25,750 research outputs found
Predicting the wake structure of the HART II rotor using the vorticity transport model
Brown’s Vorticity Transport Model has been used to predict the wake structure and resultant
blade loading of the rotor that was studied during the HART II experimental programme.
The descending flight condition of the experiment yields significant high-frequency content to
the blade loading due to the presence of blade-vortex interactions. PIV images of the wake
structure were compared against numerical predictions of the detailed geometry of the rotor
wake using three different computational resolutions of the flow. This was done to investigate
the origin of inaccuracies exposed in an earlier study of the system in capturing the effects of
blade vortex interactions on the loading on the rotor. The predicted positions of the vortex
cores agree with measured data to within a fraction of the blade chord, and the strength of the
vortices is preserved to well downstream of the rotor, essentially independently of the resolution
of the calculation. Nevertheless the amplitude of the loading impulses induced on the blade by
vortex interaction are strongly influenced by the resolution of the calculation through the effect
of cell density on the minimum vortex core size that can be supported. It would appear thus
that the inaccuracies in predicting the high-frequency loading on the rotor are not due to any
inherent deficiency in the representation of the wake, although viscous effects may need to be
considered in future in order to decouple the vortex core size from the cell size, but rather due
to the inherent deficiencies of the lifting line approach used to model the blade aerodynamics
The Relation Between Offset and Conchoid Constructions
The one-sided offset surface Fd of a given surface F is, roughly speaking,
obtained by shifting the tangent planes of F in direction of its oriented
normal vector. The conchoid surface Gd of a given surface G is roughly speaking
obtained by increasing the distance of G to a fixed reference point O by d.
Whereas the offset operation is well known and implemented in most CAD-software
systems, the conchoid operation is less known, although already mentioned by
the ancient Greeks, and recently studied by some authors. These two operations
are algebraic and create new objects from given input objects. There is a
surprisingly simple relation between the offset and the conchoid operation. As
derived there exists a rational bijective quadratic map which transforms a
given surface F and its offset surfaces Fd to a surface G and its conchoidal
surface Gd, and vice versa. Geometric properties of this map are studied and
illustrated at hand of some complete examples. Furthermore rational universal
parameterizations for offsets and conchoid surfaces are provided
Analytical study of the origin and behavior of asymmetric vortices
An hypothesis advanced originally to explain computational observations is supported by theoretical considerations: The asymmetric mean flow observed on bodies of revolution at moderate to high angles of attack is the result of a convective instability of an originally symmetric flow to a time-invariant space-fixed disturbance. Additionally, the time-dependent fluctuations characteristic of the flow at higher angles of attack (up to 90 deg) are the result of an absolute instability of an originally steady flow to a small temporal disturbance of finite duration. Within a common domain, the instability mechanisms may coexist. The experimentally confirmed existence of bistable states, wherein the side-force variation with nose roll angle approaches a square-wave distribution, is attributed to the dominant influence of a pair of trailing vortices from the ogival forebody. Their existence is made possible by the appearance of foci of separation in the skin-friction line pattern beyond a critical angle of attack. The extreme sensitivity of the asymmetric flow orientation to nose geometry, demonstrated experimentally, is attributed to the presence of an indeterminate phase in the family of possible solutions for the three-dimensional wave system
Depletion interactions of non-spherical colloidal particles in polymer solutions
We consider anisotropic colloidal particles immersed in a solution of long,
flexible, and nonadsorbing polymers. For the dumbbell shapes of recently
synthesized particles consisting of two intersecting spheres and for
lens-shaped particles with spherical surfaces we calculate the isotropic and
anisotropic interaction parameters that determine the immersion free energy and
the orientation-dependent depletion interaction between particles that are
induced by the polymers. Exact results are obtained for random-walk like
(ideal) polymer chains
A wide field X-ray telescope for astronomical survey purposes: from theory to practice
X-ray mirrors are usually built in the Wolter I (paraboloid-hyperboloid)
configuration. This design exhibits no spherical aberration on-axis but suffers
from field curvature, coma and astigmatism, therefore the angular resolution
degrades rapidly with increasing off-axis angles. Different mirror designs
exist in which the primary and secondary mirror profiles are expanded as a
power series in order to increase the angular resolution at large off-axis
positions, at the expanses of the on-axis performances. Here we present the
design and global trade off study of an X-ray mirror systems based on
polynomial optics in view of the Wide Field X-ray Telescope (WFXT) mission.
WFXT aims at performing an extended cosmological survey in the soft X-ray band
with unprecedented flux sensitivity. To achieve these goals the angular
resolution required for the mission is very demanding ~5 arcsec mean resolution
across a 1-deg field of view. In addition an effective area of 5-9000 cm^2 at 1
keV is needed.Comment: Accepted for publication in the MNRAS (11pages, 3 table, 13 figures
Unconventional string-like singularities in flat spacetime
The conical singularity in flat spacetime is mostly known as a model of the
cosmic string or the wedge disclination in solids. Its another, equally
important, function is to be a representative of quasiregular singularities.
From all these of views it seems interesting to find out whether there exist
other similar singularities. To specify what "similar" means I introduce the
notion of the string-like singularity, which is, roughly speaking, an
absolutely mild singularity concentrated on a curve or on a 2-surface S
(depending on whether the space is three- of four-dimensional). A few such
singularities are already known: the aforementioned conical singularity, two
its Lorentzian versions, the "spinning string", the "screw dislocation", and
Tod's spacetime. In all these spacetimes S is a straight line (or a plane) and
one may wonder if this is an inherent property of the string-like
singularities. The aim of this paper is to construct string-like singularities
with less trivial S. These include flat spacetimes in which S is a spiral, or
even a loop. If such singularities exist in nature (in particular, as an
approximation to gravitational field of strings) their cosmological and
astrophysical manifestations must differ drastically from those of the
conventional cosmic strings. Likewise, being realized as topological defects in
crystals such loops and spirals will probably also have rather unusual
properties.Comment: Draft. References and comments are welcome. v2. Section 3 is intact,
the rest is made briefer and clearer. A couple of references are added. v3.
Insignificant correstions. The published versio
Hausdorff dimension of a quantum string
In the path integral formulation of quantum mechanics, Feynman and Hibbs
noted that the trajectory of a particle is continuous but nowhere
differentiable. We extend this result to the quantum mechanical path of a
relativistic string and find that the ``trajectory'', in this case, is a
fractal surface with Hausdorff dimension three. Depending on the resolution of
the detecting apparatus, the extra dimension is perceived as ``fuzziness'' of
the string world-surface. We give an interpretation of this phenomenon in terms
of a new form of the uncertainty principle for strings, and study the
transition from the smooth to the fractal phase.Comment: 18 pages, non figures, ReVTeX 3.0, in print on Phys.Rev.
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