11,963 research outputs found
Bursts in discontinuous Aeolian saltation
Close to the onset of Aeolian particle transport through saltation we find in
wind tunnel experiments a regime of discontinuous flux characterized by bursts
of activity. Scaling laws are observed in the time delay between each burst and
in the measurements of the wind fluctuations at the fluid threshold Shields
number . The time delay between each burst decreases on average with
the increase of the Shields number until sand flux becomes continuous. A
numerical model for saltation including the wind-entrainment from the turbulent
fluctuations can reproduce these observations and gives insight about their
origin. We present here also for the first time measurements showing that with
feeding it becomes possible to sustain discontinuous flux even below the fluid
threshold
Bent-Double Radio Sources as Probes of Intergalactic Gas
As the most common environment in the universe, groups of galaxies are likely
to contain a significant fraction of the missing baryons in the form of
intergalactic gas. The density of this gas is an important factor in whether
ram pressure stripping and strangulation affect the evolution of galaxies in
these systems. We present a method for measuring the density of intergalactic
gas using bent-double radio sources that is independent of temperature, making
it complementary to current absorption line measurements. We use this method to
probe intergalactic gas in two different environments: inside a small group of
galaxies as well as outside of a larger group at a 2 Mpc radius and measure
total gas densities of and per cubic centimeter (random and systematic
errors) respectively. We use X-ray data to place an upper limit of K on the temperature of the intragroup gas in the small group.Comment: 6 pages, 1 figure, accepted for publication in Ap
Intrinsic localized modes in the charge-transfer solid PtCl
We report a theoretical analysis of intrinsic localized modes in a
quasi-one-dimensional charge-transfer-solid (PtCl). We discuss strongly nonlinear features of resonant Raman
overtone scattering measurements on PtCl, arising from quantum intrinsic
localized (multiphonon) modes (ILMs) and ILM-plus-phonon states. We show, that
Raman scattering data displays clear signs of a non-thermalization of lattice
degrees-of-freedom, manifested in a nonequilibrium density of intrinsic
localized modes.Comment: 4 pages, 4 figures, REVTE
Single wall carbon nanotube double quantum dot
We report on two top-gate defined, coupled quantum dots in a semiconducting
single wall carbon nanotube, constituting a tunable double quantum dot system.
The single wall carbon nanotubes are contacted by titanium electrodes, and
gated by three narrow top-gate electrodes as well as a back-gate. We show that
a bias spectroscopy plot on just one of the two quantum dots can be used to
extract the addition energy of both quantum dots. Furthermore, honeycomb charge
stability diagrams are analyzed by an electrostatic capacitor model that
includes cross capacitances, and we extract the coupling energy of the double
quantum dot.Comment: Published in Applied Physics Letters 4 December 2006.
http://link.aip.org/link/?APL/89/23211
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
Probing the mechanical unzipping of DNA
A study of the micromechanical unzipping of DNA in the framework of the
Peyrard-Bishop-Dauxois model is presented. We introduce a Monte Carlo technique
that allows accurate determination of the dependence of the unzipping forces on
unzipping speed and temperature. Our findings agree quantitatively with
experimental results for homogeneous DNA, and for -phage DNA we
reproduce the recently obtained experimental force-temperature phase diagram.
Finally, we argue that there may be fundamental differences between {\em in
vivo} and {\em in vitro} DNA unzipping
3D Viscoelastic Finite Element Modelling of Polymer Flow in the Fiber Drawing Process for Microstructured Polymer Optical Fiber Fabrication
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