44,187 research outputs found
Time-Frequency Tracking of Spectral Structures Estimated by a Data-Driven Method
International audience—The installation of a condition monitoring system aims to reduce the operating costs of the monitored system by applying a predictive maintenance strategy. However, a system-driven configuration of the condition monitoring system requires the knowledge of the system kinematics and could induce lots a false alarms because of predefined thresholds. The purpose of this paper is to propose a complete data-driven method to automatically generate system health indicators without any a priori on the monitored system or the acquired signals. This method is composed of two steps. First, every acquired signal is analysed: the spectral peaks are detected and then grouped in more complex structure as harmonic series or modulation sidebands. Then, a time-frequency tracking operation is applied on all available signals: the spectral peaks and the spectral structures are tracked over time and grouped in trajectories, which will be used to generate the system health indicators. The proposed method is tested on real-world signals coming from a wind turbine test rig. The detection of a harmonic series and a modulation sideband reports the birth of a fault on the main bearing inner ring. The evolution of the fault severity is characterised by three automatically generated health indicators and is confirmed by experts
The spectrum of kink-like oscillations of solar photospheric magnetic elements
Recently, the availability of new high-spatial and -temporal resolution
observations of the solar photosphere has allowed the study of the oscillations
in small magnetic elements. Small magnetic elements have been found to host a
rich variety of oscillations detectable as intensity, longitudinal or
transverse velocity fluctuations which have been interpreted as MHD waves.
Small magnetic elements, at or below the current spatial resolution achieved by
modern solar telescopes, are though to play a relevant role in the energy
budget of the upper layers of the Sun's atmosphere, as they are found to cover
a significant fraction of the solar photosphere. Unfortunately, the limited
temporal length and/or cadence of the data sets, or the presence of
seeing-induced effects have prevented, so far, the estimation of the power
spectra of kink-like oscillations in small magnetic elements with good
accuracy. Motivated by this, we studied kink-like oscillations in small
magnetic elements, by exploiting very long duration and high-cadence data
acquired with the Solar Optical Telescope on board the Hinode satellite. In
this work we present the results of this analysis, by studying the power
spectral density of kink-like oscillations on a statistical basis. We found
that small magnetic elements exhibit a large number of spectral features in the
range 1-12 mHz. More interestingly, most of these spectral features are not
shared among magnetic elements but represent a unique signature of each
magnetic element itself.Comment: A&A accepted for publication. 8 pages, 5 figure
Nonlinear propagation of Alfven waves driven by observed photospheric motions: Application to the coronal heating and spicule formation
We have performed MHD simulations of Alfven wave propagation along an open
flux tube in the solar atmosphere. In our numerical model, Alfven waves are
generated by the photospheric granular motion. As the wave generator, we used a
derived temporal spectrum of the photospheric granular motion from G-band
movies of Hinode/SOT. It is shown that the total energy flux at the corona
becomes larger and the transition region height becomes higher in the case when
we use the observed spectrum rather than white/pink noise spectrum as the wave
generator. This difference can be explained by the Alfven wave resonance
between the photosphere and the transition region. After performing Fourier
analysis on our numerical results, we have found that the region between the
photosphere and the transition region becomes an Alfven wave resonant cavity.
We have confirmed that there are at least three resonant frequencies, 1, 3 and
5 mHz, in our numerical model. Alfven wave resonance is one of the most
effective mechanisms to explain the dynamics of the spicules and the sufficient
energy flux to heat the corona
Time resolved tracking of a sound scatterer in a turbulent flow: non-stationary signal analysis and applications
It is known that ultrasound techniques yield non-intrusive measurements of
hydrodynamic flows. For example, the study of the echoes produced by a large
number of particle insonified by pulsed wavetrains has led to a now standard
velocimetry technique. In this paper, we propose to extend the method to the
continuous tracking of one single particle embedded in a complex flow. This
gives a Lagrangian measurement of the fluid motion, which is of importance in
mixing and turbulence studies. The method relies on the ability to resolve in
time the Doppler shift of the sound scattered by the continuously insonfied
particle.
For this signal processing problem two classes of approaches are used:
time-frequency analysis and parametric high resolution methods. In the first
class we consider the spectrogram and reassigned spectrogram, and we apply it
to detect the motion of a small bead settling in a fluid at rest. In more
non-stationary turbulent flows where methods in the second class are more
robust, we have adapted an Approximated Maximum Likelihood technique coupled
with a generalized Kalman filter.Comment: 16 pages 9 figure
The relation between photospheric supergranular flows and magnetic flux emergence
A recent study carried out on high sensitivity SUNRISE/IMAX data has reported
about the existence of areas of limited flux emergence in the quiet Sun. By
exploiting an independent and longer (4 hours) data set acquired by HINODE/SOT,
we further investigate these regions by analysing their spatial distribution
and relation with the supergranular flow. Our findings, while confirming the
presence of these calm areas, also show that the rate of emergence of small
magnetic elements is largely suppressed at the locations where the divergence
of the supergranular plasma flows is positive. This means that the dead calm
areas previously reported in literature are not randomly distributed over the
solar photosphere but they are linked to the supergranular cells themselves.
These results are discussed in the framework of the recent literature.Comment: Accepted as A&A Lette
The Band Excitation Method in Scanning Probe Microscopy for Rapid Mapping of Energy Dissipation on the Nanoscale
Mapping energy transformation pathways and dissipation on the nanoscale and
understanding the role of local structure on dissipative behavior is a
challenge for imaging in areas ranging from electronics and information
technologies to efficient energy production. Here we develop a novel Scanning
Probe Microscopy (SPM) technique in which the cantilever is excited and the
response is recorded over a band of frequencies simultaneously rather than at a
single frequency as in conventional SPMs. This band excitation (BE) SPM allows
very rapid acquisition of the full frequency response at each point (i.e.
transfer function) in an image and in particular enables the direct measurement
of energy dissipation through the determination of the Q-factor of the
cantilever-sample system. The BE method is demonstrated for force-distance and
voltage spectroscopies and for magnetic dissipation imaging with sensitivity
close to the thermomechanical limit. The applicability of BE for various SPMs
is analyzed, and the method is expected to be universally applicable to all
ambient and liquid SPMs.Comment: 32 pages, 9 figures, accepted for publication in Nanotechnolog
Ocean swell within the kinetic equation for water waves
Effects of wave-wave interactions on ocean swell are studied. Results of
extensive simulations of swell evolution within the duration-limited setup for
the kinetic Hasselmann equation at long times up to seconds are
presented. Basic solutions of the theory of weak turbulence, the so-called
Kolmogorov-Zakharov solutions, are shown to be relevant to the results of the
simulations. Features of self-similarity of wave spectra are detailed and their
impact on methods of ocean swell monitoring are discussed. Essential drop of
wave energy (wave height) due to wave-wave interactions is found to be
pronounced at initial stages of swell evolution (of order of 1000 km for
typical parameters of the ocean swell). At longer times wave-wave interactions
are responsible for a universal angular distribution of wave spectra in a wide
range of initial conditions.Comment: Submitted to Journal of Geophysical Research 18 July 201
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