16,671 research outputs found
An intuitive handheld acoustic noise source finder
ABSTRACT - An apparatus has been developed to find acoustic sound sources in the near field of a radiating object operating in a noisy environment. It is based on two orthogonally placed particle velocity probes (two Microflowns[1], [2]). The complete signal processing is done in real time with battery powered analogue circuitry, resulting in a very small and handheld measurement device. One Microflown is used to display the sound level and to listen to the source whilst rejecting the background noise and another Microflown is used to create a stereo\ud
indication in which direction the device must be moved to pinpoint the noise source
Shock finding on a moving-mesh: I. Shock statistics in non-radiative cosmological simulations
Cosmological shock waves play an important role in hierarchical structure
formation by dissipating and thermalizing kinetic energy of gas flows, thereby
heating the universe. Furthermore, identifying shocks in hydrodynamical
simulations and measuring their Mach number accurately is critical for
calculating the production of non-thermal particle components through diffusive
shock acceleration. However, shocks are often significantly broadened in
numerical simulations, making it challenging to implement an accurate shock
finder. We here introduce a refined methodology for detecting shocks in the
moving-mesh code AREPO, and show that results for shock statistics can be
sensitive to implementation details. We put special emphasis on filtering
against spurious shock detections due to tangential discontinuities and
contacts. Both of them are omnipresent in cosmological simulations, for example
in the form of shear-induced Kelvin-Helmholtz instabilities and cold fronts. As
an initial application of our new implementation, we analyse shock statistics
in non-radiative cosmological simulations of dark matter and baryons. We find
that the bulk of energy dissipation at redshift zero occurs in shocks with Mach
numbers around . Furthermore, almost of the
thermalization is contributed by shocks in the warm hot intergalactic medium
(WHIM), whereas occurs in clusters, groups and smaller halos.
Compared to previous studies, these findings revise the characterization of the
most important shocks towards higher Mach numbers and lower density structures.
Our results also suggest that regions with densities above and below
should be roughly equally important for the energetics of cosmic
ray acceleration through large-scale structure shocks.Comment: 16 pages, 13 figures, published in MNRAS, January 201
Finding faint HI structure in and around galaxies: scraping the barrel
Soon to be operational HI survey instruments such as APERTIF and ASKAP will
produce large datasets. These surveys will provide information about the HI in
and around hundreds of galaxies with a typical signal-to-noise ratio of
10 in the inner regions and 1 in the outer regions. In addition, such
surveys will make it possible to probe faint HI structures, typically located
in the vicinity of galaxies, such as extra-planar-gas, tails and filaments.
These structures are crucial for understanding galaxy evolution, particularly
when they are studied in relation to the local environment. Our aim is to find
optimized kernels for the discovery of faint and morphologically complex HI
structures. Therefore, using HI data from a variety of galaxies, we explore
state-of-the-art filtering algorithms. We show that the intensity-driven
gradient filter, due to its adaptive characteristics, is the optimal choice. In
fact, this filter requires only minimal tuning of the input parameters to
enhance the signal-to-noise ratio of faint components. In addition, it does not
degrade the resolution of the high signal-to-noise component of a source. The
filtering process must be fast and be embedded in an interactive visualization
tool in order to support fast inspection of a large number of sources. To
achieve such interactive exploration, we implemented a multi-core CPU (OpenMP)
and a GPU (OpenGL) version of this filter in a 3D visualization environment
().Comment: 17 pages, 9 figures, 4 tables. Astronomy and Computing, accepte
Support Vector Machine classification of strong gravitational lenses
The imminent advent of very large-scale optical sky surveys, such as Euclid
and LSST, makes it important to find efficient ways of discovering rare objects
such as strong gravitational lens systems, where a background object is
multiply gravitationally imaged by a foreground mass. As well as finding the
lens systems, it is important to reject false positives due to intrinsic
structure in galaxies, and much work is in progress with machine learning
algorithms such as neural networks in order to achieve both these aims. We
present and discuss a Support Vector Machine (SVM) algorithm which makes use of
a Gabor filterbank in order to provide learning criteria for separation of
lenses and non-lenses, and demonstrate using blind challenges that under
certain circumstances it is a particularly efficient algorithm for rejecting
false positives. We compare the SVM engine with a large-scale human examination
of 100000 simulated lenses in a challenge dataset, and also apply the SVM
method to survey images from the Kilo-Degree Survey.Comment: Accepted by MNRA
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