186 research outputs found
On the influence of drag force modeling in long-span suspension bridge flutter analysis
The present study aimed at investigating the role played by the description of the drag component on the predicted flutter velocity (and frequency) of very long-span suspension bridges. Based on a detailed finite element model of the central span of the Akashi Kaikyo Bridge, implemented in ANSYS, flutter analyses were run according to the following descriptions of the wind aerodynamic actions: (a) unsteady lift, moment and drag; (b) unsteady lift and moment plus steady drag, and (c) unsteady lift and moment, without drag. The finite element results are compared with those obtained by an in-house MATLAB code based on a semi-analytic continuum model. The latter includes flexural-torsional second-order effects induced by steady drag force in the bridge’s equations of motion, in addition to the unsteady lift and moment actions
An investigation into the mechanical damping characteristics of catenary contact wires and their effect on aerodynamic galloping instability
Measurement of the damped oscillation of a section of the UK East Coast Main Line (ECML) catenary/contact wire system was undertaken, and the natural frequency and mechanical damping were found to be 1.4Hz and 0.05 respectively. This information was used to assess the effect of increasing the mechanical damping ratio on the susceptibility of the system to an aerodynamic galloping instability. The section of line tested was known to gallop at wind speeds of approximately 40 mile/h, and theoretical and experimental work verified this. A friction damper arm was designed and three units were fitted to the section of line affected. The introduction of increased mechanical damping was found to raise the mechanical damping coefficient of the line to between 0.095 and 0.18, and the mathematical analysis produced a theoretical wind speed for galloping oscillation of between 75 and 141 mile/h respectively. For over a year since the units were fitted, no problems with galloping instability have been observed
Measuring and Correcting Wind-Induced Pointing Errors of the Green Bank Telescope Using an Optical Quadrant Detector
Wind-induced pointing errors are a serious concern for large-aperture
high-frequency radio telescopes. In this paper, we describe the implementation
of an optical quadrant detector instrument that can detect and provide a
correction signal for wind-induced pointing errors on the 100m diameter Green
Bank Telescope (GBT). The instrument was calibrated using a combination of
astronomical measurements and metrology. We find that the main wind-induced
pointing errors on time scales of minutes are caused by the feedarm being blown
along the direction of the wind vector. We also find that wind-induced
structural excitation is virtually non-existent. We have implemented offline
software to apply pointing corrections to the data from imaging instruments
such as the MUSTANG 3.3 mm bolometer array, which can recover ~70% of
sensitivity lost due to wind-induced pointing errors. We have also performed
preliminary tests that show great promise for correcting these pointing errors
in real-time using the telescope's subreflector servo system in combination
with the quadrant detector signal.Comment: 17 pages, 11 figures; accepted for publication in PAS
The research of the maximum wind speed in Tomsk and calculations of dynamic load on antenna systems
The work is concerned with calculations and analysis of the maximum wind speed in Tomsk city. The data for analysis were taken from the TOR-station located in the north-eastern part of the city. The TOR-station sensors to measure a speed and a direction of wind are installed on the 10-meter meteorological mast. Wind is measured by M-63, which uses the standard approach and the program with one-minute averaging for wind gusts recording as well. According to the measured results in the research performed, the estimation of the dynamic and wind load on different types of antenna systems was performed. The work shows the calculations of wind load on ten types of antenna systems, distinguished by their different constructions and antenna areas. For implementation of calculations, we used methods developed in the Central Research and Development Institute of Building Constructions named after V.A. Kucherenko. The research results could be used for design engineering of the static antenna systems and mobile tracking systems for the distant objects
Ubiquitous Crossmodal Stochastic Resonance in Humans: Auditory Noise Facilitates Tactile, Visual and Proprioceptive Sensations
BACKGROUND: Stochastic resonance is a nonlinear phenomenon whereby the addition of noise can improve the detection of weak stimuli. An optimal amount of added noise results in the maximum enhancement, whereas further increases in noise intensity only degrade detection or information content. The phenomenon does not occur in linear systems, where the addition of noise to either the system or the stimulus only degrades the signal quality. Stochastic Resonance (SR) has been extensively studied in different physical systems. It has been extended to human sensory systems where it can be classified as unimodal, central, behavioral and recently crossmodal. However what has not been explored is the extension of this crossmodal SR in humans. For instance, if under the same auditory noise conditions the crossmodal SR persists among different sensory systems. METHODOLOGY/PRINCIPAL FINDINGS: Using physiological and psychophysical techniques we demonstrate that the same auditory noise can enhance the sensitivity of tactile, visual and propioceptive system responses to weak signals. Specifically, we show that the effective auditory noise significantly increased tactile sensations of the finger, decreased luminance and contrast visual thresholds and significantly changed EMG recordings of the leg muscles during posture maintenance. CONCLUSIONS/SIGNIFICANCE: We conclude that crossmodal SR is a ubiquitous phenomenon in humans that can be interpreted within an energy and frequency model of multisensory neurons spontaneous activity. Initially the energy and frequency content of the multisensory neurons' activity (supplied by the weak signals) is not enough to be detected but when the auditory noise enters the brain, it generates a general activation among multisensory neurons of different regions, modifying their original activity. The result is an integrated activation that promotes sensitivity transitions and the signals are then perceived. A physiologically plausible model for crossmodal stochastic resonance is presented
Universal behavior of extreme value statistics for selected observables of dynamical systems
The main results of the extreme value theory developed for the investigation
of the observables of dynamical systems rely, up to now, on the Gnedenko
approach. In this framework, extremes are basically identified with the block
maxima of the time series of the chosen observable, in the limit of infinitely
long blocks. It has been proved that, assuming suitable mixing conditions for
the underlying dynamical systems, the extremes of a specific class of
observables are distributed according to the so called Generalized Extreme
Value (GEV) distribution. Direct calculations show that in the case of
quasi-periodic dynamics the block maxima are not distributed according to the
GEV distribution. In this paper we show that, in order to obtain a universal
behaviour of the extremes, the requirement of a mixing dynamics can be relaxed
if the Pareto approach is used, based upon considering the exceedances over a
given threshold. Requiring that the invariant measure locally scales with a
well defined exponent - the local dimension -, we show that the limiting
distribution for the exceedances of the observables previously studied with the
Gnedenko approach is a Generalized Pareto distribution where the parameters
depends only on the local dimensions and the value of the threshold. This
result allows to extend the extreme value theory for dynamical systems to the
case of regular motions. We also provide connections with the results obtained
with the Gnedenko approach. In order to provide further support to our
findings, we present the results of numerical experiments carried out
considering the well-known Chirikov standard map.Comment: 7 pages, 1 figur
Wind Power Persistence Characterized by Superstatistics
Mitigating climate change demands a transition towards renewable electricity generation, with wind power being a particularly promising technology. Long periods either of high or of low wind therefore essentially define the necessary amount of storage to balance the power system. While the general statistics of wind velocities have been studied extensively, persistence (waiting) time statistics of wind is far from well understood. Here, we investigate the statistics of both high- and low-wind persistence. We find heavy tails and explain them as a superposition of different wind conditions, requiring q-exponential distributions instead of exponential distributions. Persistent wind conditions are not necessarily caused by stationary atmospheric circulation patterns nor by recurring individual weather types but may emerge as a combination of multiple weather types and circulation patterns. This also leads to Fréchet instead of Gumbel extreme value statistics. Understanding wind persistence statistically and synoptically may help to ensure a reliable and economically feasible future energy system, which uses a high share of wind generation
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