177,101 research outputs found
On-demand Aerodynamics in Integrally Actuated Membranes with Feedback Control
This paper is a numerical investigation on model reduction and control system design of integrally actuated membrane wings. A high-fidelity electro-aeromechanical model is used for the simulation of the dynamic fluid-structure interaction between a low-Reynolds-number flow and a dielectric elastomeric wing. Two reduced-order models with different levels of complexity are then derived. They are based on the projection of the fullorder discretisation of fluid and structure on modal shapes obtained from eigenvalue analysis and Proper Orthogonal Decomposition. The low-order systems are then used for the design of Proportional-Integral-Derivative and Linear Quadratic Gaussian feedback schemes to control wing lift. When implemented in the full-order model, closed-loop dynamics are in very good agreement with the reduced-order model for both tracking and gust rejection, demonstrating the suitability of the approach. The control laws selected in this work were found to be effective only for low-frequency disturbances due to the large phase delay introduced by the fluid convective time-scales, but results demonstrate the potential for the aerodynamic control of membrane wings in outdoor flight using dielectric elastomers
Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the CDM Universe
This study analyzes the sizes, shapes and correlations of \lya clouds
produced by a hydrodynamic simulation of a spatially flat CDM universe with a
non-zero cosmological constant (, , ), over the redshift range . The \lya clouds range in
size from several kiloparsecs to about a hundred kiloparsecs in proper units,
and they range in shape from roundish, high column density regions with
\nhi\ge 10^{15} cm^{-2} to low column density sheet-like structures with
\nhi \le 10^{13} cm^{-2} at z=3. The most common shape found in the
simulation resembles that of a flattened cigar. The physical size of a typical
cloud grows with time roughly as while its shape hardly evolves
(except for the most dense regions ). Our result indicates that
any simple model with a population of spheres (or other shapes) of a uniform
size is oversimplified; if such a model agrees with observational evidence, it
is probably only by coincidence. We also illustrate why the use of double
quasar sightlines to set lower limits on cloud sizes is useful only when the
perpendicular sightline separation is small ( kpc).
Finally, we conjecture that high column density \lya clouds (\nhi\ge 10^{15}
cm^{-2}) may be the progenitors of the lower redshift faint blue galaxies.
This seems plausible because their correlation length, number density
(extrapolated to lower redshift) and their masses are in fair agreement with
those observed.Comment: ApJ, in press, 34 pages, 21 figures, figs (1a,b,c) can be at
http://astro.princeton.edu/~cen/LYASSC/lyassc.htm
The effect of imperfect corrections of PSF anisotropy on cosmic shear measurements
Current measurements of the weak lensing signal induced by large scale
structure provide useful constraints on a range of cosmological parameters.
However, the ultimate succes of this technique depends on the accuracy with
which one can correct for the effect of the Point Spread Function (PSF). In
this paper we examine the accuracy of the PSF anisotropy correction using
images of fields with a large number of stars. The ellipticity correlation
function of the residuals is studied to quantify the effect of imperfect
corrections for PSF anisotropy on cosmic shear studies. These imperfections
occur on the chip scale and consequently the systematic signal decreases
rapidly with increasing angular scale. Separation of the signal into ``E''
(curl-free) and ``B'' (curl) components can help to identify the presence of
residual systematics, but in general, the amplitude of the ``B''-mode is
different from that of the ``E''-mode. The study of fields with many stars can
be beneficial in finding a proper description of the variation of PSF
anisotropy, and consequently help to significantly improve the accuracy with
which the cosmic shear signal can be measured. We show that with such an
approach it is feasible that the accuracy of future cosmic shear studies is
limited by the statistical noise introduced by the intrinsic shapes of the
sources. In particular, the prospects for accurate measurements of the cosmic
shear signal on scales larger than ~10 arcminutes are excellent.Comment: submitted to MNRAS, 8 page
Dynamics of Primordial Black Hole Formation
We present a numerical investigation of the gravitational collapse of
horizon-size density fluctuations to primordial black holes (PBHs) during the
radiation-dominated phase of the Early Universe. The collapse dynamics of three
different families of initial perturbation shapes, imposed at the time of
horizon crossing, is computed. The perturbation threshold for black hole
formation, needed for estimations of the cosmological PBH mass function, is
found to be rather than the generally employed
, if is defined as \Delta M/\mh, the
relative excess mass within the initial horizon volume. In order to study the
accretion onto the newly formed black holes, we use a numerical scheme that
allows us to follow the evolution for long times after formation of the event
horizon. In general, small black holes (compared to the horizon mass at the
onset of the collapse) give rise to a fluid bounce that effectively shuts off
accretion onto the black hole, while large ones do not. In both cases, the
growth of the black hole mass owing to accretion is insignificant. Furthermore,
the scaling of black hole mass with distance from the formation threshold,
known to occur in near-critical gravitational collapse, is demonstrated to
apply to primordial black hole formation.Comment: 10 pages, 8 figures, revtex style, submitted to PR
Profile instabilities of the millisecond pulsar PSR J1022+1001
We present evidence that the integrated profiles of some millisecond pulsars
exhibit severe changes that are inconsistent with the moding phenomenon as
known from slowly rotating pulsars. We study these profile instabilities in
particular for PSR J1022+1001 and show that they occur smoothly, exhibiting
longer time constants than those associated with moding. In addition, the
profile changes of this pulsar seem to be associated with a relatively
narrow-band variation of the pulse shape. Only parts of the integrated profile
participate in this process which suggests that the origin of this phenomenon
is intrinsic to the pulsar magnetosphere and unrelated to the interstellar
medium. A polarization study rules out profile changes due to geometrical
effects produced by any sort of precession. However, changes are observed in
the circularly polarized radiation component. In total we identify four
recycled pulsars which also exhibit instabilities in the total power or
polarization profiles due to an unknown phenomenon (PSRs J1022+1001,
J1730-2304, B1821-24, J2145-0750).
The consequences for high precision pulsar timing are discussed in view of
the standard assumption that the integrated profiles of millisecond pulsars are
stable. As a result we present a new method to determine pulse times-of-arrival
that involves an adjustment of relative component amplitudes of the template
profile. Applying this method to PSR J1022+1001, we obtain an improved timing
solution with a proper motion measurement of -17 \pm 2 mas/yr in ecliptic
longitude. Assuming a distance to the pulsar as inferred from the dispersion
measure this corresponds to an one-dimensional space velocity of 50 km/s.Comment: 29 pages, 12 figures, accepted for publication in Ap
Two Sets of Simple Formulae to Estimating Fractal Dimension of Irregular Boundaries
Irregular boundary lines can be characterized by fractal dimension, which
provides important information for spatial analysis of complex geographical
phenomena such as cities. However, it is difficult to calculate fractal
dimension of boundaries systematically when image data is limited. An
approximation estimation formulae of boundary dimension based on square is
widely applied in urban and ecological studies. However, the boundary dimension
is sometimes overestimated. This paper is devoted to developing a series of
practicable formulae for boundary dimension estimation using ideas from
fractals. A number of regular figures are employed as reference shapes, from
which the corresponding geometric measure relations are constructed; from these
measure relations, two sets of fractal dimension estimation formulae are
derived for describing fractal-like boundaries. Correspondingly, a group of
shape indexes can be defined. A finding is that different formulae have
different merits and spheres of application, and the second set of boundary
dimensions is a function of the shape indexes. Under condition of data
shortage, these formulae can be utilized to estimate boundary dimension values
rapidly. Moreover, the relationships between boundary dimension and shape
indexes are instructive to understand the association and differences between
characteristic scales and scaling. The formulae may be useful for the
pre-fractal studies in geography, geomorphology, ecology, landscape science,
and especially, urban science.Comment: 28 pages, 2 figures, 9 table
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