15,304 research outputs found
Continuous measurement of shock velocity using a microwave technique
Microwave technique for continuous measurement of shock wave velocit
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
Low-velocity collision behaviour of clusters composed of sub-mm sized dust aggregates
The experiments presented aim to measure the outcome of collisions between
sub-mm sized protoplanetary dust aggregate analogues. We also observed the
clusters formed from these aggregates and their collision behaviour. The
experiments were performed at the drop tower in Bremen. The protoplanetary dust
analogue materials were micrometre-sized monodisperse and polydisperse SiO
particles prepared into aggregates with sizes between 120~m and
250~m. One of the dust samples contained aggregates that were previously
compacted through repeated bouncing. During three flights of 9~s of
microgravity each, individual collisions between aggregates and the formation
of clusters of up to a few millimetres in size were observed. In addition, the
collisions of clusters with the experiment cell walls leading to compaction or
fragmentation were recorded. We observed collisions amongst dust aggregates and
collisions between dust clusters and the cell aluminium walls at speeds ranging
from about 0.1 cm/s to 20 cm/s. The velocities at which sticking occurred
ranged from 0.18 to 5.0 cm/s for aggregates composed of monodisperse dust, with
an average value of 2.1 cm/s for reduced masses ranging from 1.2x10-6 to
1.8x10-3 g with an average value of 2.2x10-4 g. From the restructuring and
fragmentation of clusters composed of dust aggregates colliding with the
aluminium cell walls, we derived a collision recipe for dust aggregates
(100 m) following the model of Dominik \& Thielens (1997) developed
for microscopic particles. We measured a critical rolling energy of 1.8x10-13 J
and a critical breaking energy of 3.5x10-13 J for 100 m-sized
non-compacted aggregates.Comment: 12 pages, 13 figure
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
Bio-inspired swing leg control for spring-mass robots running on ground with unexpected height disturbance
We proposed three swing leg control policies for spring-mass running robots, inspired by experimental data from our recent collaborative work on ground running birds. Previous investigations suggest that animals may prioritize injury avoidance and/or efficiency as their objective function during running rather than maintaining limit-cycle stability. Therefore, in this study we targeted structural capacity (maximum leg force to avoid damage) and efficiency as the main goals for our control policies, since these objective functions are crucial to reduce motor size and structure weight. Each proposed policy controls the leg angle as a function of time during flight phase such that its objective function during the subsequent stance phase is regulated. The three objective functions that are regulated in the control policies are (i) the leg peak force, (ii) the axial impulse, and (iii) the leg actuator work. It should be noted that each control policy regulates one single objective function. Surprisingly, all three swing leg control policies result in nearly identical subsequent stance phase dynamics. This implies that the implementation of any of the proposed control policies would satisfy both goals (damage avoidance and efficiency) at once. Furthermore, all three control policies require a surprisingly simple leg angle adjustment: leg retraction with constant angular acceleration
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
A cold cathode ion source mass spectrometer employing ion counting techniques
Design and construction of mass spectrometer using cold cathode source of ions, quadrupole mass analyzer, and ion counting detector
The four-populations model: a new classification scheme for pre-planetesimal collisions
Within the collision growth scenario for planetesimal formation, the growth
step from centimetre sized pre-planetesimals to kilometre sized planetesimals
is still unclear. The formation of larger objects from the highly porous
pre-planetesimals may be halted by a combination of fragmentation in disruptive
collisions and mutual rebound with compaction. However, the right amount of
fragmentation is necessary to explain the observed dust features in late T
Tauri discs. Therefore, detailed data on the outcome of pre-planetesimal
collisions is required and has to be presented in a suitable and precise
format. We propose and apply a new classification scheme for pre-planetesimal
collisions based on the quantitative aspects of four fragment populations: the
largest and second largest fragment, a power-law population, and a
sub-resolution population. For the simulations of pre-planetesimal collisions,
we adopt the SPH numerical scheme with extensions for the simulation of porous
solid bodies. By means of laboratory benchmark experiments, this model was
previously calibrated and tested for the correct simulation of the compaction,
bouncing, and fragmentation behaviour of macroscopic highly porous silica dust
aggregates. It is shown that previous attempts to map collision data were much
too oriented on qualitatively categorising into sticking, bouncing, and
fragmentation events. We show that the four-populations model encompasses all
previous categorisations and in addition allows for transitions. This is
because it is based on quantitative characteristic attributes of each
population such as the mass, kinetic energy, and filling factor. As a
demonstration of the applicability and the power of the four-populations model,
we utilise it to present the results of a study on the influence of collision
velocity in head-on collisions of intermediate porosity aggregates.Comment: 14 pages, 11 figures, 5 tables, to be published in Astronomy and
Astrophysic
Resonant Processes in a Frozen Gas
We present a theory of resonant processes in a frozen gas of atoms
interacting via dipole-dipole potentials that vary as , where is
the interatomic separation. We supply an exact result for a single atom in a
given state interacting resonantly with a random gas of atoms in a different
state. The time development of the transition process is calculated both on-
and off-resonance, and the linewidth with respect to detuning is obtained as a
function of time . We introduce a random spin Hamiltonian to model a dense
system of resonators and show how it reduces to the previous model in the limit
of a sparse system. We derive approximate equations for the average effective
spin, and we use them to model the behavior seen in the experiments of Anderson
et al. and Lowell et al. The approach to equilibrium is found to be
proportional to ), where the constant is explicitly related to the system's parameters.Comment: 30 pages, 6 figure
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