1,660 research outputs found
Catastrophic disruptions revisited
We use a smooth particle hydrodynamics method (SPH) to simulate colliding
rocky and icy bodies from cm-scale to hundreds of km in diameter, in an effort
to define self-consistently the threshold for catastrophic disruption. Unlike
previous efforts, this analysis incorporates the combined effects of material
strength (using a brittle fragmentation model) and self-gravitation, thereby
providing results in the ``strength regime'' and the ``gravity regime'', and in
between. In each case, the structural properties of the largest remnant are
examined.Comment: To appear in Icaru
Submillimetre-sized dust aggregate collision and growth properties
The collisional and sticking properties of sub-mm-sized aggregates composed
of protoplanetary dust analogue material are measured, including the
statistical threshold velocity between sticking and bouncing, their surface
energy and tensile strength within aggregate clusters. We performed an
experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue
materials were micrometre-sized monodisperse and polydisperse SiO2 particles
prepared into aggregates with sizes around 120 m and 330 m,
respectively and volume filling factors around 0.37. During the experimental
run of 150 s under reduced gravity conditions, the sticking of aggregates and
the formation and fragmentation of clusters of up to a few millimetres in size
was observed. The sticking probability of the sub-mm-sized dust aggregates
could be derived for velocities decreasing from 22 to 3 cm/s. The transition
from bouncing to sticking collisions happened at 12.7 cm/s for the smaller
aggregates composed of monodisperse particles and at 11.5 and 11.7 cm/s for the
larger aggregates composed of mono- and polydisperse dust particles,
respectively. Using the pull-off force of sub-mm-sized dust aggregates from the
clusters, the surface energy of the aggregates composed of monodisperse dust
was derived to be 1.6x10-5 J/m2, which can be scaled down to 1.7x10-2 J/m2 for
the micrometre-sized monomer particles and is in good agreement with previous
measurements for silica particles. The tensile strengths of these aggregates
within the clusters were derived to be 1.9 Pa and 1.6 Pa for the small and
large dust aggregates, respectively. These values are in good agreement with
recent tensile strength measurements for mm-sized silica aggregates. Using our
data on the sticking-bouncing threshold, estimates of the maximum aggregate
size can be given. For a minimum mass solar nebula model, aggregates can reach
sizes of 1 cm.Comment: 21 pages (incl. 6 pages of appendix), 23 figure
Transport of a dilute active suspension in pressure-driven channel flow
Confined suspensions of active particles show peculiar dynamics characterized
by wall accumulation, as well as upstream swimming, centerline depletion and
shear-trapping when a pressure-driven flow is imposed. We use theory and
numerical simulations to investigate the effects of confinement and non-uniform
shear on the dynamics of a dilute suspension of Brownian active swimmers by
incorporating a detailed treatment of boundary conditions within a simple
kinetic model where the configuration of the suspension is described using a
conservation equation for the probability distribution function of particle
positions and orientations, and where particle-particle and particle-wall
hydrodynamic interactions are neglected. Based on this model, we first
investigate the effects of confinement in the absence of flow, in which case
the dynamics is governed by a swimming Peclet number, or ratio of the
persistence length of particle trajectories over the channel width, and a
second swimmer-specific parameter whose inverse measures the strength of
propulsion. In the limit of weak and strong propulsion, asymptotic expressions
for the full distribution function are derived. For finite propulsion,
analytical expressions for the concentration and polarization profiles are also
obtained using a truncated moment expansion of the distribution function. In
agreement with experimental observations, the existence of a
concentration/polarization boundary layer in wide channels is reported and
characterized, suggesting that wall accumulation in active suspensions is
primarily a kinematic effect which does not require hydrodynamic interactions.
Next, we show that application of a pressure-driven Poiseuille flow leads to
net upstream swimming of the particles relative to the flow, and an analytical
expression for the mean upstream velocity is derived in the weak flow limit. In
stronger imposed flows .....
Meeting Review: Airborne Aerosol Inlet Workshop
Proceedings from the Airborne Aerosol Inlet Workshop are presented. The two central topics of discussion were the role of aerosols in atmospheric processes and the difficulties in characterizing aerosols. The following topics were discussed during the working sessions: airborne observations to date; identification of inlet design issues; inlet modeling needs and directions; objectives for aircraft experiments; and future laboratory and wind tunnel studies
Kicked Burgers Turbulence
Burgers turbulence subject to a force ,
where the 's are ``kicking times'' and the ``impulses'' have
arbitrary space dependence, combines features of the purely decaying and the
continuously forced cases. With large-scale forcing this ``kicked'' Burgers
turbulence presents many of the regimes proposed by E, Khanin, Mazel and Sinai
(1997) for the case of random white-in-time forcing. It is also amenable to
efficient numerical simulations in the inviscid limit, using a modification of
the Fast Legendre Transform method developed for decaying Burgers turbulence by
Noullez and Vergassola (1994). For the kicked case, concepts such as
``minimizers'' and ``main shock'', which play crucial roles in recent
developments for forced Burgers turbulence, become elementary since everything
can be constructed from simple two-dimensional area-preserving Euler--Lagrange
maps.
One key result is for the case of identical deterministic kicks which are
periodic and analytic in space and are applied periodically in time: the
probability densities of large negative velocity gradients and of
(not-too-large) negative velocity increments follow the power law with -7/2
exponent proposed by E {\it et al}. (1997) in the inviscid limit, whose
existence is still controversial in the case of white-in-time forcing. (More in
the full-length abstract at the beginning of the paper.)Comment: LATEX 30 pages, 11 figures, J. Fluid Mech, in pres
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