76,970 research outputs found
Thermodynamic properties of interstitial elements in the refractory metals Semiannual report, 1 Jun. - 1 Dec. 1966
Thermodynamic properties of interstitial elements in refractory metal
Electric multipole response of the halo nucleus He
The role of different continuum components in the weakly-bound nucleus He
is studied by coupling unbound spd-waves of He by means of simple pairing
contact-delta interaction. The results of our previous investigations in a
model space containing only p-waves, showed the collective nature of the ground
state and allowed the calculation of the electric quadrupole transitions. We
extend this simple model by including also sd-continuum neutron states and we
investigate the electric monopole, dipole and octupole response of the system
for transitions to the continuum, discussing the contribution of different
configurations.Comment: 22 pages, 10 figure
Pairing in the continuum: the quadrupole response of the Borromean nucleus 6He
The ground state and low-lying continuum states of 6He are found within a
shell model scheme, in a basis of two-particle states built out of continuum
p-states of the unbound 5He nucleus, using a simple pairing contact-delta
interaction. This accounts for the Borromean character of the bound ground
state, revealing its composition. We investigate the quadrupole response of the
system and we put our calculations into perspective with the latest
experimental results. The calculated quadrupole strength distribution
reproduces the narrow 2+ resonance, while a second wider peak is found at about
3.9 MeV above the g.s. energy.Comment: 5 pages, 5 figure
Evaluating force field accuracy with long-time simulations of a tryptophan zipper peptide
We have combined a custom implementation of the fast multiple-time-stepping
LN integrator with parallel tempering to explore folding properties of small
peptides in implicit solvent on the time scale of microseconds. We applied this
algorithm to the synthetic {\beta}-hairpin trpzip2 and one of its sequence
variants W2W9. Each simulation consisted of over 12 {\mu}s of aggregated
virtual time. Several measures of folding behavior showed convergence, allowing
comparison with experimental equilibrium properties. Our simulations suggest
that the electrostatic interaction of tryptophan sidechains is responsible for
much of the stability of the native fold. We conclude that the ff99 force field
combined with ff96 {\phi} and {\psi} dihedral energies and implicit solvent can
reproduce plausible folding behavior in both trpzip2 and W2W9.Comment: 10 pages, 11 figures, submitted to the Journal of Chemical Physics on
June 28, 201
Stochastic axial compressor variable geometry schedule optimisation
The design of axial compressors is dictated by the maximisation of flow
efficiency at on design conditions whereas at part speed the requirement for
operation stability prevails. Among other stability aids, compressor variable
geometry is employed to rise the surge line for the provision of an adequate
surge margin. The schedule of the variable vanes is in turn typically obtained
from expensive and time consuming rig tests that go through a vast combination
of possible settings. The present paper explores the suitability of stochastic
approaches to derive the most flow efficient schedule of an axial compressor for
a minimum variable user defined value of the surge margin. A genetic algorithm
has been purposely developed and its satisfactory performance validated against
four representative benchmark functions. The work carries on with the necessary
thorough investigation of the impact of the different genetic operators employed
on the ability of the algorithm to find the global extremities in an effective
and efficient manner. This deems fundamental to guarantee that the algorithm is
not trapped in local extremities. The algorithm is then coupled with a
compressor performance prediction tool that evaluates each individual's
performance through a user defined fitness function. The most flow efficient
schedule that conforms to a prescribed surge margin can be obtained thereby fast
and inexpensively. Results are produced for a modern eight stage high bypass
ratio compressor and compared with experimental data available to the research.
The study concludes with the analysis of the existent relationship between surge
margin and flow efficiency for the particular compressor under scrutiny. The
study concludes with the analysis of the existent relationship between surge
margin and flow efficiency for the particular compressor under scrutiny
New technique for calibrating hydrocarbon gas flowmeters
A technique for measuring calibration correction factors for hydrocarbon mass flowmeters is described. It is based on the Nernst theorem for matching the partial pressure of oxygen in the combustion products of the test hydrocarbon, burned in oxygen-enriched air, with that in normal air. It is applied to a widely used type of commercial thermal mass flowmeter for a number of hydrocarbons. The calibration correction factors measured using this technique are in good agreement with the values obtained by other independent procedures. The technique is successfully applied to the measurement of differences as low as one percent of the effective hydrocarbon content of the natural gas test samples
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