4,602 research outputs found
The effect of swingarm stiffness on motorcycle stability: Experimental measurements and numerical simulations
This paper focuses on the effect of swingarm deformability on motorcycle stability and in particular on the weave mode. Multibody models for the analysis of stability and handling of single track vehicles require a lumped element representation of the deformability of the critical structural elements of the vehicle. The twist axis method is used to identify lumped stiffness and damping elements able to represent bending and torsion deformability of the swingarm. Experimental tests and identification results dealing with two different swingarms are presented. The identified lumped stiffness and damping elements are implemented in a multibody code and some numerical stability analyses are carried out. Calculated results show that swingarm deformability has a small effect on the stability of super sport motorcycles, whereas the stability of the weave mode of enduro motorcycles is affected by swingarm deformability in a specific range of speeds
Extensive search for bias in SNIa data
The use of advanced statistical analysis tools is crucial in order to improve
cosmological parameter estimates via removal of systematic errors and
identification of previously unaccounted for cosmological signals. Here we
demonstrate the application of a new fully-Bayesian method, the internal
robustness formalism, to scan for systematics and new signals in the recent
supernova Ia Union compilations. Our analysis is tailored to maximize chances
of detecting the anomalous subsets by means of a variety of sorting algorithms.
We analyse supernova Ia distance moduli for effects depending on angular
separation, redshift, surveys and hemispherical directions. The data have
proven to be robust within 2 sigma, giving an independent confirmation of
successful removal of systematics-contaminated supernovae. Hints of new
cosmology, as for example the anisotropies reported by Planck, do not seem to
be reflected in the supernova Ia data.Comment: 11 pages, 7 figures, matches version accepted for publication in
MNRA
Parametric analysis of the stability of a bicycle taking into account geometrical, mass and compliance properties
Some studies of bicycle dynamics have applied the Whipple
Carvallo bicycle model (WCBM) for the stability analysis. The WCBM is
limited, since structural elements are assumed to be rigid bodies. In this paper,
the WCBM is extended to include the front assembly lateral compliance,
and analysis focuses on the study of the open loop stability of a benchmark
bicycle. Experimental tests to identify fork and wheel properties are performed,
this data is used in the stability analysis for ranking the influence of design
parameters. Indexes from the eigenvalues analysis are applied in a full factorial
approach. The results show that introducing front assembly compliance
generates a wobble mode with little effect on self-stability. The forward
displacement of the centre of mass of the rear frame and the increment in trail
lead to large increments in the self-stability, whereas increments in front wheel
radius and wheelbase reduce stability
Cosmic variance and the measurement of the local Hubble parameter
There is an approximately 9% discrepancy, corresponding to 2.4sigma, between
two independent constraints on the expansion rate of the universe: one
indirectly arising from the cosmic microwave background and baryon acoustic
oscillations, and one more directly obtained from local measurements of the
relation between redshifts and distances to sources. We argue that by taking
into account the local gravitational potential at the position of the observer
this tension - strengthened by the recent Planck results - is partially
relieved and the concordance of the standard model of cosmology increased. We
estimate that measurements of the local Hubble constant are subject to a cosmic
variance of about 2.4% (limiting the local sample to redshifts z>0.010) or 1.3%
(limiting it to z>0.023), a more significant correction than that taken into
account already. Nonetheless, we show that one would need a very rare
fluctuation to fully explain the offset in the Hubble rates. If this tension is
further strengthened, a cosmology beyond the standard model may prove
necessary.Comment: 5 pages, 4 figures. v2: added one figure, improved results and
discussion, added references. v3: updated to Planck results. v4: matches
version accepted for publication in PR
Supernova constraints on Multi-coupled Dark Energy
The persisting consistency of ever more accurate observational data with the
predictions of the standard LCDM cosmological model puts severe constraints on
possible alternative scenarios, but still does not shed any light on the
fundamental nature of the cosmic dark sector.As large deviations from a LCDM
cosmology are ruled out by data, the path to detect possible features of
alternative models goes necessarily through the definition of cosmological
scenarios that leave almost unaffected the background and -- to a lesser extent
-- the linear perturbations evolution of the universe. In this context,the
Multi-coupled DE (McDE) model was proposed by Baldi 2012 as a particular
realization of an interacting Dark Energy field characterized by an effective
screening mechanism capable of suppressing the effects of the coupling at the
background and linear perturbation level. In the present paper, for the first
time, we challenge the McDE scenario through a direct comparison with real
data, in particular with the luminosity distance of Type Ia supernovae. By
studying the existence and stability conditions of the critical points of the
associated background dynamical system, we select only the cosmologically
consistent solutions, and confront their background expansion history with
data. Confirming previous qualitative results, the McDE scenario appears to be
fully consistent with the adopted sample of Type Ia supernovae, even for
coupling values corresponding to an associated scalar fifth-force about four
orders of magnitude stronger than standard gravity. Our analysis demonstrates
the effectiveness of the McDE background screening, and shows some new
non-trivial asymptotic solutions for the future evolution of the universe. Our
results show how the background expansion history might be highly insensitive
to the fundamental nature and to the internal complexity of the dark sector.
[Abridged]Comment: 10 pages, 7 figures. Matches version accepted for publication in JCA
Linear Perturbation constraints on Multi-coupled Dark Energy
The Multi-coupled Dark Energy (McDE) scenario has been recently proposed as a
specific example of a cosmological model characterized by a non-standard
physics of the dark sector of the universe that nevertheless gives an expansion
history which does not significantly differ from the one of the standard
CDM model. In this work, we present the first constraints on the McDE
scenario obtained by comparing the predicted evolution of linear density
perturbations with a large compilation of recent data sets for the growth rate
, including 6dFGS, LRG, BOSS, WiggleZ and VIPERS. Confirming
qualitative expectations, growth rate data provide much tighter bounds on the
model parameters as compared to the extremely loose bounds that can be obtained
when only the background expansion history is considered. In particular, the
confidence level on the coupling strength is reduced from
(background constraints only) to
(background and linear perturbation constraints). We also investigate how these
constraints further improve when using data from future wide-field surveys such
as supernova data from LSST and growth rate data from Euclid-type missions. In
this case the confidence level on the coupling further reduce to . Such constraints are in any case still consistent with a scalar
fifth-force of gravitational strength, and we foresee that tighter bounds might
be possibly obtained from the investigation of nonlinear structure formation in
McDE cosmologies.[Abridged]Comment: 24 pages, 12 figure
Constraining the growth of perturbations with lensing of supernovae
A recently proposed technique allows one to constrain both the background and
perturbation cosmological parameters through the distribution function of
supernova Ia apparent magnitudes. Here we extend this technique to alternative
cosmological scenarios, in which the growth of structure does not follow the
CDM prescription. We apply the method first to the supernova data
provided by the JLA catalog combined with all the current independent redshift
distortion data and with low-redshift cluster data from Chandra and show that
although the supernovae alone are not very constraining, they help in reducing
the confidence regions. Then we apply our method to future data from LSST and
from a survey that approximates the Euclid satellite mission. In this case we
show that the combined data are nicely complementary and can constrain the
normalization and the growth rate index to within
and , respectively. In particular, the LSST supernova catalog is forecast
to give the constraint . We also
report on constraints relative to a step-wise parametrization of the growth
rate of structures. These results show that supernova lensing serves as a good
cross-check on the measurement of perturbation parameters from more standard
techniques.Comment: 9 pages, 7 figures, matches version accepted for publication in
MNRAS. Added appendix with fits of lensing moments as function of (z,
\sigma_8, \gamma). The updated turboGL 3.0 code can be downloaded at
http://www.turbogl.or
New technology based on clamping for high gradient radio frequency photogun
High gradient rf photoguns have been a key development to enable several applications of high quality electron beams. They allow the generation of beams with very high peak current and low transverse emittance, satisfying the tight demands for free-electron lasers, energy recovery linacs, Compton/Thomson sources and high-energy linear colliders. In the present paper we present the design of a new rf photogun recently developed in the framework of the SPARC_LAB photoinjector activities at the laboratories of the National Institute of Nuclear Physics in Frascati (LNF-INFN, Italy). This design implements several new features from the electromagnetic point of view and, more important, a novel technology for its realization that does not involve any brazing process. From the electromagnetic point of view the gun presents high mode separation, low peak surface electric field at the iris and minimized pulsed heating on the coupler. For the realization, we have implemented a novel fabrication design that, avoiding brazing, strongly reduces the cost, the realization time and the risk of failure. Details on the electromagnetic design, low power rf measurements and high power radiofrequency and beam tests performed at the University of California in Los Angeles (UCLA) are discussed in the paper
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