59,023 research outputs found
Magneto--Acoustic Energetics Study of the Seismically Active Flare of 15 February 2011
Multi--wavelength studies of energetic solar flares with seismic emissions
have revealed interesting common features between them. We studied the first
GOES X--class flare of the 24th solar cycle, as detected by the Solar Dynamics
Observatory (SDO). For context, seismic activity from this flare
(SOL2011-02-15T01:55-X2.2, in NOAA AR 11158) has been reported in the
literature (Kosovichev, 2011; Zharkov et al., 2011). Based on Dopplergram data
from the Helioseismic and Magnetic Imager (HMI), we applied standard methods of
local helioseismology in order to identify the seismic sources in this event.
RHESSI hard X-ray data are used to check the correlation between the location
of the seismic sources and the particle precipitation sites in during the
flare. Using HMI magnetogram data, the temporal profile of fluctuations in the
photospheric line-of-sight magnetic field is used to estimate the magnetic
field change in the region where the seismic signal was observed. This leads to
an estimate of the work done by the Lorentz-force transient on the photosphere
of the source region. In this instance this is found to be a significant
fraction of the acoustic energy in the attendant seismic emission, suggesting
that Lorentz forces can contribute significantly to the generation of
sunquakes. However, there are regions in which the signature of the
Lorentz-force is much stronger, but from which no significant acoustic emission
emanates.Comment: Submitted to Solar Physic
mfEGRA: Multifidelity Efficient Global Reliability Analysis through Active Learning for Failure Boundary Location
This paper develops mfEGRA, a multifidelity active learning method using
data-driven adaptively refined surrogates for failure boundary location in
reliability analysis. This work addresses the issue of prohibitive cost of
reliability analysis using Monte Carlo sampling for expensive-to-evaluate
high-fidelity models by using cheaper-to-evaluate approximations of the
high-fidelity model. The method builds on the Efficient Global Reliability
Analysis (EGRA) method, which is a surrogate-based method that uses adaptive
sampling for refining Gaussian process surrogates for failure boundary location
using a single-fidelity model. Our method introduces a two-stage adaptive
sampling criterion that uses a multifidelity Gaussian process surrogate to
leverage multiple information sources with different fidelities. The method
combines expected feasibility criterion from EGRA with one-step lookahead
information gain to refine the surrogate around the failure boundary. The
computational savings from mfEGRA depends on the discrepancy between the
different models, and the relative cost of evaluating the different models as
compared to the high-fidelity model. We show that accurate estimation of
reliability using mfEGRA leads to computational savings of 46% for an
analytic multimodal test problem and 24% for a three-dimensional acoustic horn
problem, when compared to single-fidelity EGRA. We also show the effect of
using a priori drawn Monte Carlo samples in the implementation for the acoustic
horn problem, where mfEGRA leads to computational savings of 45% for the
three-dimensional case and 48% for a rarer event four-dimensional case as
compared to single-fidelity EGRA
Thermodynamics of acoustic black holes in two dimensions
It is well-known that the thermal Hawking-like radiation can be emitted from
the acoustic horizon, but the thermodynamic-like understanding for acoustic
black holes was rarely made. In this paper, we will show that the kinematic
connection can lead to the dynamic connection at the horizon between the fluid
and gravitational models in two dimension, which implies that there exists the
thermodynamic-like description for acoustic black holes. Then, we discuss the
first law of thermodynamics for the acoustic black hole via an intriguing
connection between the gravitational-like dynamics of the acoustic horizon and
thermodynamics. We obtain a universal form for the entropy of acoustic black
holes, which has an interpretation similar to the entropic gravity. We also
discuss the specific heat, and find that the derivative of the velocity of
background fluid can be regarded as a novel acoustic analogue of the
two-dimensional dilaton potential, which interprets why the two-dimensional
fluid dynamics can be connected to the gravitational dynamics but difficult for
four-dimensional case. In particular, when a constraint is added for the fluid,
the analogue of a Schwarzschild black hole can be realized
Excitation of acoustic waves by vortices in the quiet Sun
Five-minutes oscillations is one of the basic properties of solar convection.
Observations show mixture of a large number of acoustic wave fronts propagating
from their sources. We investigate the process of acoustic waves excitation
from the point of view of individual events, by using realistic 3D radiative
hydrodynamic simulation of the quiet Sun. The results show that the excitation
events are related to dynamics vortex tubes (or swirls) in the intergranular
lanes. These whirlpool-like flows are characterized by very strong horizontal
velocities (7 - 11 km/s) and downflows (~ 7 km/s), and are accompanied by
strong decreases of the temperature, density and pressure at the surface and in
a ~ 0.5-1 Mm deep layer below the surface. High-speed whirlpool flows can
attract and capture other vortices. According to our simulation results, the
processes of the vortex interaction, such as vortex annihilation, can cause the
excitation of acoustic waves.Comment: 10 pages, 5 figure, submitted to ApJ
Deciphering acoustic emission signals in drought stressed branches: the missing link between source and sensor
When drought occurs in plants, acoustic emission (AE) signals can be detected, but the actual causes of these signals are still unknown. By analyzing the waveforms of the measured signals, it should, however, be possible to trace the characteristics of the AE source and get information about the underlying physiological processes. A problem encountered during this analysis is that the waveform changes significantly from source to sensor and lack of knowledge on wave propagation impedes research progress made in this field. We used finite element modeling and the well-known pencil lead break source to investigate wave propagation in a branch. A cylindrical rod of polyvinyl chloride was first used to identify the theoretical propagation modes. Two wave propagation modes could be distinguished and we used the finite element model to interpret their behavior in terms of source position for both the PVC rod and a wooden rod. Both wave propagation modes were also identified in drying-induced signals from woody branches, and we used the obtained insights to provide recommendations for further AE research in plant science
Review: Acoustic emission technique - Opportunities, challenges and current work at QUT
Acoustic emission (AE) is the phenomenon where high frequency stress waves are generated by rapid release of energy within a material by sources such as crack initiation or growth. AE technique involves recording these stress waves by means of sensors placed on the surface and subsequent analysis of the recorded signals to gather information such as the nature and location of the source. AE is one of the several non-destructive testing (NDT) techniques currently used for structural health monitoring (SHM) of civil, mechanical and aerospace structures. Some of its advantages include ability to provide continuous in-situ monitoring and high sensitivity to crack activity. Despite these advantages, several challenges still exist in successful application of AE monitoring. Accurate localization of AE sources, discrimination between genuine AE sources and spurious noise sources and damage quantification for severity assessment are some of the important issues in AE testing and will be discussed in this paper. Various data analysis and processing approaches will be applied to manage those issues
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