11,607 research outputs found
Coupling iterated Kolmogorov diffusions
The Kolmogorov (1934) diffusion is the two-dimensional diffusion generated by real Brownian motion B and its time integral integral B d t. In this paper we construct successful co-adapted couplings for iterated Kolmogorov diffusions defined by adding iterated time integrals integral integral B d s d t,... as further components to the original Kolmogorov diffusion. A Laplace-transform argument shows it is not possible successfully to couple all iterated time integrals at once; however we give an explicit construction of a successful co-adapted coupling method for (B, integral B d t, integral integral B d s d t); and a more implicit construction of a successful co-adapted coupling method which works for finite sets of iterated time integrals
Conceptual design studies and experiments related to cavity exhaust systems for nuclear light bulb configurations
Investigations of various phases of gaseous nuclear rocket technology have been conducted. The principal research efforts have recently been directed toward the closed-cycle, vortex-stabilized nuclear light bulb engine and toward a small-scale fissioning uranium plasma experiment that could be conducted in the Los Alamos Scientific Laboratory's Nuclear Furnace. The engine concept is based on the transfer of energy by thermal radiation from gaseous fissioning uranium, through a transparent wall, to hydrogen propellant. The reference engine configuration is comprised of seven unit cavities, each having its own fuel transparent wall and propellant duct. The basic design of the engine is described. Subsequent studies performed to supplement and investigate the basic design are reported. Summaries of other nuclear light bulb research programs are included
Solar sustained plasma/absorber conceptual design
A space power system concept was evaluated which uses concentrated solar energy to heat a working fluid to temperatures as high as 4000 K. The high temperature working fluid could be used for efficient electric power production in advanced thermal or magnetohydrodynamic conversion cycles. Energy absorber configurations utilizing particles or cesium vapor absorber material were investigaed. Results of detailed radiant heat transfer calculations indicated approximately 86 percent of the incident solar energy could be absorbed within a 12-cm-dia flowing stream of gas borne carbon particles. Calculated total energy absorption in the cesium vapor seeded absorber configuration ranged from 34 percent to 64 percent of the incident solar energy. Solar flux concentration ratios of between approximately 3000 and 10,000 will be required to sustain absorber temperatures in the range from 3000 K to 4000 K
MEXIT: Maximal un-coupling times for stochastic processes
Classical coupling constructions arrange for copies of the \emph{same} Markov
process started at two \emph{different} initial states to become equal as soon
as possible. In this paper, we consider an alternative coupling framework in
which one seeks to arrange for two \emph{different} Markov (or other
stochastic) processes to remain equal for as long as possible, when started in
the \emph{same} state. We refer to this "un-coupling" or "maximal agreement"
construction as \emph{MEXIT}, standing for "maximal exit". After highlighting
the importance of un-coupling arguments in a few key statistical and
probabilistic settings, we develop an explicit \MEXIT construction for
stochastic processes in discrete time with countable state-space. This
construction is generalized to random processes on general state-space running
in continuous time, and then exemplified by discussion of \MEXIT for Brownian
motions with two different constant drifts.Comment: 28 page
Initial radio-frequency gas heating experiments to simulate the thermal environment in a nuclear light bulb reactor
Initial radio frequency gas heating experiments to simulate thermal environment in nuclear light bulb reacto
Containment experiments in vortex tubes with radial outflow and large superimposed axial flows
Containment tests of vortexes with radial outflow in basic vortex tube, and in axial-flow vortex tub
Experimental investigation of containment in constant-temperature radial-inflow vortexes
Containment characteristics of radial inflow gases in vortex tube
New, nearby bright southern ultracool dwarfs
We report the discovery of twenty-one hitherto unknown bright southern
ultracool dwarfs with spectral types in the range M7 to L5.5, together with new
observations of a further three late M dwarfs previously confirmed. Three more
objects are already identified in the literature as high proper motion stars;we
derive their spectral types for the first time. All objects were selected from
the 2MASS All Sky and SuperCOSMOS point source databases on the basis of their
optical/near-infrared colours, -band magnitudes and proper motions. Low
resolution (R 1000) spectroscopy with the ESO/NTT SOFI spectrograph
has confirmed the ultracool nature of 24 targets, out of a total of 25
candidates observed. Spectral types are derived by direct comparison with
template objects and compared to results from HO and FeH indices. We also
report the discovery of one binary, as revealed by SOFI acquisition imaging;
spectra were taken for both components. The spectral types of the two
components are L2 and L4 and the distance 19 pc. Spectroscopic distances
and transverse velocities are derived for the sample. Two L5 objects lie
only 10 pc distant. Such nearby objects are excellent targets for
further study to derive their parallaxes and to search for fainter, later
companions with AO and/or methane imaging.Comment: 11 pages, 10 figures, accepted to MNRA
Spectral centrality measures in complex networks
Complex networks are characterized by heterogeneous distributions of the
degree of nodes, which produce a large diversification of the roles of the
nodes within the network. Several centrality measures have been introduced to
rank nodes based on their topological importance within a graph. Here we review
and compare centrality measures based on spectral properties of graph matrices.
We shall focus on PageRank, eigenvector centrality and the hub/authority scores
of HITS. We derive simple relations between the measures and the (in)degree of
the nodes, in some limits. We also compare the rankings obtained with different
centrality measures.Comment: 11 pages, 10 figures, 5 tables. Final version published in Physical
Review
Hidden Order in Crackling Noise during Peeling of an Adhesive Tape
We address the long standing problem of recovering dynamical information from
noisy acoustic emission signals arising from peeling of an adhesive tape
subject to constant traction velocity. Using phase space reconstruction
procedure we demonstrate the deterministic chaotic dynamics by establishing the
existence of correlation dimension as also a positive Lyapunov exponent in a
mid range of traction velocities. The results are explained on the basis of the
model that also emphasizes the deterministic origin of acoustic emission by
clarifying its connection to sticks-slip dynamics.Comment: 5 pages, 10 figure
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