4,395 research outputs found
Stabilizing periodic orbits above the elliptic plane in the solar sail 3-body problem
We consider periodic orbits high above the ecliptic plane in the Elliptic Restricted Three-Body Problem where the third massless body is a solar sail. Periodic orbits above the ecliptic are of practical interest as they are ideally positioned for the year-round constant imaging of, and communication with, the poles. Initially we identify an unstable periodic orbit by using a numerical continuation from a known periodic orbit above the ecliptic in the circular case with the eccentricity as the varying parameter. This orbit is then used to construct a reference trajectory for the sail to track. In addition we illustrate an alternative method for constructing a periodic reference trajectory based on a time-delayed feedback mechanism. The reference trajectories are then tracked using a linear feedback regulator (LQR) where the control actuation is delivered by varying the solar sails orientation. Using this method it is shown that a 'near term' solar sail is capable of performing stable periodic motions high above the ecliptic
Solar sail formation flying for deep-space remote sensing
In this paper we consider how 'near' term solar sails can be used in formation above the ecliptic plane to provide platforms for accurate and continuous remote sensing of the polar regions of the Earth. The dynamics of the solar sail elliptical restricted three-body problem (ERTBP) are exploited for formation flying by identifying a family of periodic orbits above the ecliptic plane. Moreover, we find a family of 1 year periodic orbits where each orbit corresponds to a unique solar sail orientation using a numerical continuation method. It is found through a number of example numerical simulations that this family of orbits can be used for solar sail formation flying. Furthermore, it is illustrated numerically that Solar Sails can provide stable formation keeping platforms that are robust to injection errors. In addition practical trajectories that pass close to the Earth and wind onto these periodic orbits above the ecliptic are identified
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Embedding formulae for scattering in a waveguide containing polygonal obstacles
For certain wave scattering problems embedding formulae can be derived, which express the solution, or far-field behaviour of the solution, for arbitrary plane wave incident angle in terms of the corresponding quantities for a finite number of other related problems. Their scope has so far been limited to scattering in R^2, and to a lesser extent R^3; in this paper we derive embedding formulae for wave scattering in a class of two-dimensional waveguide. The waveguide is straight and of uniform width outside a finite length region within which the boundaries are piecewise-linear and the waveguide can contain polygonal obstacles, a restriction being that all boundaries of the waveguide and obstacles must be inclined at a rational angle to the axis of the waveguide. Once solutions are determined for a finite set of incident propagating modes, the embedding formulae provide expressions for reflection and transmission
coefficients for all remaining incident propagating modes. The precise number of solutions required is a function of the number and nature of the corners of the
boundaries and obstacles. The formulae are illustrated for a particular waveguide geometry for which the problem can be formulated as an integral equation and approximate numerical solutions determined using the Galerkin method
Some Exact Results on the Potts Model Partition Function in a Magnetic Field
We consider the Potts model in a magnetic field on an arbitrary graph .
Using a formula of F. Y. Wu for the partition function of this model as a
sum over spanning subgraphs of , we prove some properties of concerning
factorization, monotonicity, and zeros. A generalization of the Tutte
polynomial is presented that corresponds to this partition function. In this
context we formulate and discuss two weighted graph-coloring problems. We also
give a general structural result for for cyclic strip graphs.Comment: 5 pages, late
The importance of integrated projects in historic conservation education
The Erasmus Mundus Advanced Masters in Structural Analysis of Monuments and
Historical Constructions (SAHC) is a unique European program. It is a Master Program
jointly run by four Universities on the conservation of architectural heritage structures, with
the goal of producing an international platform of competence.
The educational objective of SAHC is to offer an advanced engineering program on the
conservation of structures, with a focus on cultural heritage buildings. The program is composed
of eight units (SA-1 through SA-8). SA-7 is the Integrated Project which is a projectbased
unit that includes group projects to solve real engineering problems with heritage
structures.
This paper will describe and demonstrate the importance of the integrated project as part
of the overall education of emerging engineering professionals in the field of conservation of
the built cultural heritage. While successful in Europe, the SAHC program with its Integrated
Project could be used as a model for other countries to implement a quality education program
for engineers in the conservation of heritage structures
Characterization of rapidly solidified commercial grey cast iron in drop-tube
This study presents containerless solidification of BS 1452 grade 250 commercial grey cast iron using 6.5m drop-tube apparatus. It gives a comparative summary of microstructural changes that occur between the rapidly cooled droplet particles as against its conventional slowly cooled control as-cast sample. The bulk as-received sample was melted and rapidly cooled during free fall in high vacuum containerless equipment. These rapidly solidified samples were collected and sieved into size ranges from >850 ÎŒm to <53 ÎŒm diameter, corresponding to estimated cooling rate of 500 K s-1 to 75,000 K s-1 with each sieve fraction being prepared for metallographic characterization. The analytical methods used include; light optical microscope and scanning electron microscopy, x-ray diffraction and differential thermal analyses. The result of this investigation reveals that the microstructure of the as-cast sample shows flake graphite randomly dispersed in ferrite matrix which is typical of slowly cooled grey cast iron. In contrast, flake graphite was absent in virtually all drop-tube samples even those with modest cooling rate. The evolved microstructure clearly shows the effect of cooling rate on the transformation from the conventional to rapidly solidified droplet particles in terms of microsegregation
Partition Function Zeros of a Restricted Potts Model on Lattice Strips and Effects of Boundary Conditions
We calculate the partition function of the -state Potts model
exactly for strips of the square and triangular lattices of various widths
and arbitrarily great lengths , with a variety of boundary
conditions, and with and restricted to satisfy conditions corresponding
to the ferromagnetic phase transition on the associated two-dimensional
lattices. From these calculations, in the limit , we determine
the continuous accumulation loci of the partition function zeros in
the and planes. Strips of the honeycomb lattice are also considered. We
discuss some general features of these loci.Comment: 12 pages, 12 figure
Distribution of the molecular absorption in front of the quasar B0218+357
The line of sight to the quasar B0218+357, one of the most studied lensed
systems, intercepts a z=0.68 spiral galaxy, which splits its image into two
main components A and B, separated by ca. 0.3'', and gives rise to molecular
absorption. Although the main absorption component has been shown to arise in
front of image A, it is not established whether some absorption from other
velocity components is also occuring in front of image B. To tackle this
question, we have observed the HCO+(2-1) absorption line during the
commissioning phase of the new very extended configuration of the Plateau de
Bure Interferometer, in order to trace the position of the absorption as a
function of frequency. Visibility fitting of the self-calibrated data allowed
us to achieve position accuracy between ~12 and 80 mas per velocity component.
Our results clearly demonstrate that all the different velocity components of
the HCO+(2-1) absorption arise in front of the south-west image A of the
quasar. We estimate a flux ratio fA/fB = 4.2 (-1.0;+1.8 at 106 GHz.Comment: accepted for publication in A&A Letter special issue for the new
extended configuration of the Plateau de Bure Interferomete
MERLIN/VLA imaging of the gravitational lens system B0218+357
Gravitational lenses offer the possibility of accurately determining the
Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the
most promising systems for an application of this technique. In particular this
system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999)
and preliminary mass modelling has given a value for H_0 of 69 +13/-19
km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the
mass modelling. As this system contains an Einstein ring it should be possible
to constrain the model better by imaging the ring at high resolution. To
achieve this we have combined data from MERLIN and the VLA at a frequency of 5
GHz. In particular MERLIN has been used in multi-frequency mode in order to
improve substantially the aperture coverage of the combined data set. The
resulting map is the best that has been made of the ring and contains many new
and interesting features. Efforts are currently underway to exploit the new
data for lensing constraints using the LensClean algorithm (Kochanek & Narayan
1992).Comment: Accepted for publication in MNRAS. 6 pages, 4 included PostScript
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Did local civil rights protest liberalize whitesâ racial attitudes?
Mazumder investigates the long-term effect of protest on political attitudes. He finds that whites have more liberal views on race and are more likely to be Democrats in counties where Civil Rights protest was reported in the early 1960s. The analysis omits a crucial predictor of protest and of racial attitudes: college education. We include the proportion of adults with a college degree and the number of college students at the county level. The inclusion of these variables, along with some other contextual variables from the original dataset, cuts the effect of protest by about half. Protest is no longer statistically significant in eight out of nine combinations of outcome variables and protest measures. The size of the effect remains trivial when we shift analysis from the county to the individual level. Even accounting for the individualâs own education, the countyâs proportion of college graduates is strongly associated with racial liberalism. This finding emphasizes the importance of education as a contextual variable. Our conclusion highlights two methodological lessons. First, causal inference should be paired with sustained historical inquiry that specifies plausible mechanisms. Second, statistical tests for sensitivity can induce complacency about the risk of confounding
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