458 research outputs found
A Mechanism to Produce the Small Dust Observed in Protoplanetary Disks
Small (sub)-micron dust is present over the entire lifetime of protoplanetary
disks. As aggregation readily depletes small particles, one explanation might
be that dust is continuously generated by larger bodies in the midplane and
transported to the surface of the disks. In general, in a first step of this
scenario, the larger bodies have to be destroyed again and different mechanisms
exist with the potential to accomplish this. Possible destructive mechanisms
are fragmentation in collisions, erosion by gas drag or light induced erosion.
In laboratory experiments we find that the latter, light induced erosion by
Knudsen compression and photophoresis, can provide small particles. It might be
a preferred candidate as the dust is released into a low particle density
region. The working principle of this mechanism prevents or decreases the
likelihood for instant re-accretion or re-growth of large dense aggregates.
Provided that there is a particle lift, e.g. turbulence, these particles might
readily reach the surface of the disk.Comment: 7 pages, 6 figure
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases of planet formation. It is for example currently unclear whether there is a bouncing barrier where millimeter aggregates no longer grow by sticking. We developed a laboratory setup that allowed us to observe collisions of dust aggregates levitating at mbar pressures and elevated temperatures of 800 K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller than 25 μm in size. We find that for all impact energies in the studied range sticking occurs at a probability of 32.1 ± 2.5% on average. In general, the sticking probability decreases with increasing impact parameter. The sticking probability increases with energy density (impact energy per contact area). We also observe collisions of aggregates that were formed by a previous sticking of two larger aggregates. Partners of these aggregates can be detached by a second collision with a probability of on average 19.8 ± 4.0%. The measured accretion efficiencies are remarkably high compared to other experimental results. We attribute this to the relatively large dust grains used in our experiments, which make aggregates more susceptible to restructuring and energy dissipation. Collisional hardening by compaction might not occur as the aggregates are already very compact with only 54 ± 1% porosity. The disassembly of previously grown aggregates in collisions might stall further aggregate growth. However, owing to the levitation technique and the limited data statistics, no conclusive statement about this aspect can yet be given. We find that the detachment efficiency decreases with increasing velocities and accretion dominates in the higher velocity range. For high accretion efficiencies, our experiments suggest that continued growth in the mm-range with larger constituent grains would be a viable way to produce larger aggregates, which might in turn form the seeds to proceed to growing planetesimals. © 2012 ESO
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases
of planet formation. We developed a laboratory setup to observe collisions of
dust aggregates levitating at mbar pressures and elevated temperatures of 800
K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm
in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller
than 25 \mum in size. We find that for all impact energies in the studied range
sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the
sticking probability decreases with increasing impact parameter. The sticking
probability increases with energy density (impact energy per contact area). We
also observe collisions of aggregates that were formed by a previous sticking
of two larger aggregates. Partners of these aggregates can be detached by a
second collision with a probability of on average 19.8 \pm 4.0%. The measured
accretion efficiencies are remarkably high compared to other experimental
results. We attribute this to the rel. large dust grains used in our
experiments, which make aggregates more susceptible to restructuring and energy
dissipation. Collisional hardening by compaction might not occur as the
aggregates are already very compact with only 54 \pm 1% porosity. The
disassembly of previously grown aggregates in collisions might stall further
aggregate growth. However, owing to the levitation technique and the limited
data statistics, no conclusive statement about this aspect can yet be given. We
find that the detachment efficiency decreases with increasing velocities and
accretion dominates in the higher velocity range. For high accretion
efficiencies, our experiments suggest that continued growth in the mm-range
with larger constituent grains would be a viable way to produce larger
aggregates, which might in turn form the seeds to proceed to growing
planetesimals.Comment: 9 pages, 20 figure
Comparison of Different Parallel Implementations of the 2+1-Dimensional KPZ Model and the 3-Dimensional KMC Model
We show that efficient simulations of the Kardar-Parisi-Zhang interface
growth in 2 + 1 dimensions and of the 3-dimensional Kinetic Monte Carlo of
thermally activated diffusion can be realized both on GPUs and modern CPUs. In
this article we present results of different implementations on GPUs using CUDA
and OpenCL and also on CPUs using OpenCL and MPI. We investigate the runtime
and scaling behavior on different architectures to find optimal solutions for
solving current simulation problems in the field of statistical physics and
materials science.Comment: 14 pages, 8 figures, to be published in a forthcoming EPJST special
issue on "Computer simulations on GPU
Community‐Engaged Neighborhood Revitalization and Empowerment: Busy Streets Theory in Action
Busy streets theory predicts that engaging residents in physical revitalization of neighborhoods will facilitate community empowerment through the development of sense of community, social cohesion, collective efficacy, social capital, and behavioral action. Establishing safe environments fosters positive street activity, which reinforces neighborhood social relationships. A community‐engaged approach to crime prevention through environmental design (CE‐CPTED) is one promising approach to creating busy streets because it engages residents in collaborative interactions to promote safer environments. Yet, few researchers have studied how CE‐CPTED may be associated with busy streets. We interviewed 18 residents and stakeholders implementing CE‐CPTED in Flint, Michigan. We studied three neighborhoods with different levels of resident control over CE‐CPTED. Participants described how CE‐CPTED implementation affected their neighborhood. Participants from all three neighborhoods reported that CE‐CPTED was associated with positive street activity, sense of community, and collective efficacy. Participants from neighborhoods with higher resident control of CE‐CPTED reported more social capital and behavioral action than those from neighborhoods with less resident control. Our findings support busy streets theory: Community engagement in neighborhood improvement enhanced community empowerment. CE‐CPTED that combines physical revitalization with resident engagement and control creates a potent synergy for promoting safe and healthy neighborhoods.HighlightsBusy streets theory supported in qualitative study of neighborhoods in a rust belt city.Community engaged neighborhood improvement enhances psychological empowerment.Resident control of neighborhood revitalization results in most empowered outcomes of busy streets.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154635/1/ajcp12358_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154635/2/ajcp12358.pd
Analytic traveling-wave solutions of the Kardar-Parisi-Zhang interface growing equation with different kind of noise terms
The one-dimensional Kardar-Parisi-Zhang dynamic interface growth equation
with the traveling-wave Ansatz is analyzed. As a new feature additional
analytic terms are added. From the mathematical point of view, these can be
considered as various noise distribution functions. Six different cases were
investigated among others Gaussian, Lorentzian, white or even pink noise.
Analytic solutions are evaluated and analyzed for all cases. All results are
expressible with various special functions Mathieu, Bessel, Airy or Whittaker
functions showing a very rich mathematical structure with some common general
characteristics. This study is the continuation of our former work, where the
same physical phenomena was investigated with the self-similar Ansatz. The
differences and similarities among the various solutions are enlightened.Comment: 14 pages,14 figures. arXiv admin note: text overlap with
arXiv:1904.0183
From dynamical scaling to local scale-invariance: a tutorial
Dynamical scaling arises naturally in various many-body systems far from
equilibrium. After a short historical overview, the elements of possible
extensions of dynamical scaling to a local scale-invariance will be introduced.
Schr\"odinger-invariance, the most simple example of local scale-invariance,
will be introduced as a dynamical symmetry in the Edwards-Wilkinson
universality class of interface growth. The Lie algebra construction, its
representations and the Bargman superselection rules will be combined with
non-equilibrium Janssen-de Dominicis field-theory to produce explicit
predictions for responses and correlators, which can be compared to the results
of explicit model studies.
At the next level, the study of non-stationary states requires to go over,
from Schr\"odinger-invariance, to ageing-invariance. The ageing algebra admits
new representations, which acts as dynamical symmetries on more general
equations, and imply that each non-equilibrium scaling operator is
characterised by two distinct, independent scaling dimensions. Tests of
ageing-invariance are described, in the Glauber-Ising and spherical models of a
phase-ordering ferromagnet and the Arcetri model of interface growth.Comment: 1+ 23 pages, 2 figures, final for
Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth
The formation of planetesimals is often accredited to collisional sticking of
dust grains. The exact process is unknown, as collisions between larger
aggregates tend to lead to fragmentation or bouncing rather than sticking.
Recent laboratory experiments have however made great progress in the
understanding and mapping of the complex physics involved in dust collisions.
We want to study the possibility of planetesimal formation using the results
from the latest laboratory experiments, particularly by including the
fragmentation with mass transfer effect, which might lead to growth even at
high impact velocities. We present a new experimentally and physically
motivated dust collision model capable of predicting the outcome of a collision
between two particles of arbitrary masses and velocities. It is used together
with a continuum dust-size evolution code that is both fast in terms of
execution time and able to resolve the dust well at all sizes, allowing for all
types of interactions to be studied without biases. We find that for the
general dust population, bouncing collisions prevent the growth above
millimeter-sizes. However, if a small number of cm-sized particles are
introduced, for example due to vertical mixing or radial drift, they can act as
a catalyst and start to sweep up the smaller particles. At a distance of 3 AU,
100-meter-sized bodies are formed on a timescale of 1 Myr. We conclude that
direct growth of planetesimals might be a possibility thanks to a combination
of the existence of a bouncing barrier and the fragmentation with mass transfer
effect. The bouncing barrier is here even beneficial, as it prevents the growth
of too many large particles that would otherwise only fragment among each
other, and creates a reservoir of small particles that can be swept up by
larger bodies. However, for this process to work, a few seeds of cm in size or
larger have to be introduced.Comment: 17 pages, 13 figures. Accepted for publication in Astronomy and
Astrophysic
Citizen science breathes new life into participatory agricultural research : A review
Participatory research can improve the efficiency, effectiveness, and scope of research processes, and foster social inclusion, empowerment and sustainability. Yet despite four decades of agricultural research institutions exploring and developing methods for participatory research, it has never become mainstream in the agricultural technology development cycle. Citizen science promises an innovative approach to participation in research, using the unique facilities of new digital technologies, but its potential in agricultural research participation has not been systematically probed. To this end, we conducted a critical literature review. We found that citizen science opens up four opportunities for creatively reshaping research: i) new possibilities for interdisciplinary collaboration, ii) rethinking configurations of socio-computational systems, iii) research on democratization of science more broadly, and iv) new accountabilities. Citizen science also brings a fresh perspective on the barriers to institutionalizing participation in the agricultural sciences. Specifically, we show how citizen science can reconfigure cost-motivation-accountability combinations using digital tools, open up a larger conceptual space of experimentation, and stimulate new collaborations. With appropriate and persistent institutional support and investment, citizen science can therefore have a lasting impact on how agricultural science engages with farming communities and wider society, and more fully realize the promises of participation
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