837 research outputs found
Eigenvalue assignment strategies in rotor systems
The work done to establish the control and direction of effective eigenvalue excursions of lightly damped, speed dependent rotor systems using passive control is discussed. Both second order and sixth order bi-axis, quasi-linear, speed dependent generic models were investigated. In every case a single, bi-directional control bearing was used in a passive feedback stabilization loop to resist modal destabilization above the rotor critical speed. Assuming incomplete state measurement, sub-optimal control strategies were used to define the preferred location of the control bearing, the most effective measurement locations, and the best set of control gains to extend the speed range of stable operation. Speed dependent control gains were found by Powell's method to maximize the minimum modal damping ratio for the speed dependent linear model. An increase of 300 percent in stable speed operation was obtained for the sixth order linear system using passive control. Simulations were run to examine the effectiveness of the linear control law on nonlinear rotor models with bearing deadband. The maximum level of control effort (force) required by the control bearing to stabilize the rotor at speeds above the critical was determined for the models with bearing deadband
Fermionic Molecular Dynamics for nuclear dynamics and thermodynamics
A new Fermionic Molecular Dynamics (FMD) model based on a Skyrme functional
is proposed in this paper. After introducing the basic formalism, some first
applications to nuclear structure and nuclear thermodynamics are presentedComment: 5 pages, Proceedings of the French-Japanese Symposium, September
2008. To be published in Int. J. of Mod. Phys.
The long journey from the giant-monopole resonance to the nuclear-matter incompressibility
Differences in the density dependence of the symmetry energy predicted by
nonrelativistic and relativistic models are suggested, at least in part, as the
culprit for the discrepancy in the values of the compression modulus of
symmetric nuclear matter extracted from the energy of the giant monopole
resonance in 208Pb. ``Best-fit'' relativistic models, with stiffer symmetry
energies than Skyrme interactions, consistently predict higher compression
moduli than nonrelativistic approaches. Relativistic models with compression
moduli in the physically acceptable range of K=200-300 MeV are used to compute
the distribution of isoscalar monopole strength in 208Pb. When the symmetry
energy is artificially softened in one of these models, in an attempt to
simulate the symmetry energy of Skyrme interactions, a lower value for the
compression modulus is indeed obtained. It is concluded that the proposed
measurement of the neutron skin in 208Pb, aimed at constraining the density
dependence of the symmetry energy and recently correlated to the structure of
neutron stars, will also become instrumental in the determination of the
compression modulus of nuclear matter.Comment: 9 pages with 2 (eps) figure
Diagnostics Using Nuclear Plant Cyber Attack Analysis Toolkit
A Python interface is developed for the GPWR Simulator to automatically
simulate cyber-spoofing of different steam generator parameters and plant
operation. Specifically, steam generator water level, feedwater flowrate, steam
flowrate, valve position, and steam generator controller parameters, including
controller gain and time constant, can be directly attacked using command
inject, denial of service, and man-in-the-middle type attacks. Plant operation
can be initialized to any of the initial conditions provided by the GPWR
simulator. Several different diagnostics algorithms have been implemented for
anomaly detection, including physics-based diagnostics with Kalman filtering,
data-driven diagnostics, noise profiling, and online sensor validation.
Industry-standard safety analysis code RELAP5 is also available as a part of
the toolkit. Diagnostics algorithms are analyzed based on accuracy and
efficiency. Our observations indicate that physics-based diagnostics with
Kalman filtering are the most robust. An experimental quantum kernel has been
added to the framework for preliminary testing. Our first impressions suggest
that while quantum kernels can be accurate, just like any other kernels, their
applicability is problem/data dependent, and can be prone to overfitting.Comment: Paper has been submitted to ANS for revie
Self-consistent description of nuclear compressional modes
Isoscalar monopole and dipole compressional modes are computed for a variety
of closed-shell nuclei in a relativistic random-phase approximation to three
different parametrizations of the Walecka model with scalar self-interactions.
Particular emphasis is placed on the role of self-consistency which by itself,
and with little else, guarantees the decoupling of the spurious
isoscalar-dipole strength from the physical response and the conservation of
the vector current. A powerful new relation is introduced to quantify the
violation of the vector current in terms of various ground-state form-factors.
For the isoscalar-dipole mode two distinct regions are clearly identified: (i)
a high-energy component that is sensitive to the size of the nucleus and scales
with the compressibility of the model and (ii) a low-energy component that is
insensitivity to the nuclear compressibility. A fairly good description of both
compressional modes is obtained by using a ``soft'' parametrization having a
compression modulus of K=224 MeV.Comment: 28 pages and 10 figures; submitted to PR
Tests of Transfer Reaction Determinations of Astrophysical S-Factors
The reaction has been used to determine
asymptotic normalization coefficients for transitions to the ground and first
excited states of . The coefficients provide the normalization for
the tails of the overlap functions for and allow us
to calculate the S-factors for at astrophysical
energies. The calculated S-factors are compared to measurements and found to be
in very good agreement. This provides the first test of this indirect method to
determine astrophysical direct capture rates using transfer reactions. In
addition, our results yield S(0) for capture to the ground and first excited
states in , without the uncertainty associated with extrapolation from
higher energies.Comment: 6 pages, 2 figure
Conductance anomalies in quantum wires
We study the conductance threshold of clean nearly straight quantum wires in
the magnetic field. As a quantitative example we solve exactly the scattering
problem for two-electrons in a wire with planar geometry and a weak bulge. From
the scattering matrix we determine conductance via the Landauer-Buettiker
formalism. The conductance anomalies found near 0.25(2e^2/h) and 0.75(2e^2/h)
are related to a singlet resonance and a triplet resonance, respectively, and
survive to temperatures of a few degrees. With increasing in-plane magnetic
field the conductance exhibits a plateau at e^2/h, consistent with recent
experiments.Comment: Quantum wire with planar geometry; in-plane magnetic fiel
Continuum QRPA response for deformed neutron-rich nuclei
We discuss properties of the quadrupole collective excitation of the deformed
neutron-rich nucleus Mg within the framework of quasi-particle random
phase approximation (QRPA). We first solve the coupled-channels equations to
obtain the single-particle levels, and construct the ground state by treating
the pairing correlations in the BCS approximation. We then solve the QRPA
equation using the response function formalism, by including the continuum
spectra with the box dicscretization method. We show that the collectivity of
the gamma vibration (the lowest mode) is significantly enhanced if
protons and neutrons have different deformations. We also discuss an attempt
towards full continuum QRPA calculations for deformed nuclei.Comment: 8 pages, 2 eps figures. Use espcrc1.sty. A talk given at the
International Conference on Collective Motion in Nuclei Under Extreme
Conditions (COMEX1), June 10 - 13, 2003, Paris, France. To be published in
the proceedings (Nucl. Phys. A
Isoscalar Giant Dipole Resonance and Nuclear Matter Incompressibility Coefficient
We present results of microscopic calculations of the strength function,
S(E), and alpha-particle excitation cross sections sigma(E) for the isoscalar
giant dipole resonance (ISGDR). An accurate and a general method to eliminate
the contributions of spurious state mixing is presented and used in the
calculations. Our results provide a resolution to the long standing problem
that the nuclear matter incompressibility coefficient, K, deduced from sigma(E)
data for the ISGDR is significantly smaller than that deduced from data for the
isoscalar giant monopole resonance (ISGMR).Comment: 4 pages using revtex 3.0, 3 postscript figures created by Mathematica
4.
Three-dimensional hydrodynamic simulations of the upper atmosphere of Men c: comparison with Ly transit observations
Aims: We aim at constraining the conditions of the wind and high-energy
emission of the host star reproducing the non-detection of Ly planetary
absorption. Methods: We model the escaping planetary atmosphere, the stellar
wind, and their interaction employing a multi-fluid, three-dimensional
hydrodynamic code. We assume a planetary atmosphere composed of hydrogen and
helium. We run models varying the stellar high-energy emission and stellar
mass-loss rate, further computing for each case the Ly synthetic
planetary atmospheric absorption and comparing it with the observations.
Results: We find that a non-detection of Ly in absorption employing the
stellar high-energy emission estimated from far-ultraviolet and X-ray data
requires a stellar wind with a stellar mass-loss rate about six times lower
than solar. This result is a consequence of the fact that, for Men c,
detectable Ly absorption can be caused exclusively by energetic neutral
atoms, which become more abundant with increasing the velocity and/or the
density of the stellar wind. By considering, instead, that the star has a
solar-like wind, the non-detection requires a stellar ionising radiation about
four times higher than estimated. This is because, despite the fact that a
stronger stellar high-energy emission ionises hydrogen more rapidly, it also
increases the upper atmosphere heating and expansion, pushing the interaction
region with the stellar wind farther away from the planet, where the planet
atmospheric density that remains neutral becomes smaller and the production of
energetic neutral atoms less efficient. Conclusions: Comparing the results of
our grid of models with what is expected and estimated for the stellar wind and
high-energy emission, respectively, we support the idea that the atmosphere of
Men c is likely not hydrogen-dominated.Comment: Accepted for publication in A&A. The abstract has been shortened to
fit the arXiv for
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