8,579 research outputs found
The scenario of two-dimensional instabilities of the cylinder wake under EHD forcing: A linear stability analysis
We propose to study the stability properties of an air flow wake forced by a dielectric barrier discharge (DBD) actuator, which is a type of electrohydrodynamic (EHD) actuator. These actuators add momentum to the flow around a cylinder in regions close to the wall and, in our case, are symmetrically disposed near the boundary layer separation point.
Since the forcing frequencies, typical of DBD, are much higher than the natural shedding frequency of the flow, we will be considering the forcing actuation as stationary.
In the first part, the flow around a circular cylinder modified by EHD actuators will be experimentally studied by means of particle image velocimetry (PIV). In the second part, the EHD actuators have been numerically implemented as a boundary condition on the cylinder surface. Using this boundary condition, the computationally obtained base flow is then compared with the experimental one in order to relate the control parameters from both methodologies.
After validating the obtained agreement, we study the Hopf bifurcation that appears once the flow starts the vortex shedding through experimental and computational approaches. For the base flow derived from experimentally obtained snapshots, we monitor the evolution of the velocity amplitude oscillations. As to the computationally obtained base flow, its stability is analyzed by solving a global eigenvalue problem obtained from the linearized Navier–Stokes equations. Finally, the critical parameters obtained from both approaches are compared
Extent of force indeterminacy in packings of frictional rigid disks
Static packings of frictional rigid particles are investigated by means of
discrete element simulations. We explore the ensemble of allowed force
realizations in the space of contact forces for a given packing structure. We
estimate the extent of force indeterminacy with different methods. The
indeterminacy exhibits a nonmonotonic dependence on the interparticle friction
coefficient. We verify directly that larger force-indeterminacy is accompanied
by a more robust behavior against local perturbations. We also investigate the
local indeterminacy of individual contact forces. The probability distribution
of local indeterminacy changes its shape depending on friction. We find that
local indeterminacy tends to be larger on force chains for intermediate
friction. This correlation disappears in the large friction limit.Comment: 5 pages, 6 figure
Pore Stabilization in Cohesive Granular Systems
Cohesive powders tend to form porous aggregates which can be compacted by
applying an external pressure. This process is modelled using the Contact
Dynamics method supplemented with a cohesion law and rolling friction. Starting
with ballistic deposits of varying density, we investigate how the porosity of
the compacted sample depends on the cohesion strength and the friction
coefficients. This allows to explain different pore stabilization mechanisms.
The final porosity depends on the cohesion force scaled by the external
pressure and on the lateral distance between branches of the ballistic deposit
r_capt. Even if cohesion is switched off, pores can be stabilized by Coulomb
friction alone. This effect is weak for round particles, as long as the
friction coefficient is smaller than 1. However, for nonspherical particles the
effect is much stronger.Comment: 10 pages, 15 figure
Stress-strain behavior and geometrical properties of packings of elongated particles
We present a numerical analysis of the effect of particle elongation on the
quasistatic behavior of sheared granular media by means of the Contact Dynamics
method. The particle shapes are rounded-cap rectangles characterized by their
elongation. The macroscopic and microstructural properties of several packings
subjected to biaxial compression are analyzed as a function of particle
elongation. We find that the shear strength is an increasing linear function of
elongation. Performing an additive decomposition of the stress tensor based on
a harmonic approximation of the angular dependence of branch vectors, contact
normals and forces, we show that the increasing mobilization of friction force
and the associated anisotropy are key effects of particle elongation. These
effects are correlated with partial nematic ordering of the particles which
tend to be oriented perpendicular to the major principal stress direction and
form side-to-side contacts. However, the force transmission is found to be
mainly guided by cap-to-side contacts, which represent the largest fraction of
contacts for the most elongated particles. Another interesting finding is that,
in contrast to shear strength, the solid fraction first increases with particle
elongation, but declines as the particles become more elongated. It is also
remarkable that the coordination number does not follow this trend so that the
packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review
Spatial fluctuations of a surviving particle in the trapping reaction
We consider the trapping reaction, , where and particles
have a diffusive dynamics characterized by diffusion constants and .
The interaction with particles can be formally incorporated in an effective
dynamics for one particle as was recently shown by Bray {\it et al}. [Phys.
Rev. E {\bf 67}, 060102 (2003)]. We use this method to compute, in space
dimension , the asymptotic behaviour of the spatial fluctuation,
, for a surviving particle in the perturbative regime,
, for the case of an initially uniform distribution of
particles. We show that, for , with
. By contrast, the fluctuations of paths constrained to return to
their starting point at time grow with the larger exponent 1/3. Numerical
tests are consistent with these predictions.Comment: 10 pages, 5 figure
DEEP: a provenance-aware executable document system
The concept of executable documents is attracting growing interest from both academics and publishers since it is a promising technology for the dissemination of scientific results. Provenance is a kind of metadata that provides a rich description of the derivation history of data products starting from their original sources. It has been used in many different e-Science domains and has shown great potential in enabling reproducibility of scientific results. However, while both executable documents and provenance are aimed at enhancing the dissemination of scientific results, little has been done to explore the integration of both techniques. In this paper, we introduce the design and development of DEEP, an executable document environment that generates scientific results dynamically and interactively, and also records the provenance for these results in the document. In this system, provenance is exposed to users via an interface that provides them with an alternative way of navigating the executable document. In addition, we make use of the provenance to offer a document rollback facility to users and help to manage the system's dynamic resources
Efficiency of a thermodynamic motor at maximum power
Several recent theories address the efficiency of a macroscopic thermodynamic
motor at maximum power and question the so-called "Curzon-Ahlborn (CA)
efficiency." Considering the entropy exchanges and productions in an n-sources
motor, we study the maximization of its power and show that the controversies
are partly due to some imprecision in the maximization variables. When power is
maximized with respect to the system temperatures, these temperatures are
proportional to the square root of the corresponding source temperatures, which
leads to the CA formula for a bi-thermal motor. On the other hand, when power
is maximized with respect to the transitions durations, the Carnot efficiency
of a bi-thermal motor admits the CA efficiency as a lower bound, which is
attained if the duration of the adiabatic transitions can be neglected.
Additionally, we compute the energetic efficiency, or "sustainable efficiency,"
which can be defined for n sources, and we show that it has no other universal
upper bound than 1, but that in certain situations, favorable for power
production, it does not exceed 1/2
Force transmission in a packing of pentagonal particles
We perform a detailed analysis of the contact force network in a dense
confined packing of pentagonal particles simulated by means of the contact
dynamics method. The effect of particle shape is evidenced by comparing the
data from pentagon packing and from a packing with identical characteristics
except for the circular shape of the particles. A counterintuitive finding of
this work is that, under steady shearing, the pentagon packing develops a lower
structural anisotropy than the disk packing. We show that this weakness is
compensated by a higher force anisotropy, leading to enhanced shear strength of
the pentagon packing. We revisit "strong" and "weak" force networks in the
pentagon packing, but our simulation data provide also evidence for a large
class of "very weak" forces carried mainly by vertex-to-edge contacts. The
strong force chains are mostly composed of edge-to-edge contacts with a marked
zig-zag aspect and a decreasing exponential probability distribution as in a
disk packing
Generalized Forward-Backward Splitting
This paper introduces the generalized forward-backward splitting algorithm
for minimizing convex functions of the form , where
has a Lipschitz-continuous gradient and the 's are simple in the sense
that their Moreau proximity operators are easy to compute. While the
forward-backward algorithm cannot deal with more than non-smooth
function, our method generalizes it to the case of arbitrary . Our method
makes an explicit use of the regularity of in the forward step, and the
proximity operators of the 's are applied in parallel in the backward
step. This allows the generalized forward backward to efficiently address an
important class of convex problems. We prove its convergence in infinite
dimension, and its robustness to errors on the computation of the proximity
operators and of the gradient of . Examples on inverse problems in imaging
demonstrate the advantage of the proposed methods in comparison to other
splitting algorithms.Comment: 24 pages, 4 figure
Flavour physics of the RS model with KK masses reachable at LHC
The version of the higher-dimensional Randall-Sundrum (RS) model with matter
in the bulk, which addresses the gauge hierarchy problem, has additional
attractive features. In particular, it provides an intrinsic geometrical
mechanism that can explain the origin of the large mass hierarchies among the
Standard Model fermions. Within this context, a good solution for the gauge
hierarchy problem corresponds to low masses for the Kaluza-Klein (KK)
excitations of the gauge bosons. Some scenarios have been proposed in order to
render these low masses (down to a few TeV) consistent with precision
electroweak measurements. Here, we give specific and complete realizations of
this RS version with small KK masses, down to 1 TeV, which are consistent with
the entire structure of the fermions in flavour space: (1) all the last
experimental data on quark/lepton masses and mixing angles (including massive
neutrinos of Dirac type) are reproduced, (2) flavour changing neutral current
constraints are satisfied and (3) the effective suppression scales of
non-renormalizable interactions (in the physical basis) are within the bounds
set by low energy flavour phenomenology. Our result, on the possibility of
having KK gauge boson modes as light as a few TeV, constitutes one of the first
theoretical motivations for experimental searches of direct signatures at the
LHC collider, of this interesting version of the RS model which accommodates
fermion masses.Comment: 27 pages, Latex file. References and comments adde
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