12,436 research outputs found
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
The overlap lattice Dirac operator and dynamical fermions
I show how to avoid a two level nested conjugate gradient procedure in the
context of Hybrid Monte Carlo with the overlap fermionic action. The resulting
procedure is quite similar to Hybrid Monte Carlo with domain wall fermions, but
is more flexible and therefore has some potential worth exploring.Comment: Further expanded version. 12 pages, plain Te
A quantum protocol for cheat-sensitive weak coin flipping
We present a quantum protocol for the task of weak coin flipping. We find
that, for one choice of parameters in the protocol, the maximum probability of
a dishonest party winning the coin flip if the other party is honest is
1/sqrt(2). We also show that if parties restrict themselves to strategies
wherein they cannot be caught cheating, their maximum probability of winning
can be even smaller. As such, the protocol offers additional security in the
form of cheat sensitivity.Comment: 4 pages RevTex. Differs from the journal version only in that the
sentences: "The ordering of the authors on this paper was chosen by a coin
flip implemented by a trusted third party. TR lost." have not been remove
Polarization dependent photoionization cross-sections and radiative lifetimes of atomic states in Ba
The photoionization cross-sections of two even-parity excited states, and , of atomic Ba at the ionization-laser wavelength of
556.6 nm were measured. We found that the total cross-section depends on the
relative polarization of the atoms and the ionization-laser light. With
density-matrix algebra, we show that, in general, there are at most three
parameters in the photoionization cross-section. Some of these parameters are
determined in this work. We also present the measurement of the radiative
lifetime of five even-parity excited states of barium.Comment: 11 pages, 7 figure
Experiments on Column Base Stiffness of Long-Span Cold-Formed Steel Portal Frames Composed of Double Channels
Cold-formed steel haunched portal frames are popular structures in industrial and housing applications. They are mostly used as sheds, garages, and shelters, and are common in rural areas. Cold-formed steel portal frames with spans of up to 30m (100 ft) are now being constructed in Australia. As these large structures are fairly new to the market, there is limited data on their feasibility and design recommendations. An experimental program was carried out on a series of portal frame systems composed of back-to-back channels for the columns, rafters, and knee braces. The system consisted of three frames connected in parallel with purlins to simulate a free standing structure, with an approximate span of 14 m (46 ft), column height of 5.3 m (17 ft), and apex height of 7 m (23 ft). Several configurations were tested including variations in the knee connection, sleeve stiffeners in the columns and rafters, and loading of either vertical or combined horizontal and vertical loads. Deflections were recorded at various locations to measure global and local movements of the structural members, as well as column base reactions and base rotations. It was determined that the column bases are semi-rigid and further column base connection tests were completed to quantify column base connection stiffness for bending about the column major and minor axes, as well as twist. Results of the column base connection stiffness are presented as well as the implications for frame design
One loop calculation in lattice QCD with domain-wall quarks
One loop corrections to the domain-wall quark propagator are calculated in
massless QCD. It is shown that no additative counter term to the current quark
mass is generated in this theory, and the wave function renormalization factor
of the massless quark is explicitly evaluated. We also show that an analysis
with a simple mean-field approximation can explain properties of the massless
quark in numerical simulations of QCD with domain-wall quarks.Comment: 24 pages, REVTeX, with 3 epsf figure
Extending holographic LEED to ordered small unit cell superstructures
Following on the success of the recent application of holographic LEED to the
determination of the 3D atomic geometry of Si adatoms on a SiC(111) p(3x3)
surface, which enabled that structure to be solved, we show in this paper that
a similar technique allows the direct recovery of the local geometry of
adsorbates forming superstructures as small as p(2x2), even in the presence of
a local substrate reconstruction.Comment: 10 pages, 5 figures postscript included, revtex, Phys. Rev. B in
pres
Domain-Wall Induced Quark Masses in Topologically-Nontrivial Background
In the domain-wall formulation of chiral fermion, the finite separation
between domain-walls () induces an effective quark mass ()
which complicates the chiral limit. In this work, we study the size of the
effective mass as the function of and the domain-wall height by
calculating the smallest eigenvalue of the hermitian domain-wall Dirac operator
in the topologically-nontrivial background fields. We find that, just like in
the free case, decreases exponentially in with a rate
depending on . However, quantum fluctuations amplify the wall effects
significantly. Our numerical result is consistent with a previous study of the
effective mass from the Gell-Mann-Oakes-Renner relation.Comment: 10 pages, an appendix and minor changes adde
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