10,783 research outputs found
Instabilities of geared couplings: Theory and practice
The use of couplings for high speed turbocompressors or pumps is essential to transmit power from the driver. Typical couplings are either of the lubricated gear or dry diaphragm type design. Gear couplings have been the standard design for many years and recent advances in power and speed requirements have pushed the standard design criteria to the limit. Recent test stand and field data on continuous lube gear type couplings have forced a closer examination of design tolerances and concepts to avoid operational instabilities. Two types of mechanical instabilities are reviewed in this paper: (1) entrapped fluid, and (2) gear mesh instability resulting in spacer throw-out onset. Test stand results of these types of instabilities and other directly related problems are presented together with criteria for proper coupling design to avoid these conditions. An additional test case discussed shows the importance of proper material selection and processing and what can happen to an otherwise good design
A Structural Analysis of Star-Forming Region AFGL 490
We present Spitzer IRAC and MIPS observations of the star-forming region
containing intermediate-mass young stellar object (YSO) AFGL 490. We supplement
these data with near-IR 2MASS photometry and with deep SQIID observations off
the central high extinction region. We have more than doubled the known
membership of this region to 57 Class I and 303 Class II YSOs via the combined
1-24 um photometric catalog derived from these data. We construct and analyze
the minimum spanning tree of their projected positions, isolating one locally
over-dense cluster core containing 219 YSOs (60.8% of the region's members). We
find this cluster core to be larger yet less dense than similarly analyzed
clusters. Although the structure of this cluster core appears irregular, we
demonstrate that the parsec-scale surface densities of both YSOs and gas are
correlated with a power law slope of 2.8, as found for other similarly analyzed
nearby molecular clouds. We also explore the mass segregation implications of
AFGL 490's offset from the center of its core, finding that it has no apparent
preferential central position relative to the low-mass members.Comment: 44 pages, 13 figures, accepted to Ap
A modal model for diffraction gratings
A description of an algorithm for a rather general modal grating calculation
is presented. Arbitrary profiles, depth, and permittivity are allowed. Gratings
built up from sub-gratings are allowed, as are coatings on the sidewalls of
lines, and arbitrary complex structure. Conical angles and good conductors are
supported
Dissipation in Poynting-flux Dominated Flows: the Sigma-Problem of the Crab Pulsar Wind
Flows in which energy is transported predominantly as Poynting flux are
thought to occur in pulsars, gamma-ray bursts and relativistic jets from
compact objects. The fluctuating component of the magnetic field in such a flow
can in principle be dissipated by magnetic reconnection, and used to accelerate
the flow. We investigate how rapidly this transition can take place, by
implementing into a global MHD model, that uses a thermodynamic description of
the plasma, explicit, physically motivated prescriptions for the dissipation
rate: a lower limit on this rate is given by limiting the maximum drift speed
of the current carriers to that of light, an upper limit follows from demanding
that the dissipation zone expand only subsonically in the comoving frame and a
further prescription is obtained by assuming that the expansion speed is
limited by the growth rate of the relativistic tearing mode. In each case,
solutions are presented which give the Lorentz factor of a spherical wind
containing a transverse, oscillating magnetic field component as a function of
radius. In the case of the Crab pulsar, we find that the Poynting flux can be
dissipated before the wind reaches the inner edge of the Nebula if the pulsar
emits electron positron pairs at a rate >1.E40 per second, thus providing a
possible solution to the sigma-problem.Comment: Accepted for publication in Ap
Ohm's Law for a Relativistic Pair Plasma
We derive the fully relativistic Ohm's law for an electron-positron plasma.
The absence of non-resistive terms in Ohm's law and the natural substitution of
the 4-velocity for the velocity flux in the relativistic bulk plasma equations
do not require the field gradient length scale to be much larger than the
lepton inertial lengths, or the existence of a frame in which the distribution
functions are isotropic.Comment: 12 pages, plain TeX, Phys. Rev. Lett. 71 3481 (1993
Understanding the effect resonant magnetic perturbations have on ELMs
All current estimations of the energy released by type I ELMs indicate that,
in order to ensure an adequate lifetime of the divertor targets on ITER, a
mechanism is required to decrease the amount of energy released by an ELM, or
to eliminate ELMs altogether. One such amelioration mechanism relies on
perturbing the magnetic field in the edge plasma region, either leading to more
frequent, smaller ELMs (ELM mitigation) or ELM suppression. This technique of
Resonant Magnetic Perturbations (RMPs) has been employed to suppress type I
ELMs at high collisionality/density on DIII-D, ASDEX Upgrade, KSTAR and JET and
at low collisionality on DIII-D. At ITER-like collisionality the RMPs enhance
the transport of particles or energy and keep the edge pressure gradient below
the 2D linear ideal MHD critical value that would trigger an ELM, whereas at
high collisionality/density the type I ELMs are replaced by small type II ELMs.
Although ELM suppression only occurs within limitied operational ranges, ELM
mitigation is much more easily achieved. The exact parameters that determine
the onset of ELM suppression are unknown but in all cases the magnetic
perturbations produce 3D distortions to the plasma and enhanced particle
transport. The incorporation of these 3D effects in codes will be essential in
order to make quantitative predictions for future devices.Comment: 32 pages, 9 figure
Errors in quantum optimal control and strategy for the search of easily implementable control pulses
We introduce a new approach to assess the error of control problems we aim to
optimize. The method offers a strategy to define new control pulses that are
not necessarily optimal but still able to yield an error not larger than some
fixed a priori threshold, and therefore provide control pulses that might be
more amenable for an experimental implementation. The formalism is applied to
an exactly solvable model and to the Landau-Zener model, whose optimal control
problem is solvable only numerically. The presented method is of importance for
applications where a high degree of controllability of the dynamics of quantum
systems is required.Comment: 13 pages, 3 figure
Seeding rates for precision seeded canola
Non-Peer Reviewe
Modeling the Multiwavelength Spectra and Variability of BL Lacertae in 2000
BL Lacertae was the target of an extensive multiwavelength monitoring
campaign in the second half of 2000. In this paper, we are using leptonic and
hadronic jet models to fit the observed broadband spectra and spectral
variability patterns. We start out with global spectral models. Subsequently,
we investigate various flaring scenarios for comparison with the observed
short-term variability. For our leptonic jet model, we find that the short-term
variability, in particular the optical and X-ray spectral variability, can be
best represented with a flaring scenario dominated by a spectral-index change
of the spectrum of ultrarelativistic electrons injected into the jet. Based on
this result, a detailed model simulation, reproducing the observed optical and
X-ray spectral variability and broadband SED of BL Lacertae simultaneously, is
presented. Our leptonic modeling results are compared to fits using the
hadronic synchrotron-proton blazar (SPB) model. That model can reproduce the
observed SEDs of BL Lacertae in a scenario with muon-synchrotron dominated
high-energy emission. It requires a significantly higher magnetic field than
the leptonic model (~ 40 G vs. ~ 2 G in the leptonic model) and a lower Doppler
factor associated with the bulk motion of the emission region (D ~ 8 vs. D ~ 18
in the leptonic model). The hadronic model predicts a significantly larger >
100 GeV flux than the leptonic models, well within the anticipated capabilities
of VERITAS and MAGIC.Comment: Accepted for publication in ApJ. Uses AASTEX LaTeX macros.41 pages,
including 12 figure
Reconstruction of metabolic networks from high-throughput metabolite profiling data: in silico analysis of red blood cell metabolism
We investigate the ability of algorithms developed for reverse engineering of
transcriptional regulatory networks to reconstruct metabolic networks from
high-throughput metabolite profiling data. For this, we generate synthetic
metabolic profiles for benchmarking purposes based on a well-established model
for red blood cell metabolism. A variety of data sets is generated, accounting
for different properties of real metabolic networks, such as experimental
noise, metabolite correlations, and temporal dynamics. These data sets are made
available online. We apply ARACNE, a mainstream transcriptional networks
reverse engineering algorithm, to these data sets and observe performance
comparable to that obtained in the transcriptional domain, for which the
algorithm was originally designed.Comment: 14 pages, 3 figures. Presented at the DIMACS Workshop on Dialogue on
Reverse Engineering Assessment and Methods (DREAM), Sep 200
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