wireless Networks Without Trusted Authorities
Wireless networks are vulnerable to Sybil attacks, in which a malicious node
poses as many identities in order to gain disproportionate influence. Many
defenses based on spatial variability of wireless channels exist, but depend
either on detailed, multi-tap channel estimation - something not exposed on
commodity 802.11 devices - or valid RSSI observations from multiple trusted
sources, e.g., corporate access points - something not directly available in ad
hoc and delay-tolerant networks with potentially malicious neighbors. We extend
these techniques to be practical for wireless ad hoc networks of commodity
802.11 devices. Specifically, we propose two efficient methods for separating
the valid RSSI observations of behaving nodes from those falsified by malicious
participants. Further, we note that prior signalprint methods are easily
defeated by mobile attackers and develop an appropriate challenge-response
defense. Finally, we present the Mason test, the first implementation of these
techniques for ad hoc and delay-tolerant networks of commodity 802.11 devices.
We illustrate its performance in several real-world scenarios
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