338 research outputs found
Cyclic pitch for the control of wind turbine noise amplitude modulation
ABSTRACT Using experimental data acquired during a wind turbine measurement campaign, it is shown that amplitude modulation of aerodynamic noise can be generated by the rotating blades in conjunction with the atmospheric wind shear. As an attempt to alleviate this phenomenon, a control strategy is designed in form of a cyclic pitch of the blades. As a side effect, it is shown that it is also possible to reduce fatigue load on the blade using this cyclic pitch. The main goal is to reduce both amplitude modulation and fatigue load without compromising the energy harvested from the wind. A simulation tool that can model the different aerodynamic and aeroacoustic aspects of the study is presented. Parameters controlling the cyclic pitch are optimized in order to reduce amplitude modulation and/or fatigue load to a minimum. It is shown that such a minimum can be found and that benefit may be achieved if such a strategy is to be implemented on an actual wind turbine, though at the expense of an increased wear and tear of the pitch control system
Approaching criticality via the zero dissipation limit in the abelian avalanche model
The discrete height abelian sandpile model was introduced by Bak, Tang &
Wiesenfeld and Dhar as an example for the concept of self-organized
criticality. When the model is modified to allow grains to disappear on each
toppling, it is called bulk-dissipative. We provide a detailed study of a
continuous height version of the abelian sandpile model, called the abelian
avalanche model, which allows an arbitrarily small amount of dissipation to
take place on every toppling. We prove that for non-zero dissipation, the
infinite volume limit of the stationary measure of the abelian avalanche model
exists and can be obtained via a weighted spanning tree measure. We show that
in the whole non-zero dissipation regime, the model is not critical, i.e.,
spatial covariances of local observables decay exponentially. We then study the
zero dissipation limit and prove that the self-organized critical model is
recovered, both for the stationary measure and for the dynamics. We obtain
rigorous bounds on toppling probabilities and introduce an exponent describing
their scaling at criticality. We rigorously establish the mean-field value of
this exponent for .Comment: 46 pages, substantially revised 4th version, title has been changed.
The main new material is Section 6 on toppling probabilities and the toppling
probability exponen
Gravitational collapse with tachyon field and barotropic fluid
A particular class of space-time, with a tachyon field, \phi, and a
barotropic fluid constituting the matter content, is considered herein as a
model for gravitational collapse. For simplicity, the tachyon potential is
assumed to be of inverse square form i.e., V(\phi) \sim \phi^{-2}. Our purpose,
by making use of the specific kinematical features of the tachyon, which are
rather different from a standard scalar field, is to establish the several
types of asymptotic behavior that our matter content induces. Employing a
dynamical system analysis, complemented by a thorough numerical study, we find
classical solutions corresponding to a naked singularity or a black hole
formation. In particular, there is a subset where the fluid and tachyon
participate in an interesting tracking behaviour, depending sensitively on the
initial conditions for the energy densities of the tachyon field and barotropic
fluid. Two other classes of solutions are present, corresponding respectively,
to either a tachyon or a barotropic fluid regime. Which of these emerges as
dominant, will depend on the choice of the barotropic parameter, \gamma.
Furthermore, these collapsing scenarios both have as final state the formation
of a black hole.Comment: 18 pages, 7 figures. v3: minor changes. Final version to appear in
GR
More three-point correlators of giant magnons with finite size
In the framework of the semiclassical approach, we compute the normalized
structure constants in three-point correlation functions, when two of the
vertex operators correspond to heavy string states, while the third vertex
corresponds to a light state. This is done for the case when the heavy string
states are finite-size giant magnons with one or two angular momenta, and for
two different choices of the light state, corresponding to dilaton operator and
primary scalar operator. The relevant operators in the dual gauge theory are
Tr(F_{\mu\nu}^2 Z^j+...) and Tr(Z^j). We first consider the case of AdS_5 x S^5
and N = 4 super Yang-Mills. Then we extend the obtained results to the
gamma-deformed AdS_5 x S^5_\gamma, dual to N = 1 super Yang-Mills theory,
arising as an exactly marginal deformation of N = 4 super Yang-Mills.Comment: 14 pages, no figure
Holographic three-point functions of giant gravitons
Working within the AdS/CFT correspondence we calculate the three-point
function of two giant gravitons and one pointlike graviton using methods of
semiclassical string theory and considering both the case where the giant
gravitons wrap an S^3 in S^5 and the case where the giant gravitons wrap an S^3
in AdS_5. We likewise calculate the correlation function in N=4 SYM using two
Schur polynomials and a single trace chiral primary. We find that the gauge and
string theory results have structural similarities but do not match perfectly,
and interpret this in terms of the Schur polynomials' inability to interpolate
between dual giant and pointlike gravitons.Comment: 21 page
Aging and Holography
Aging phenomena are examples of `non-equilibrium criticality' and can be
exemplified by systems with Galilean and scaling symmetries but no time
translation invariance. We realize aging holographically using a deformation of
a non-relativistic version of gauge/gravity duality. Correlation functions of
scalar operators are computed using holographic real-time techniques, and agree
with field theory expectations. At least in this setup, general aging phenomena
are reproduced holographically by complexifying the bulk space-time geometry,
even in Lorentzian signature.Comment: 1 pdf figur
On holographic three point functions for GKP strings from integrability
Adapting the powerful integrability-based formalism invented previously for
the calculation of gluon scattering amplitudes at strong coupling, we develop a
method for computing the holographic three point functions for the large spin
limit of Gubser-Klebanov- Polyakov (GKP) strings. Although many of the ideas
from the gluon scattering problem can be transplanted with minor modifications,
the fact that the information of the external states is now encoded in the
singularities at the vertex insertion points necessitates several new
techniques. Notably, we develop a new generalized Riemann bilinear identity,
which allows one to express the area integral in terms of appropriate contour
integrals in the presence of such singularities. We also give some general
discussions on how semiclassical vertex operators for heavy string states
should be constructed systematically from the solutions of the Hamilton-Jacobi
equation.Comment: 62 pages;v2 Typos and equation (3.7) corrected. Clarifying remarks
added in Section 4.1. Published version;v3 Minor errors found in version 2
are corrected. For explanation of the revision, see Erratum published in
http://www.springerlink.com/content/m67055235407vx67/?MUD=M
Universal features of correlated bursty behaviour
Inhomogeneous temporal processes, like those appearing in human
communications, neuron spike trains, and seismic signals, consist of
high-activity bursty intervals alternating with long low-activity periods. In
recent studies such bursty behavior has been characterized by a fat-tailed
inter-event time distribution, while temporal correlations were measured by the
autocorrelation function. However, these characteristic functions are not
capable to fully characterize temporally correlated heterogenous behavior. Here
we show that the distribution of the number of events in a bursty period serves
as a good indicator of the dependencies, leading to the universal observation
of power-law distribution in a broad class of phenomena. We find that the
correlations in these quite different systems can be commonly interpreted by
memory effects and described by a simple phenomenological model, which displays
temporal behavior qualitatively similar to that in real systems
Rare coding SNP in DZIP1 gene associated with late-onset sporadic Parkinson's disease
We present the first application of the hypothesis-rich mathematical theory
to genome-wide association data. The Hamza et al. late-onset sporadic
Parkinson's disease genome-wide association study dataset was analyzed. We
found a rare, coding, non-synonymous SNP variant in the gene DZIP1 that confers
increased susceptibility to Parkinson's disease. The association of DZIP1 with
Parkinson's disease is consistent with a Parkinson's disease stem-cell ageing
theory.Comment: 14 page
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