921 research outputs found
Coupled orbit and attitude dynamics of a reconfigurable spacecraft with solar radiation pressure
This work investigates the orbital and attitude dynamics of future reconfigurable multi-panel solar sails able to change
their shape during a mission. This can be enabled either by changing the relative position of the individual panels, or
by using articulated mechanisms and deployable, retractable and/or inflatable structures. Such a model introduces the
concept of modular spacecraft of variable morphology to large gossamer spacecraft. However, this joint concept is
complex in nature and requires equations for coupled orbit/attitude dynamics. Therefore, as a starting point, the system
is modelled as a rigid-body dumbbell consisting of two tip masses connected by a rigid, massless panel. The system
is subjected to a central gravitational force field under consideration of solar radiation pressure forces. Therefore, we
assign reflectivity coefficients to the tip masses and a high area-to-mass ratio. An analytical Hamiltonian approach
is used to describe the planar motion of the system in Sun-centred Keplerian and non-Keplerian circular orbits. The
stability and controllability of the system is enabled through changing the reflectivity coefficients, for example through
the use of electro-chromic coating on its surface. The creation of artificial unstable equilibria of the system due to the
presence of solar radiation pressure and heteroclinic connections between the equilibria are investigated. We further
derive a constraint for the solar radiation pressure forces to maintain the system on a circular Sun-centred orbit. It is
planned that the structure is eventually capable of reconfiguring between the equilibria by a minimum actuation effort
Probing defects and correlations in the hydrogen-bond network of ab initio water
The hydrogen-bond network of water is characterized by the presence of
coordination defects relative to the ideal tetrahedral network of ice, whose
fluctuations determine the static and time-dependent properties of the liquid.
Because of topological constraints, such defects do not come alone, but are
highly correlated coming in a plethora of different pairs. Here we discuss in
detail such correlations in the case of ab initio water models and show that
they have interesting similarities to regular and defective solid phases of
water. Although defect correlations involve deviations from idealized
tetrahedrality, they can still be regarded as weaker hydrogen bonds that retain
a high degree of directionality. We also investigate how the structure and
population of coordination defects is affected by approximations to the
inter-atomic potential, finding that in most cases, the qualitative features of
the hydrogen bond network are remarkably robust
Using Gaussian Process Regression to Simulate the Vibrational Raman Spectra of Molecular Crystals
Vibrational properties of molecular crystals are constantly used as structural fingerprints, in order to identify both the chemical nature and the structural arrangement of molecules. The simulation of these properties is typically very costly, especially when dealing with response properties of materials to e.g. electric fields, which require a good description of the perturbed electronic density. In this work, we use Gaussian process regression (GPR) to predict the static polarizability and dielectric susceptibility of molecules and molecular crystals. We combine this framework with ab initio molecular dynamics to predict their anharmonic vibrational Raman spectra. We stress the importance of data representation, symmetry, and locality, by comparing the performance of different flavors of GPR. In particular, we show the advantages of using a recently developed symmetry-adapted version of GPR. As an examplary application, we choose Paracetamol as an isolated molecule and in different crystal forms. We obtain accurate vibrational Raman spectra in all cases with fewer than 1000 training points, and obtain improvements when using a GPR trained on the molecular monomer as a baseline for the crystal GPR models. Finally, we show that our methodology is transferable across polymorphic forms: we can train the model on data for one structure, and still be able to accurately predict the spectrum for a second polymorph. This procedure provides an independent route to access electronic structure properties when performing force-evaluations on empirical force-fields or machine-learned potential energy surfaces
Shape-changing solar sails for novel mission applications
In order to increase the range of potential mission applications of solar sail technology, this paper introduces the
concepts of shape change and continuously variable optical properties to large gossamer spacecraft. Merging the two
concepts leads to the idea of solar sails as multi-functional platforms that can have potential benefits over conventional
solar sails by delivering additional key mission functions such as power collection, sensing and communications. To
this aim, the paper investigates the static deflection of a thin inelastic circular sail film with a variable surface reflectivity
distribution. The sail film is modelled as a single surface framed by a rigid supporting hoop structure. When changing
the reflectivity coefficient across the sail surface, the forces acting on the sail can be controlled without changing the
incidence angle relative to the Sun. In addition, by assigning an appropriate reflectivity function across the sail, the
load distribution due to solar radiation pressure can also be manipulated to control the billowing of the film. By an
appropriate choice of reflectivity across the sail, specific geometries can be generated, such as a parabolic reflector,
thus enabling a multi-functional sail. This novel concept of optical reconfiguration can potentially extend solar sail
mission applications
Ab initio Modelling of the Early Stages of Precipitation in Al-6000 Alloys
Age hardening induced by the formation of (semi)-coherent precipitate phases
is crucial for the processing and final properties of the widely used Al-6000
alloys. Early stages of precipitation are particularly important from the
fundamental and technological side, but are still far from being fully
understood. Here, an analysis of the energetics of nanometric precipitates of
the meta-stable phases is performed, identifying the bulk, elastic
strain and interface energies that contribute to the stability of a nucleating
cluster. Results show that needle-shape precipitates are unstable to growth
even at the smallest size formula unit, i.e. there is no energy
barrier to growth. The small differences between different compositions points
toward the need for the study of possible precipitate/matrix interface
reconstruction. A classical semi-quantitative nucleation theory approach
including elastic strain energy captures the trends in precipitate energy
versus size and composition. This validates the use of mesoscale models to
assess stability and interactions of precipitates. Studies of smaller 3d
clusters also show stability relative to the solid solution state, indicating
that the early stages of precipitation may be diffusion-limited. Overall, these
results demonstrate the important interplay among composition-dependent bulk,
interface, and elastic strain energies in determining nanoscale precipitate
stability and growth
The Gibbs free energy of homogeneous nucleation: from atomistic nuclei to the planar limit
In this paper we discuss how the information contained in atomistic
simulations of homogeneous nucleation should be used when fitting the
parameters in macroscopic nucleation models. We show how the number of solid
and liquid atoms in such simulations can be determined unambiguously by using a
Gibbs dividing surface and how the free energy as a function of the number of
solid atoms in the nucleus can thus be extracted. We then show that the
parameters of a model based on classical nucleation theory can be fit using the
information contained in these free-energy profiles but that the parameters in
such models are highly correlated. This correlation is unfortunate as it
ensures that small errors in the computed free energy surface can give rise to
large errors in the extrapolated properties of the fitted model. To resolve
this problem we thus propose a method for fitting macroscopic nucleation models
that uses simulations of planar interfaces and simulations of three-dimensional
nuclei in tandem. We show that when the parameters of the macroscopic model are
fitted in this way the numerical errors for the final fitted model are smaller
and that the extrapolated predictions for large nuclei are thus more reliable
The inefficiency of re-weighted sampling and the curse of system size in high order path integration
Computing averages over a target probability density by statistical
re-weighting of a set of samples with a different distribution is a strategy
which is commonly adopted in fields as diverse as atomistic simulation and
finance. Here we present a very general analysis of the accuracy and efficiency
of this approach, highlighting some of its weaknesses. We then give an example
of how our results can be used, specifically to assess the feasibility of
high-order path integral methods. We demonstrate that the most promising of
these techniques -- which is based on re-weighted sampling -- is bound to fail
as the size of the system is increased, because of the exponential growth of
the statistical uncertainty in the re-weighted average
Optimisation of Low-Thrust and Hybrid Earth-Moon Transfers
This paper presents an optimization procedure to generate fast and low-∆v Earth-Moon transfer trajectories, by exploiting the multi-body dynamics of the Sun-Earth-Moon system. Ideal (first-guess) trajectories are generated at first, using two coupled planar circular restricted three-body problems, one representing the Earth-Moon system, and one representing the Sun-Earth. The trajectories consist of a first ballistic arc in the Sun-Earth system, and a second ballistic arc in the Earth-Moon system. The two are connected at a patching point at one end (with an instantaneous ∆v), and they are bounded at Earth and Moon respectively at the other end. Families of these trajectories are found by means of an evolutionary optimization method. Subsequently, they are used as first-guess for solving an optimal control problem, in which the full three-dimensional 4-body problem is introduced and the patching point is set free. The objective of the optimisation is to reduce the total ∆v, and the time of flight, together with introducing the constraints on the transfer boundary conditions and of the considered propulsion technology. Sets of different optimal trajectories are presented, which represents trade-off options between ∆v and time of flight. These optimal transfers include conventional solar-electric low-thrust and hybrid chemical/solar-electric high/low-thrust, envisaging future spacecraft that can carry both systems. A final comparison is made between the optimal transfers found and only chemical high-thrust optimal solutions retrieved from literature
A self-learning algorithm for biased molecular dynamics
A new self-learning algorithm for accelerated dynamics, reconnaissance
metadynamics, is proposed that is able to work with a very large number of
collective coordinates. Acceleration of the dynamics is achieved by
constructing a bias potential in terms of a patchwork of one-dimensional,
locally valid collective coordinates. These collective coordinates are obtained
from trajectory analyses so that they adapt to any new features encountered
during the simulation. We show how this methodology can be used to enhance
sampling in real chemical systems citing examples both from the physics of
clusters and from the biological sciences.Comment: 6 pages, 5 figures + 9 pages of supplementary informatio
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