89,657 research outputs found
From Averaging to Acceleration, There is Only a Step-size
We show that accelerated gradient descent, averaged gradient descent and the
heavy-ball method for non-strongly-convex problems may be reformulated as
constant parameter second-order difference equation algorithms, where stability
of the system is equivalent to convergence at rate O(1/n 2), where n is the
number of iterations. We provide a detailed analysis of the eigenvalues of the
corresponding linear dynamical system , showing various oscillatory and
non-oscillatory behaviors, together with a sharp stability result with explicit
constants. We also consider the situation where noisy gradients are available,
where we extend our general convergence result, which suggests an alternative
algorithm (i.e., with different step sizes) that exhibits the good aspects of
both averaging and acceleration
DNS of vertical plane channel flow with finite-size particles: Voronoi analysis, acceleration statistics and particle-conditioned averaging
We have performed a direct numerical simulation of dilute turbulent
particulate flow in a vertical plane channel, fully resolving the phase
interfaces. The flow conditions are the same as those in the main case of
"Uhlmann, M., Phys. Fluids, vol. 20, 2008, 053305", with the exception of the
computational domain length which has been doubled in the present study. The
statistics of flow and particle motion are not significantly altered by the
elongation of the domain. The large-scale columnar-like structures which had
previously been identified do persist and they are still only marginally
decorrelated in the prolonged domain. Voronoi analysis of the spatial particle
distribution shows that the state of the dispersed phase can be characterized
as slightly more ordered than random tending towards a homogeneous spatial
distribution. It is also found that the p.d.f.'s of Lagrangian particle
accelerations for wall-normal and spanwise directions follow a lognormal
distribution as observed in previous experiments of homogeneous flows. The
streamwise component deviates from this law presenting significant skewness.
Finally, a statistical analysis of the flow in the near field around the
particles reveals that particle wakes present two regions, a near wake where
the velocity deficit decays as 1/x and a far wake with a decay of approximately
1/(x*x).Comment: accepted for publication in Int. J. Multiphase Flo
Non-averaged regularized formulations as an alternative to semi-analytical orbit propagation methods
This paper is concerned with the comparison of semi-analytical and
non-averaged propagation methods for Earth satellite orbits. We analyse the
total integration error for semi-analytical methods and propose a novel
decomposition into dynamical, model truncation, short-periodic, and numerical
error components. The first three are attributable to distinct approximations
required by the method of averaging, which fundamentally limit the attainable
accuracy. In contrast, numerical error, the only component present in
non-averaged methods, can be significantly mitigated by employing adaptive
numerical algorithms and regularized formulations of the equations of motion.
We present a collection of non-averaged methods based on the integration of
existing regularized formulations of the equations of motion through an
adaptive solver. We implemented the collection in the orbit propagation code
THALASSA, which we make publicly available, and we compared the non-averaged
methods to the semi-analytical method implemented in the orbit propagation tool
STELA through numerical tests involving long-term propagations (on the order of
decades) of LEO, GTO, and high-altitude HEO orbits. For the test cases
considered, regularized non-averaged methods were found to be up to two times
slower than semi-analytical for the LEO orbit, to have comparable speed for the
GTO, and to be ten times as fast for the HEO (for the same accuracy). We show
for the first time that efficient implementations of non-averaged regularized
formulations of the equations of motion, and especially of non-singular element
methods, are attractive candidates for the long-term study of high-altitude and
highly elliptical Earth satellite orbits.Comment: 33 pages, 10 figures, 7 tables. Part of the CMDA Topical Collection
on "50 years of Celestial Mechanics and Dynamical Astronomy". Comments and
feedback are encourage
Attitude and orbit coupling of planar helio-stable solar sails
The coupled attitude and orbit dynamics of solar sails is studied. The shape
of the sail is a simplified quasi-rhombic-pyramid that provides the structure
helio-stablility properties. After adimensionalisation, the system is put in
the form of a fast-slow dynamical system where the different time scales are
explicitely related to the physical parameters of the system. The orientation
of the body frame with respect to the inertial orbit frame is a fast phase that
can be averaged out. This gives rise to a simplified formulation that only
consists of the orbit dynamics perturbed by a flat sail with fixed attitude
perpendicular to the direction of the sunlight. The results are exemplified
using numerical simulations.Comment: 36 pages, 31 figure
Errors in particle tracking velocimetry with high-speed cameras
Velocity errors in particle tracking velocimetry (PTV) are studied. When
using high-speed video cameras, the velocity error may increase at a high
camera frame rate. This increase in velocity error is due to particle-position
uncertainty, which is one of two sources of velocity errors studied here. The
other source of error is particle acceleration, which has the opposite trend of
diminishing at higher frame rates. Both kinds of errors can propagate into
quantities calculated from velocity, such as the kinetic temperature of
particles or correlation functions. As demonstrated in a dusty plasma
experiment, the kinetic temperature of particles has no unique value when
measured using PTV, but depends on the sampling time interval or frame rate. It
is also shown that an artifact appears in an autocorrelation function computed
from particle positions and velocities, and it becomes more severe when a small
sampling-time interval is used. Schemes to reduce these errors are
demonstrated.Comment: 6 pages, 5 figures, Review of Scientific Instruments, 2011 (In Press
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