2,390 research outputs found
Wavelet-based density estimation for noise reduction in plasma simulations using particles
For given computational resources, the accuracy of plasma simulations using
particles is mainly held back by the noise due to limited statistical sampling
in the reconstruction of the particle distribution function. A method based on
wavelet analysis is proposed and tested to reduce this noise. The method, known
as wavelet based density estimation (WBDE), was previously introduced in the
statistical literature to estimate probability densities given a finite number
of independent measurements. Its novel application to plasma simulations can be
viewed as a natural extension of the finite size particles (FSP) approach, with
the advantage of estimating more accurately distribution functions that have
localized sharp features. The proposed method preserves the moments of the
particle distribution function to a good level of accuracy, has no constraints
on the dimensionality of the system, does not require an a priori selection of
a global smoothing scale, and its able to adapt locally to the smoothness of
the density based on the given discrete particle data. Most importantly, the
computational cost of the denoising stage is of the same order as one time step
of a FSP simulation. The method is compared with a recently proposed proper
orthogonal decomposition based method, and it is tested with three particle
data sets that involve different levels of collisionality and interaction with
external and self-consistent fields
Wavelet Transform Applied to Internal Defect Detection by Means of Laser Ultrasound
Laser-generated ultrasound represents an interesting nondestructive testing technique that is being investigated in the last years as performative alternative to classical ultrasonic-based approaches. The greatest difficulty in analyzing the acoustic emission response is that an in-depth knowledge of how acoustic waves propagate through the tested composite is required. In this regard, different signal processing approaches are being applied in order to assess the significance of features extracted from the resulting analysis. In this study, the detection capabilities of internal defects in a metallic sample are proposed to be studied by means of the time-frequency analysis of the ultrasonic waves resulting from laser-induced thermal mechanism. In the proposed study, the use of the wavelet transform considering different wavelet variants is considered due to its multi-resolution time-frequency characteristics. Also, a significant time-frequency technique widely applied in other fields of research is applied, the synchrosqueezed transform
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An approach to melodic segmentation and classification based on filtering with the Haar wavelet
We present a novel method of classification and segmentation of melodies in symbolic representation. The method is based on filtering pitch as a signal over time with the Haar-wavelet, and we evaluate it on two tasks. The filtered signal corresponds to a single-scale signal ws from the continuous Haar wavelet transform. The melodies are first segmented using local maxima or zero-crossings of ws. The
segments of ws are then classified using the k–nearest neighbour algorithm with Euclidian and city-block distances. The method proves more effective than using unfiltered pitch signals and Gestalt-based segmentation when used to recognize the parent works of segments from Bach’s Two-Part Inventions (BWV 772–786). When used to classify 360 Dutch folk tunes into 26 tune families, the performance of the
method is comparable to the use of pitch signals, but not as good as that of string-matching methods based on multiple features
Sardinia Radio Telescope wide-band spectral-polarimetric observations of the galaxy cluster 3C 129
We present new observations of the galaxy cluster 3C 129 obtained with the
Sardinia Radio Telescope in the frequency range 6000-7200 MHz, with the aim to
image the large-angular-scale emission at high-frequency of the radio sources
located in this cluster of galaxies. The data were acquired using the
recently-commissioned ROACH2-based backend to produce full-Stokes image cubes
of an area of 1 deg x 1 deg centered on the radio source 3C 129. We modeled and
deconvolved the telescope beam pattern from the data. We also measured the
instrumental polarization beam patterns to correct the polarization images for
off-axis instrumental polarization. Total intensity images at an angular
resolution of 2.9 arcmin were obtained for the tailed radio galaxy 3C 129 and
for 13 more sources in the field, including 3C 129.1 at the galaxy cluster
center. These data were used, in combination with literature data at lower
frequencies, to derive the variation of the synchrotron spectrum of 3C 129
along the tail of the radio source. If the magnetic field is at the
equipartition value, we showed that the lifetimes of radiating electrons result
in a radiative age for 3C 129 of t_syn = 267 +/- 26 Myrs. Assuming a linear
projected length of 488 kpc for the tail, we deduced that 3C 129 is moving
supersonically with a Mach number of M=v_gal/c_s=1.47. Linearly polarized
emission was clearly detected for both 3C 129 and 3C 129.1. The linear
polarization measured for 3C 129 reaches levels as high as 70% in the faintest
region of the source where the magnetic field is aligned with the direction of
the tail.Comment: 19 pages, 17 figures, accepted for publication in MNRA
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