303 research outputs found
Load Balancing via Random Local Search in Closed and Open systems
In this paper, we analyze the performance of random load resampling and
migration strategies in parallel server systems. Clients initially attach to an
arbitrary server, but may switch server independently at random instants of
time in an attempt to improve their service rate. This approach to load
balancing contrasts with traditional approaches where clients make smart server
selections upon arrival (e.g., Join-the-Shortest-Queue policy and variants
thereof). Load resampling is particularly relevant in scenarios where clients
cannot predict the load of a server before being actually attached to it. An
important example is in wireless spectrum sharing where clients try to share a
set of frequency bands in a distributed manner.Comment: Accepted to Sigmetrics 201
Comparison of quasi-2-day wave amplitudes and phases over Collm (51.3°N, 13.0°E) based on two different analyses
Collm meteor radar (MR) winds have been analyzed with respect to quasi 2-day wave (QTDW) amplitudes and phases. Two methods have been considered, one of them taking into account the varying period and the other one assuming a fixed period of 48 h. While the use of a fixed period leads to a slight underestimation of amplitudes, the seasonal cycle, the inter-annual variability, and the distribution of amplitude and phase diffferences between the zonal and meridional horizontal component are similar for both methods. One may conclude that the use of a fixed period is justified for analyzing QTDW amplitudes and phases as long as only qualitative results are required
Experimental analysis of gas-sensitive Braitenberg vehicles
This article addresses the problem of localizing a static gas source in an indoor environment by a mobile robot. In contrast to previous works, the environment is not artificially ventilated to produce a strong unidirectional airflow. Here, the dominant transport mechanisms of gas molecules are turbulence and convection flow rather than diffusion, which results in a patchy, chaotically fluctuating gas distribution. Two Braitenberg-type strategies (positive and negative tropotaxis) based on the instantaneously measured spatial concentration gradient were investigated. Both strategies were shown to be of potential use for gas source localization. As a possible solution to the problem of gas source declaration (the task of determining with certainty that the gas source has been found), an indirect localization strategy based on exploration and concentration peak avoidance is suggested. Here, a gas source is located by exploiting the fact that local concentration maxima occur more frequently near the gas source compared to distant regions
Radar observations of geomagnetic disturbance effects on midlatitude mesosphere/lower thermosphere dynamics
Zeitreihen von Monatsmittelwerten des Windes in der Mesosphäre/unteren Thermosphäre über Collm werden auf mögliche Korrelationen mit der Nordatlantischen Oszillation (NAO) und der Südlichen Oszillation (SO) hin untersucht. Während eine positive Korrelation bis in die 1990er Jahre existiert, schwächt sich diese in der Folge ab und kehrt sich teilweise um. Da NAO und SO gekoppelt sind, erfolgen diese Änderungen etwa zur selben Zeit. Die Änderung der Kopplung steht wahrscheinlich in Verbindung mit einer generellen Änderung der Dynamik der mittleren Atmosphäre
Forcing mechanisms of the terdiurnal tide
Using a nonlinear mechanistic global circulation model we analyze the
migrating terdiurnal tide in the middle atmosphere with respect to its
possible forcing mechanisms, i.e., the absorption of solar radiation in the
water vapor and ozone band, nonlinear tidal interactions, and gravity
wave–tide interactions. In comparison to the forcing mechanisms of diurnal
and semidiurnal tides, these terdiurnal forcings are less well understood and
there are contradictory opinions about their respective relevance. In our
simulations we remove the wave number 3 pattern for each forcing individually
and analyze the remaining tidal wind and temperature fields. We find that the
direct solar forcing is dominant and explains most of the migrating
terdiurnal tide's amplitude. Nonlinear interactions due to other tides or
gravity waves are most important during local winter. Further analyses show
that the nonlinear forcings are locally counteracting the solar forcing due
to destructive interferences. Therefore, tidal amplitudes can become even
larger for simulations with removed nonlinear forcings.</p
Quarterdiurnal signature in sporadic E occurrence rates and comparison with neutral wind shear
The GPS radio occultation (RO) technique is used to study
sporadic E (Es) layer plasma irregularities of the Earth's ionosphere on a
global scale using GPS signal-to-noise ratio (SNR) profiles from the
COSMIC/FORMOSAT-3 satellite. The maximum deviation from the mean SNR can be
attributed to the height of the Es layer. Es are generally accepted to be
produced by ion convergence due to vertical wind shear in the presence of a
horizontal component of the Earth's magnetic field, while the wind shear is
provided mainly by the solar tides. Here we present analyses of
quarterdiurnal tide (QDT) signatures in Es occurrence rates. From a local
comparison with mesosphere/lower thermosphere wind shear obtained with a
meteor radar at Collm (51.3∘ N, 13.0∘ E), we find that the
phases of the QDT in Es agree well with those of negative vertical shear of
the zonal wind for all seasons except for summer, when the QDT amplitudes are
small. We also compare the global QDT Es signal with numerical model results.
The global distribution of the Es occurrence rates qualitatively agrees with
the modeled zonal wind shears. The results indicate that zonal wind shear is
indeed an important driving mechanism for the QDT seen in Es.</p
On the Invariant Theory of Weingarten Surfaces in Euclidean Space
We prove that any strongly regular Weingarten surface in Euclidean space
carries locally geometric principal parameters. The basic theorem states that
any strongly regular Weingarten surface is determined up to a motion by its
structural functions and the normal curvature function satisfying a geometric
differential equation. We apply these results to the special Weingarten
surfaces: minimal surfaces, surfaces of constant mean curvature and surfaces of
constant Gauss curvature.Comment: 16 page
Recommended from our members
Exploring the relation between flapping behaviour and accelerometer signal during ascending flight, and a new approach to calibration
We understand little about the energetic costs of flight in free-ranging birds; in part since current techniques for estimating flight energetics in the wild are limited. Accelerometry is known to estimate energy expenditure through body movement in terrestrial animals, once calibrated using a treadmill with chamber respirometry. The flight equivalent, a wind tunnel with mask respirometry, is particularly difficult to instigate, and has not been applied to calibrate accelerometry. We take the first steps in exploring a novel method for calibrating accelerometers with flight energy expenditure. We collected accelerometry data for Harris’s Hawks Parabuteo unicinctus flying to varying heights up to 4.1 m over a small horizontal distance; the mechanical
energy expended to gain height can be estimated from physical first principles. The relationship between accelerometry and mechanical energy expenditure was strong, and while a simple wing flapping model confirmed that accelerometry is sensitive to both changes in wing beat amplitude and frequency, the relationship was explained predominately by changes in wing beat frequency, and less so by changes in amplitude. Our study provides initial, positive evidence that accelerometry can be calibrated with body power using climbing flights, potentially providing a basis for estimating flapping flight metabolic rate at least in situations of altitude gain
A Physics-based Investigation of Pt-salt Doped Carbon Nanotubes for Local Interconnects
We investigate, by combining physical and electrical measurements together with an atomistic-to-circuit modeling approach, the conductance of doped carbon nanotubes (CNTs) and their eligibility as possible candidate for next generation back-end-of-line (BEOL) interconnects. Ab-initio simulations predict a doping-related shift of the Fermi level, which reduces shell chirality variability and improves electrical conductance up to 90% by converting semiconducting shells to metallic. Circuit-level simulations predict up to 88% signal delay improvement with doped vs. pristine CNT. Electrical measurements of Pt-salt doped CNTs provide up to 50% of resistance reduction which is a milestone result for future CNT interconnect technology
- …