7,932 research outputs found
Rigid body mode identification of the PAH-2 helicopter using the eigensystem realization algorithm
The rigid body modes of the PAH-2 'Tiger' helicopter were identified using the Eigensystem Realization Algorithm (ERA). This work complements ground vibration tests performed using DLR's traditional phase resonance technique and the ISSPA (Identification of Structural System Parameters) method. Rigid body modal parameters are important for ground resonance prediction. Time-domain data for ERA were obtained by inverse Fourier transformation of frequency response functions measured with stepped-sine excitation. Mode purity (based on the Phase Resonance Criterion) was generally equal to or greater than corresponding results obtained in the ground vibration tests. All identified natural frequencies and mode shapes correlate well with corresponding ground vibration test results. The modal identification approach discussed in this report has become increasingly attractive in recent years due to the steadily declining cost and increased performance of scientific computers. As illustrated in this application, modern time-domain methods can be successfully applied to data acquired using DLR's existing test equipment. Some suggestions are made for future applications of time domain modal identification in this manner
Chaotic provinces in the kingdom of the Red Queen
The interplay between parasites and their hosts is found in all kinds of
species and plays an important role in understanding the principles of
evolution and coevolution. Usually, the different genotypes of hosts and
parasites oscillate in their abundances. The well-established theory of
oscillatory Red Queen dynamics proposes an ongoing change in frequencies of the
different types within each species. So far, it is unclear in which way Red
Queen dynamics persists with more than two types of hosts and parasites. In our
analysis, an arbitrary number of types within two species are examined in a
deterministic framework with constant or changing population size. This general
framework allows for analytical solutions for internal fixed points and their
stability. For more than two species, apparently chaotic dynamics has been
reported. Here we show that even for two species, once more than two types are
considered per species, irregular dynamics in their frequencies can be observed
in the long run. The nature of the dynamics depends strongly on the initial
configuration of the system; the usual regular Red Queen oscillations are only
observed in some parts of the parameter region
Density functional theory for a model quantum dot: Beyond the local-density approximation
We study both static and transport properties of model quantum dots,
employing density functional theory as well as (numerically) exact methods. For
the lattice model under consideration the accuracy of the local-density
approximation generally is poor. For weak interaction, however, accurate
results are achieved within the optimized effective potential method, while for
intermediate interaction strengths a method combining the exact diagonalization
of small clusters with density functional theory is very successful. Results
obtained from the latter approach yield very good agreement with density matrix
renormalization group studies, where the full Hamiltonian consisting of the dot
and the attached leads has to be diagonalized. Furthermore we address the
question whether static density functional theory is able to predict the exact
linear conductance through the dot correctly - with, in general, negative
answer.Comment: 8 page
Eigensystem realization algorithm modal identification experiences with mini-mast
This paper summarizes work performed under a collaborative research effort between the National Aeronautics and Space Administration (NASA) and the German Aerospace Research Establishment (DLR, Deutsche Forschungsanstalt fur Luft- und Raumfahrt). The objective is to develop and demonstrate system identification technology for future large space structures. Recent experiences using the Eigensystem Realization Algorithm (ERA), for modal identification of Mini-Mast, are reported. Mini-Mast is a 20 m long deployable space truss used for structural dynamics and active vibration-control research at the Langley Research Center. A comprehensive analysis of 306 frequency response functions (3 excitation forces and 102 displacement responses) was performed. Emphasis is placed on two topics of current research: (1) gaining an improved understanding of ERA performance characteristics (theory vs. practice); and (2) developing reliable techniques to improve identification results for complex experimental data. Because of nonlinearities and numerous local modes, modal identification of Mini-Mast proved to be surprisingly difficult. Methods were available, ERA, for obtaining detailed, high-confidence results
Biomechanical factors may explain why grasping violates Weber's law
Copyright © 2015. Published by Elsevier Ltd. Acknowledgments The experiment was part of N. Aschenneller’s MD thesis. The study was funded by the Staedtler Stiftung (Nuremberg, Germany).Peer reviewedPostprin
Concept for classifying facade elements based on material, geometry and thermal radiation using multimodal UAV remote sensing
This paper presents a concept for classification of facade elements, based on the material and the geometry of the elements in addition
to the thermal radiation of the facade with the usage of a multimodal Unmanned Aerial Vehicle (UAV) system. Once the concept is
finalized and functional, the workflow can be used for energy demand estimations for buildings by exploiting existing methods for
estimation of heat transfer coefficient and the transmitted heat loss. The multimodal system consists of a thermal, a hyperspectral and
an optical sensor, which can be operational with a UAV. While dealing with sensors that operate in different spectra and have different
technical specifications, such as the radiometric and the geometric resolution, the challenges that are faced are presented. Addressed
are the different approaches of data fusion, such as image registration, generation of 3D models by performing image matching and the
means for classification based on either the geometry of the object or the pixel values. As a first step towards realizing the concept, the
result from a geometric calibration with a designed multimodal calibration pattern is presented
Measurement of the drift field in the ARGONTUBE LAr TPC with 266~nm pulsed laser beams
ARGONTUBE is a liquid argon time projection chamber (LAr TPC) with a drift
field generated in-situ by a Greinacher voltage multiplier circuit. We present
results on the measurement of the drift-field distribution inside ARGONTUBE
using straight ionization tracks generated by an intense UV laser beam. Our
analysis is based on a simplified model of the charging of a multi-stage
Greinacher circuit to describe the voltages on the field cage rings
3-Dimensional Core-Collapse
In this paper, we present the results of 3-dimensional collapse simulations
of rotating stars for a range of stellar progenitors. We find that for the
fastest spinning stars, rotation does indeed modify the convection above the
proto-neutron star, but it is not fast enough to cause core fragmentation.
Similarly, although strong magnetic fields can be produced once the
proto-neutron star cools and contracts, the proto-neutron star is not spinning
fast enough to generate strong magnetic fields quickly after collapse and, for
our simulations, magnetic fields will not dominate the supernova explosion
mechanism. Even so, the resulting pulsars for our fastest rotating models may
emit enough energy to dominate the total explosion energy of the supernova.
However, more recent stellar models predict rotation rates that are much too
slow to affect the explosion, but these models are not sophisticated enough to
determine whether the most recent, or past, stellar rotation rates are most
likely. Thus, we must rely upon observational constraints to determine the true
rotation rates of stellar cores just before collapse. We conclude with a
discussion of the possible constraints on stellar rotation which we can derive
from core-collapse supernovae.Comment: 34 pages (5 of 17 figures missing), For full paper, goto
http://qso.lanl.gov/~clf/papers/rot.ps.gz accepted by Ap
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