84,031 research outputs found
Body-rock or lift-off in flow
Conditions are investigated under which a body lying at rest or rocking on a
solid horizontal surface can be removed from the surface by hydrodynamic forces
or instead continues rocking. The investigation is motivated by recent
observations on Martian dust movement as well as other small- and large-scale
applications. The nonlinear theory of fluid-body interaction here has unsteady
motion of an inviscid fluid interacting with a moving thin body. Various shapes
of body are addressed together with a range of initial conditions. The relevant
parameter space is found to be subtle as evolution and shape play substantial
roles coupled with scaled mass and gravity effects. Lift-off of the body from
the surface generally cannot occur without fluid flow but it can occur either
immediately or within a finite time once the fluid flow starts up: parameters
for this are found and comparisons are made with Martian observations.Comment: 28 pages, 9 figure
Interstellar 12C/13C from CH+ absorption lines: Results from an extended survey
The 12C/13C isotope ratio in the interstellar medium (ISM), and its evolution
with time, is an important tracer of stellar yields. Spatial variations of this
ratio can be used to study mixing in the ISM. We want to determine this ratio
and its spatial variations in the local ISM from CH+ absorption lines in the
optical towards early-type stars. The aim is to determine the average value for
the local ISM and study possible spatial variations. We observed a large number
of early-type stars with Feros to extend the sample of suitable target stars
for CH+ isotope studies. The best suited targets were observed with Uves with
higher signal-to-noise ratio and spectral resolution to determine the isotope
ratio from the interstellar CH+ lines. This study significantly expands the
number of 13CH+ detections. We find an average ratio of = 76.27 +- 1.94 or,
for f = 1/R, = (120.46 +- 3.02) 10^{-4}. The scatter in f is 6.3
sigma(). This findings strengthens the case for chemical inhomogeneity in
the local ISM, with important implications for the mixing in the ISM. Given the
large scatter, the present-day value in the ISM is not significantly larger
than the solar value, which corresponds to the local value 4.5 Gyr ago.Comment: 11 pages, 16 figures, 2 tables, A&A submitte
Temperature automation for a propellant mixer
The analysis and installation of an automatic temperature controller on a propellant mixer is presented. Ultimately, the entire mixing process will come under automation, but since precise adherence to the temperature profile is very difficult to sustain manually, this was the first component to be automated. Automation is not only important for producing a uniform product, but it is necessary for envisioned space-based propellant production
Nano-Engineering Defect Structures on Graphene
We present a new way of nano-engineering graphene using defect domains. These
regions have ring structures that depart from the usual honeycomb lattice,
though each carbon atom still has three nearest neighbors. A set of stable
domain structures is identified using density functional theory (DFT),
including blisters, ridges, ribbons, and metacrystals. All such structures are
made solely out of carbon; the smallest encompasses just 16 atoms. Blisters,
ridges and metacrystals rise up out of the sheet, while ribbons remain flat. In
the vicinity of vacancies, the reaction barriers to formation are sufficiently
low that such defects could be synthesized through the thermally activated
restructuring of coalesced adatoms.Comment: 4 pages, 5 figure
Molecular Gas Around Young Stellar Clusters
We have begun a survey of the molecular gas surrounding 31 young clusters in
order to investigate the link between environment and the resulting cluster. We
present here a preliminary comparison of two clusters in our sample: GGD12-15
and Mon R2. Since both clusters are located in the MonR2 molecular cloud at a
distance of 830 pc, observational biases due to differing sensitivities and
angular resolutions are avoided.Comment: 2 pages, 2 figures, uses newpasp.sty. To appear in "Hot Star Workshop
III: The Earliest Phases of Massive Star Birth" (ed. P.A. Crowther
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