32,257 research outputs found
Flame Instability and Transition to Detonation in Supersonic Reactive Flows
Multidimensional numerical simulations of a homogeneous, chemically reactive
gas were used to study ignition, flame stability, and
deflagration-to-detonation transition (DDT) in a supersonic combustor. The
configuration studied was a rectangular channel with a supersonic inflow of
stoichiometric ethylene-oxygen and a transimissive outflow boundary. The
calculation is initialized with a velocity in the computational domain equal to
that of the inflow, which is held constant for the duration of the calculation.
The compressible reactive Navier-Stokes equations were solved by a high-order
numerical algorithm on an adapting mesh. This paper describes two calculations,
one with a Mach 3 inflow and one with Mach 5.25. In the Mach 3 case, the
fuel-oxidizer mixture does not ignite and the flow reaches a steady-state
oblique shock train structure. In the Mach 5.25 case, ignition occurs in the
boundary layers and the flame front becomes unstable due to a Rayleigh-Taylor
instability at the interface between the burned and unburned gas. Growth of the
reaction front and expansion of the burned gas compress and preheat the
unburned gas. DDT occurs in several locations, initiating both at the flame
front and in the unburned gas, due to an energy-focusing mechanism. The growth
of the flame instability that leads to DDT is analyzed using the Atwood number
parameter
Timing the Nearby Isolated Neutron Star RX J1856.5-3754
RX J1856.5-3754 is the X-ray brightest among the nearby isolated neutron
stars. Its X-ray spectrum is thermal, and is reproduced remarkably well by a
black-body, but its interpretation has remained puzzling. One reason is that
the source did not exhibit pulsations, and hence a magnetic field
strength--vital input to atmosphere models--could not be estimated. Recently,
however, very weak pulsations were discovered. Here, we analyze these in
detail, using all available data from the XMM-Newton and Chandra X-ray
observatories. From frequency measurements, we set a 2-sigma upper limit to the
frequency derivative of \dot\nu<1.3e-14 Hz/s. Trying possible phase-connected
timing solutions, we find that one solution is far more likely than the others,
and we infer a most probable value of \dot\nu=(-5.98+/-0.14)e-16 Hz/s. The
inferred magnetic field strength is 1.5e13 G, comparable to what was found for
similar neutron stars. From models, the field seems too strong to be consistent
with the absence of spectral features for non-condensed atmospheres. It is
sufficiently strong, however, that the surface could be condensed, but only if
it is consists of heavy elements like iron. Our measurements imply a
characteristic age of about 4 Myr. This is longer than the cooling and
kinematic ages, as was found for similar objects, but at almost a factor ten,
the discrepancy is more extreme. A puzzle raised by our measurement is that the
implied rotational energy loss rate of about 3e30 erg/s is orders of magnitude
smaller than what was inferred from the H-alpha nebula surrounding the source.Comment: 4 pages, 2 figures, 2 tables; accepted for publication in
Astrophysical Journal (Letters
The development of absorptive capacity-based innovation in a construction SME
Traditionally, construction has been a transaction-oriented industry. However, it is changing from the design-bid-build process into a business based on innovation capability and performance management, in which contracts are awarded on the basis of factors such as knowledge, intellectual capital and skills. This change presents a challenge to construction-sector SMEs with scarce resources, which must find ways to innovate based on those attributes to ensure their future competitiveness. This paper explores how dynamic capability, using an absorptive capacity framework in response to these challenges, has been developed in a construction-based SME. The paper also contributes to the literature on absorptive capacity and innovation by showing how the construct can be operationalized within an organization. The company studied formed a Knowledge Transfer Partnership using action research over a two-year period with a local university. The aim was to increase its absorptive capacity and hence its ability to meet the changing market challenges. The findings show that absorptive capacity can be operationalized into a change management approach for improving capability-based competitiveness. Moreover, it is important for absorptive capacity constructs and language to be contextualized within a given organizational setting (as in the case of the construction-based SME in the present study)
Eigenstate Structure in Graphs and Disordered Lattices
We study wave function structure for quantum graphs in the chaotic and
disordered regime, using measures such as the wave function intensity
distribution and the inverse participation ratio. The result is much less
ergodicity than expected from random matrix theory, even though the spectral
statistics are in agreement with random matrix predictions. Instead, analytical
calculations based on short-time semiclassical behavior correctly describe the
eigenstate structure.Comment: 4 pages, including 2 figure
Chiral Fermions and Multigrid
Lattice regularization of chiral fermions is an important development of the
theory of elementary particles. Nontheless, brute force computer simulations
are very expensive, if not prohibitive. In this letter I exploit the
non-interacting character of the lattice theory in the flavor space and propose
a multigrid approach for the simulation of the theory. Already a two-grid
algorithm saves an order of magnitude of computer time for fermion propagator
calculations.Comment: Latex, 6 pages, 1 figur
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
The dynamics of laser droplet generation
We propose an experimental setup allowing for the characterization of laser
droplet generation in terms of the underlying dynamics, primarily showing that
the latter is deterministically chaotic by means of nonlinear time series
analysis methods. In particular, we use a laser pulse to melt the end of a
properly fed vertically placed metal wire. Due to the interplay of surface
tension, gravity force and light-metal interaction, undulating pendant droplets
are formed at the molten end, which eventually completely detach from the wire
as a consequence of their increasing mass. We capture the dynamics of this
process by employing a high-speed infrared camera, thereby indirectly measuring
the temperature of the wire end and the pendant droplets. The time series is
subsequently generated as the mean value over the pixel intensity of every
infrared snapshot. Finally, we employ methods of nonlinear time series analysis
to reconstruct the phase space from the observed variable and test it against
determinism and stationarity. After establishing that the observed laser
droplet generation is a deterministic and dynamically stationary process, we
calculate the spectra of Lyapunov exponents. We obtain a positive largest
Lyapunov exponent and a negative divergence, i.e., sum of all the exponents,
thus indicating that the observed dynamics is deterministically chaotic with an
attractor as solution in the phase space. In addition to characterizing the
dynamics of laser droplet generation, we outline industrial applications of the
process and point out the significance of our findings for future attempts at
mathematical modeling.Comment: 7 two-column pages, 8 figures; accepted for publication in Chaos
[supplementary material available at
http://www.matjazperc.com/chaos/laser.html
A Coherent Timing Solution for the Nearby Isolated Neutron Star RX J0720.4-3125
We present the results of a dedicated effort to measure the spin-down rate of
the nearby isolated neutron star RX J0720.4-3125. Comparing arrival times of
the 8.39-sec pulsations for data from Chandra we derive an unambiguous timing
solution for RX J0720.4-3125 that is accurate to 5 years.
Adding data from XMM and ROSAT, the final solution yields
Pdot=(6.98+/-0.02)x10^(-14) s/s; for dipole spin-down, this implies a
characteristic age of 2 Myr and a magnetic field strength of 2.4e13 G. The
phase residuals are somewhat larger than those for purely regular spin-down,
but do not show conclusive evidence for higher-order terms or a glitch. From
our timing solution as well as recent X-ray spectroscopy, we concur with recent
suggestions that RX J0720.4-3125 is most likely an off-beam radio pulsar with a
moderately high magnetic field.Comment: 5 pages, 1 figure. Accepted for publication in ApJ
Research on oxygen toxicity at the cellular level Final report, 15 Apr. 1965 - 15 Jun. 1966
Oxygen toxicity at cellular level in manned spacecraf
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