14,624 research outputs found
Real time plasma equilibrium reconstruction in a Tokamak
The problem of equilibrium of a plasma in a Tokamak is a free boundary
problemdescribed by the Grad-Shafranov equation in axisymmetric configurations.
The right hand side of this equation is a non linear source, which represents
the toroidal component of the plasma current density. This paper deals with the
real time identification of this non linear source from experimental
measurements. The proposed method is based on a fixed point algorithm, a finite
element resolution, a reduced basis method and a least-square optimization
formulation
Growth of Dust as the Initial Step Toward Planet Formation
We discuss the results of laboratory measurements and theoretical models
concerning the aggregation of dust in protoplanetary disks, as the initial step
toward planet formation. Small particles easily stick when they collide and
form aggregates with an open, often fractal structure, depending on the growth
process. Larger particles are still expected to grow at collision velocities of
about 1m/s. Experiments also show that, after an intermezzo of destructive
velocities, high collision velocities above 10m/s on porous materials again
lead to net growth of the target. Considerations of dust-gas interactions show
that collision velocities for particles not too different in surface-to-mass
ratio remain limited up to sizes about 1m, and growth seems to be guaranteed to
reach these sizes quickly and easily. For meter sizes, coupling to nebula
turbulence makes destructive processes more likely. Global aggregation models
show that in a turbulent nebula, small particles are swept up too fast to be
consistent with observations of disks. An extended phase may therefore exist in
the nebula during which the small particle component is kept alive through
collisions driven by turbulence which frustrates growth to planetesimals until
conditions are more favorable for one or more reasons.Comment: Protostars and Planets V (PPV) review. 18 pages, 5 figure
Construct, Merge, Solve and Adapt: Application to the repetition-free longest common subsequence problem
In this paper we present the application of a recently proposed, general, algorithm for combinatorial optimization to the repetition-free longest common subsequence problem. The applied algorithm, which is labelled Construct, Merge, Solve & Adapt, generates sub-instances based on merging the solution components found in randomly constructed solutions. These sub-instances are subsequently solved by means of an exact solver. Moreover, the considered sub-instances are dynamically changing due to adding new solution components at each iteration, and removing existing solution components on the basis of indicators about their usefulness. The results of applying this algorithm to the repetition-free longest common subsequence problem show that the algorithm generally outperforms competing approaches from the literature. Moreover, they show that the algorithm is competitive with CPLEX for small and medium size problem instances, whereas it outperforms CPLEX for larger problem instances.Peer ReviewedPostprint (author's final draft
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments
The growth processes from protoplanetary dust to planetesimals are not fully
understood. Laboratory experiments and theoretical models have shown that
collisions among the dust aggregates can lead to sticking, bouncing, and
fragmentation. However, no systematic study on the collisional outcome of
protoplanetary dust has been performed so far so that a physical model of the
dust evolution in protoplanetary disks is still missing. We intend to map the
parameter space for the collisional interaction of arbitrarily porous dust
aggregates. This parameter space encompasses the dust-aggregate masses, their
porosities and the collision velocity. With such a complete mapping of the
collisional outcomes of protoplanetary dust aggregates, it will be possible to
follow the collisional evolution of dust in a protoplanetary disk environment.
We use literature data, perform own laboratory experiments, and apply simple
physical models to get a complete picture of the collisional interaction of
protoplanetary dust aggregates. In our study, we found four different types of
sticking, two types of bouncing, and three types of fragmentation as possible
outcomes in collisions among protoplanetary dust aggregates. We distinguish
between eight combinations of porosity and mass ratio. For each of these cases,
we present a complete collision model for dust-aggregate masses between 10^-12
and 10^2 g and collision velocities in the range 10^-4 to 10^4 cm/s for
arbitrary porosities. This model comprises the collisional outcome, the
mass(es) of the resulting aggregate(s) and their porosities. We present the
first complete collision model for protoplanetary dust. This collision model
can be used for the determination of the dust-growth rate in protoplanetary
disks.Comment: accepted by Astronomy and Astrophysic
Collisions of small ice particles under microgravity conditions (II): Does the chemical composition of the ice change the collisional properties?
Context: Understanding the collisional properties of ice is important for
understanding both the early stages of planet formation and the evolution of
planetary ring systems. Simple chemicals such as methanol and formic acid are
known to be present in cold protostellar regions alongside the dominant water
ice; they are also likely to be incorporated into planets which form in
protoplanetary disks, and planetary ring systems. However, the effect of the
chemical composition of the ice on its collisional properties has not yet been
studied. Aims: Collisions of 1.5 cm ice spheres composed of pure crystalline
water ice, water with 5% methanol, and water with 5% formic acid were
investigated to determine the effect of the ice composition on the collisional
outcomes. Methods: The collisions were conducted in a dedicated experimental
instrument, operated under microgravity conditions, at relative particle impact
velocities between 0.01 and 0.19 m s^-1, temperatures between 131 and 160 K and
a pressure of around 10^-5 mbar. Results: A range of coefficients of
restitution were found, with no correlation between this and the chemical
composition, relative impact velocity, or temperature. Conclusions: We conclude
that the chemical composition of the ice (at the level of 95% water ice and 5%
methanol or formic acid) does not affect the collisional properties at these
temperatures and pressures due to the inability of surface wetting to take
place. At a level of 5% methanol or formic acid, the structure is likely to be
dominated by crystalline water ice, leading to no change in collisional
properties. The surface roughness of the particles is the dominant factor in
explaining the range of coefficients of restitution
Heat transfer across surfaces in contact - Practical effects of transient temperature and pressure environments Semiannual report, 1 Oct. 1965 - 1 Apr. 1966
Heat transfer across metal surfaces under transient temperature and pressure environmen
The Refractory-to-Ice Mass Ratio in Comets
We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? II. Introducing the bouncing barrier
The sticking of micron sized dust particles due to surface forces in
circumstellar disks is the first stage in the production of asteroids and
planets. The key ingredients that drive this process are the relative velocity
between the dust particles in this environment and the complex physics of dust
aggregate collisions. Here we present the results of a collision model, which
is based on laboratory experiments of these aggregates. We investigate the
maximum aggregate size and mass that can be reached by coagulation in
protoplanetary disks. We model the growth of dust aggregates at 1 AU at the
midplane at three different gas densities. We find that the evolution of the
dust does not follow the previously assumed growth-fragmentation cycles.
Catastrophic fragmentation hardly occurs in the three disk models. Furthermore
we see long lived, quasi-steady states in the distribution function of the
aggregates due to bouncing. We explore how the mass and the porosity change
upon varying the turbulence parameter and by varying the critical mass ratio of
dust particles. Particles reach Stokes numbers of roughly 10^-4 during the
simulations. The particle growth is stopped by bouncing rather than
fragmentation in these models. The final Stokes number of the aggregates is
rather insensitive to the variations of the gas density and the strength of
turbulence. The maximum mass of the particles is limited to approximately 1
gram (chondrule-sized particles). Planetesimal formation can proceed via the
turbulent concentration of these aerodynamically size-sorted chondrule-sized
particles.Comment: accepted for publication in A&
Numerical Simulations of Highly Porous Dust Aggregates in the Low-Velocity Collision Regime
A highly favoured mechanism of planetesimal formation is collisional growth.
Single dust grains, which follow gas flows in the protoplanetary disc, hit each
other, stick due to van der Waals forces and form fluffy aggregates up to
centimetre size. The mechanism of further growth is unclear since the outcome
of aggregate collisions in the relevant velocity and size regime cannot be
investigated in the laboratory under protoplanetary disc conditions. Realistic
statistics of the result of dust aggregate collisions beyond decimetre size is
missing for a deeper understanding of planetary growth. Joining experimental
and numerical efforts we want to calibrate and validate a computer program that
is capable of a correct simulation of the macroscopic behaviour of highly
porous dust aggregates. After testing its numerical limitations thoroughly we
will check the program especially for a realistic reproduction of various
benchmark experiments. We adopt the smooth particle hydrodynamics (SPH)
numerical scheme with extensions for the simulation of solid bodies and a
modified version of the Sirono porosity model. Experimentally measured
macroscopic material properties of silica dust are implemented. We calibrate
and test for the compressive strength relation and the bulk modulus. SPH has
already proven to be a suitable tool to simulate collisions at rather high
velocities. In this work we demonstrate that its area of application can not
only be extended to low-velocity experiments and collisions. It can also be
used to simulate the behaviour of highly porous objects in this velocity regime
to a very high accuracy.The result of the calibration process in this work is
an SPH code that can be utilised to investigate the collisional outcome of
porous dust in the low-velocity regime.Comment: accepted by Astronomy & Astrophysic
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