7,038 research outputs found
Chemical and physical influences on aerosol activation in liquid clouds: a study based on observations from the Jungfraujoch, Switzerland
A simple statistical model to predict the number of aerosols which activate to form cloud droplets in warm clouds has been established, based on regression analysis of data from four summertime Cloud and Aerosol Characterisation Experiments (CLACE) at the high-altitude site Jungfraujoch (JFJ). It is shown that 79 % of the observed variance in droplet numbers can be represented by a model accounting only for the number of potential cloud condensation nuclei (defined as number of particles larger than 80 nm in diameter), while the mean errors in the model representation may be reduced by the addition of further explanatory variables, such as the mixing ratios of O3, CO, and the height of the measurements above cloud base. The statistical model has a similar ability to represent the observed droplet numbers in each of the individual years, as well as for the two predominant local wind directions at the JFJ (northwest and southeast). Given the central European location of the JFJ, with air masses in summer being representative of the free troposphere with regular boundary layer in-mixing via convection, we expect that this statistical model is generally applicable to warm clouds under conditions where droplet formation is aerosol limited (i.e. at relatively high updraught velocities and/or relatively low aerosol number concentrations). A comparison between the statistical model and an established microphysical parametrization shows good agreement between the two and supports the conclusion that cloud droplet formation at the JFJ is predominantly controlled by the number concentration of aerosol particles
Complete characterization of sink-strengths for 1D to 3D mobilities of defect clusters.II. Bridging between limiting cases with effective sink-strengths calculations
In a companion paper, we proposed new analytical expressions of cluster
sink-strengths (CSS) indispensable to any complete parameterization of rate
equations cluster dynamics accounting for reaction between defect clusters
populations having a 1D-mobility. In this second paper, we first establish
simulation setup rules for truly converged estimates of effective CSS by
Kinetic Monte-Carlo, and then we grid on a wide set of radii, rotation
energies, diffusion coefficients and concentrations of both reaction partners.
Symmetric roles of some parameters are used to infer a generic form for a
semi-analytical expression of CSS depending on all these interaction
parameters: it is composed of the various analytical limiting cases established
and fitted transition functions that allow a gradual switching between them.
The analysis of the residuals shows that the overall agreement is reasonably
good: it is only in the transition zones that discrepancies are located and
this is due to the asymmetry of the actual transition functions. The expression
can be easily extended to temperatures at least a few hundred degrees around
the reference. But further extending the CSS evaluations to much smaller
diffusion coefficients ratios, we see that the domain for 1D-1D mobility is
very extended: for a ratio the computed CSS is still not correctly
described by the 1D-CSS with respect to a fixed sink (1D-0), but rather by the
established 1D-1D expression. For our typical sets of conditions, it is only
when approaching a ratio of that the 1D-0 CSS starts to become more
relevant. This highlights the counter-intuitive fact that the growth kinetics
of moderately trapped 1D mobile loops, whose effective mobility is greatly
reduced, may not be described by 1D-0 kinetics but rather by appropriately
corrected 1D-1D CSS which have completely different order of magnitude and
kinetic orders.Comment: 21 pages, 12 figure
Level Set Methods for Stochastic Discontinuity Detection in Nonlinear Problems
Stochastic physical problems governed by nonlinear conservation laws are
challenging due to solution discontinuities in stochastic and physical space.
In this paper, we present a level set method to track discontinuities in
stochastic space by solving a Hamilton-Jacobi equation. By introducing a speed
function that vanishes at discontinuities, the iso-zero of the level set
problem coincide with the discontinuities of the conservation law. The level
set problem is solved on a sequence of successively finer grids in stochastic
space. The method is adaptive in the sense that costly evaluations of the
conservation law of interest are only performed in the vicinity of the
discontinuities during the refinement stage. In regions of stochastic space
where the solution is smooth, a surrogate method replaces expensive evaluations
of the conservation law. The proposed method is tested in conjunction with
different sets of localized orthogonal basis functions on simplex elements, as
well as frames based on piecewise polynomials conforming to the level set
function. The performance of the proposed method is compared to existing
adaptive multi-element generalized polynomial chaos methods
Low temperature dynamics of kinks on Ising interfaces
The anisotropic motion of an interface driven by its intrinsic curvature or
by an external field is investigated in the context of the kinetic Ising model
in both two and three dimensions. We derive in two dimensions (2d) a continuum
evolution equation for the density of kinks by a time-dependent and nonlocal
mapping to the asymmetric exclusion process. Whereas kinks execute random walks
biased by the external field and pile up vertically on the physical 2d lattice,
then execute hard-core biased random walks on a transformed 1d lattice. Their
density obeys a nonlinear diffusion equation which can be transformed into the
standard expression for the interface velocity v = M[(gamma + gamma'')kappa +
H]$, where M, gamma + gamma'', and kappa are the interface mobility, stiffness,
and curvature, respectively. In 3d, we obtain the velocity of a curved
interface near the orientation from an analysis of the self-similar
evolution of 2d shrinking terraces. We show that this velocity is consistent
with the one predicted from the 3d tensorial generalization of the law for
anisotropic curvature-driven motion. In this generalization, both the interface
stiffness tensor and the curvature tensor are singular at the
orientation. However, their product, which determines the interface velocity,
is smooth. In addition, we illustrate how this kink-based kinetic description
provides a useful framework for studying more complex situations by modeling
the effect of immobile dilute impurities.Comment: 11 pages, 10 figure
Thermally-activated Non-Schmid Glide of Screw Dislocations in W using Atomistically-informed Kinetic Monte Carlo Simulations
Thermally-activated \small{\nicefrac{1}{2}} screw dislocation motion
is the controlling plastic mechanism at low temperatures in body-centered cubic
(bcc) crystals. Motion proceeds by the nucleation and propagation of
atomic-sized kink pairs susceptible of being studied using molecular dynamics
(MD). However, MD's natural inability to properly sample thermally-activated
processes as well as to capture screw dislocation glide calls for the
development of other methods capable of overcoming these limitations. Here we
develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation
dynamics from room temperature to and at stresses
, where and are the melting point and
the Peierls stress. The method is entirely parameterized with atomistic
simulations using an embedded atom potential for tungsten. To increase the
physical fidelity of our simulations, we calculate the deviations from Schmid's
law prescribed by the interatomic potential used and we study single
dislocation kinetics using both projections. We calculate dislocation
velocities as a function of stress, temperature, and dislocation line length.
We find that considering non-Schmid effects has a strong influence on both the
magnitude of the velocities and the trajectories followed by the dislocation.
We finish by condensing all the calculated data into effective stress and
temperature dependent mobilities to be used in more homogenized numerical
methods
Urban Swarms: A new approach for autonomous waste management
Modern cities are growing ecosystems that face new challenges due to the
increasing population demands. One of the many problems they face nowadays is
waste management, which has become a pressing issue requiring new solutions.
Swarm robotics systems have been attracting an increasing amount of attention
in the past years and they are expected to become one of the main driving
factors for innovation in the field of robotics. The research presented in this
paper explores the feasibility of a swarm robotics system in an urban
environment. By using bio-inspired foraging methods such as multi-place
foraging and stigmergy-based navigation, a swarm of robots is able to improve
the efficiency and autonomy of the urban waste management system in a realistic
scenario. To achieve this, a diverse set of simulation experiments was
conducted using real-world GIS data and implementing different garbage
collection scenarios driven by robot swarms. Results presented in this research
show that the proposed system outperforms current approaches. Moreover, results
not only show the efficiency of our solution, but also give insights about how
to design and customize these systems.Comment: Manuscript accepted for publication in IEEE ICRA 201
Modeling Radiation Damage to Pixel Sensors in the ATLAS Detector
Silicon pixel detectors are at the core of the current ATLAS detector and its
planned upgrade. As the detectors in closest proximity to the interaction
point, they will be exposed to a significant amount of radiation: prior to the
HL-LHC, the innermost layers will receive a fluence in excess of 1
MeV and the HL-LHC detector upgrades must cope
with an order of magnitude higher fluence integrated over their lifetimes. This
talk presents a digitization model that includes radiation damage effects to
the ATLAS Pixel sensors for the first time. After a thorough description of the
setup, predictions for basic pixel cluster properties are presented alongside
first validation studies with Run 2 collision data.Comment: 12 pages, 13 figures; Talk presented at the APS Division of Particles
and Fields Meeting (DPF 2017), July 31-August 4, 2017, Fermilab. C17073
Agent-Based Modeling of Human-Induced Spread of Invasive Species in Agricultural Landscapes: Insights from the Potato Moth in Ecuador
Agent-based models (ABM) are ideal tools to deal with the complexity of pest invasion throughout agricultural socio-ecological systems, yet very few studies have applied them in such context. In this work we developed an ABM that simulates interactions between farmers and an invasive insect pest in an agricultural landscape of the tropical Andes. Our specific aims were to use the model 1) to assess the importance of farmers\' mobility and pest control knowledge on pest expansion and 2) to use it as an educational tool to train farmer communities facing pest risks. Our model combined an ecological sub-model, simulating pest population dynamics driven by a cellular automaton including environmental factors of the landscape, with a social model in which we incorporated agents (farmers) potentially transporting and spreading the pest through displacements among villages. Results of model simulation revealed that both agents\' movements and knowledge had a significant, non-linear, impact on invasion spread, confirming previous works on disease expansion by epidemiologists. However, heterogeneity in knowledge among agents had a low effect on invasion dynamics except at high levels of knowledge. Evaluations of the training sessions using ABM suggest that farmers would be able to better manage their crop after our implementation. Moreover, by providing farmers with evidence that pests propagated through their community not as the result of isolated decisions but rather as the result of repeated interactions between multiple individuals over time, our ABM allowed introducing them with social and psychological issues which are usually neglected in integrated pest management programs.Socio-Ecological Systems, Farmers, Invasive Pest, Long Distance Dispersion, Teaching
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