213 research outputs found
Finite element modelling in integral design strategies of form- and bending-active hybrid structures
This paper discusses form-finding and simulation strategies for form- and bending-active hybrid structures, with practical feedback from two realised projects. Next to some general aspects of computational form-finding approaches with focus on finite element methods (FEM), the influence of changing mechanical properties of elastic beams on the resultant form-found hybrid system will be discussed on an umbrella structure with integrated bendingactive beam elements. Alongside the question of simulation strategies comes the search for a practical design setup to establish an FEM environment that is cross integrating information from various other modelling environments. This is discussed through the case study project M1 where physical form-finding and vector-based spring methods are utilised to generate input data for the FEM simulation
I.C.E.: a Transportable Atomic Inertial Sensor for Test in Microgravity
We present our the construction of an atom interferometer for inertial
sensing in microgravity, as part of the I.C.E. (\textit{Interf\'{e}rom\'{e}trie
Coh\'{e}rente pour l'Espace}) collaboration. On-board laser systems have been
developed based on fibre-optic components, which are insensitive to mechanical
vibrations and acoustic noise, have sub-MHz linewidth, and remain frequency
stabilised for weeks at a time. A compact, transportable vacuum system has been
built, and used for laser cooling and magneto-optical trapping. We will use a
mixture of quantum degenerate gases, bosonic Rb and fermionic K,
in order to find the optimal conditions for precision and sensitivity of
inertial measurements. Microgravity will be realised in parabolic flights
lasting up to 20s in an Airbus. We show that the factors limiting the
sensitivity of a long-interrogation-time atomic inertial sensor are the phase
noise in reference frequency generation for Raman-pulse atomic beam-splitters
and acceleration fluctuations during free fall
NBSymple, a double parallel, symplectic N-body code running on Graphic Processing Units
We present and discuss the characteristics and performances, both in term of
computational speed and precision, of a numerical code which numerically
integrates the equation of motions of N 'particles' interacting via Newtonian
gravitation and move in an external galactic smooth field. The force evaluation
on every particle is done by mean of direct summation of the contribution of
all the other system's particle, avoiding truncation error. The time
integration is done with second-order and sixth-order symplectic schemes. The
code, NBSymple, has been parallelized twice, by mean of the Computer Unified
Device Architecture to make the all-pair force evaluation as fast as possible
on high-performance Graphic Processing Units NVIDIA TESLA C 1060, while the
O(N) computations are distributed on various CPUs by mean of OpenMP Application
Program. The code works both in single precision floating point arithmetics or
in double precision. The use of single precision allows the use at best of the
GPU performances but, of course, limits the precision of simulation in some
critical situations. We find a good compromise in using a software
reconstruction of double precision for those variables that are most critical
for the overall precision of the code. The code is available on the web site
astrowww.phys.uniroma1.it/dolcetta/nbsymple.htmlComment: Paper composed by 29 pages, including 9 figures. Submitted to New
Astronomy
Modelling of content-aware indicators for effective determination of shot boundaries in compressed MPEG videos
In this paper, a content-aware approach is proposed to design multiple test conditions for shot cut detection, which are organized into a multiple phase decision tree for abrupt cut detection and a finite state machine for dissolve detection. In comparison with existing approaches, our algorithm is characterized with two categories of content difference indicators and testing. While the first category indicates the content changes that are directly used for shot cut detection, the second category indicates the contexts under which the content change occurs. As a result, indications of frame differences are tested with context awareness to make the detection of shot cuts adaptive to both content and context changes. Evaluations announced by TRECVID 2007 indicate that our proposed algorithm achieved comparable performance to those using machine learning approaches, yet using a simpler feature set and straightforward design strategies. This has validated the effectiveness of modelling of content-aware indicators for decision making, which also provides a good alternative to conventional approaches in this topic
Analysis of 2D airglow imager data with respect to dynamics using machine learning
We demonstrate how machine learning can be easily applied
to support the analysis of large quantities of excited hydroxyl (OH*) airglow imager data. We use
a TCN (temporal convolutional network) classification algorithm to
automatically pre-sort images into the three categories âdynamicâ (images
where small-scale motions like turbulence are likely to be found), âcalmâ
(clear-sky images with weak airglow variations) and âcloudyâ (cloudy images
where no airglow analyses can be performed). The proposed approach is
demonstrated using image data of FAIMÂ 3 (Fast Airglow IMager), acquired at
Oberpfaffenhofen, Germany, between 11 June 2019 and 25 February 2020,
achieving a mean average precision of 0.82 in image classification. The
attached video sequence demonstrates the classification abilities of the
learned TCN.
Within the dynamic category, we find a subset of 13Â episodes of image
series showing turbulence. As FAIMÂ 3 exhibits a high spatial
(23âm per pixel) and temporal (2.8âs per image) resolution, turbulence
parameters can be derived to estimate the energy diffusion rate. Similarly to
the results the authors found for another FAIM station (Sedlak et al.,
2021), the values of the energy dissipation rate range from 0.03 to
3.18âWâkgâ1.</p
Detecting inertial effects with airborne matter-wave interferometry
Inertial sensors relying on atom interferometry offer a breakthrough advance
in a variety of applications, such as inertial navigation, gravimetry or
ground- and space-based tests of fundamental physics. These instruments require
a quiet environment to reach their performance and using them outside the
laboratory remains a challenge. Here we report the first operation of an
airborne matter-wave accelerometer set up aboard a 0g plane and operating
during the standard gravity (1g) and microgravity (0g) phases of the flight. At
1g, the sensor can detect inertial effects more than 300 times weaker than the
typical acceleration fluctuations of the aircraft. We describe the improvement
of the interferometer sensitivity in 0g, which reaches 2 x 10-4 ms-2 / \surdHz
with our current setup. We finally discuss the extension of our method to
airborne and spaceborne tests of the Universality of free fall with matter
waves.Comment: 7 pages, 6 figures. The final version of this article is available in
OPEN access (free) from the editor website at
http://www.nature.com/ncomms/journal/v2/n9/full/ncomms1479.htm
Robust 3D face capture using example-based photometric stereo
We show that using example-based photometric stereo, it is possible to achieve realistic reconstructions of the human face. The method can handle non-Lambertian reflectance and attached shadows after a simple calibration step. We use spherical harmonics to model and de-noise the illumination functions from images of a reference object with known shape, and a fast grid technique to invert those functions and recover the surface normal for each point of the target object. The depth coordinate is obtained by weighted multi-scale integration of these normals, using an integration weight mask obtained automatically from the images themselves. We have applied these techniques to improve the PHOTOFACE system of Hansen et al. (2010). © 2013 Elsevier B.V. All rights reserved
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