2,449 research outputs found
Optimal control theory investigation of proprotor/wing response to vertical gust
Optimal control theory is used to design linear state variable feedback to improve the dynamic characteristics of a rotor and cantilever wing representing the tilting proprotor aircraft in cruise flight. The response to a vertical gust and system damping are used as criteria for the open and closed loop performance. The improvement in the dynamic characteristics achievable is examined for a gimballed rotor and for a hingeless rotor design. Several features of the design process are examined, including: (1) using only the wing or only the rotor dynamics in the control system design; (2) the use of a wing flap as well as the rotor controls for inputs; (3) and the performance of the system designed for one velocity at other forward speeds
Connection between slow and fast dynamics of molecular liquids around the glass transition
The mean-square displacement (MSD) was measured by neutron scattering at
various temperatures and pressures for a number of molecular glass-forming
liquids. The MSD is invariant along the glass-transition line at the pressure
studied, thus establishing an ``intrinsic'' Lindemann criterion for any given
liquid. A one-to-one connection between the MSD's temperature dependence and
the liquid's fragility is found when the MSD is evaluated on a time scale of
approximately 4 nanoseconds, but does not hold when the MSD is evaluated at
shorter times. The findings are discussed in terms of the elastic model and the
role of relaxations, and the correlations between slow and fast dynamics are
addressed.Comment: accepted by Phys Rev E (2010
Comparison of different methods of measuring angle of progression in prediction of labor outcome
Objective: First, to compare the manual sagittal and para-sagittal and automated para-sagittal methods of measuring the angle of progression (AOP) by transperineal ultrasound during labor, and second, to develop models for the prediction of time-to-delivery and need for cesarean section (CS) for failure to progress (FTP) in a population of patients undergoing induction of labor.
Methods: This was a prospective observational study of transperineal ultrasound on a cohort of 512 women with singleton pregnancies undergiong induction of labor. A random selection of 50 stored images was assessed for inter- and intra-observer reliability between methods. In the cases of vaginal delivery univariate linear, multivariate linear and quantile regression were performed to predict time-to-delivery. Univariate and multivariate binomial logistic regression were performed to predict CS for FTP in the first stage of labor.
Results: The intra correlation coefficients (ICC) for the manual para-sagittal method for a single observer was 0.97 (CI 0.95-0.98) and for two observers was 0.96 (CI 0.93-0.98) indicating good reliability. The ICC for the sagittal method for a single observer was 0.93 (0.88-0.96) and for two observers was 0.74 (0.58-0.84) indicating moderate reliabilty for a single observer and poor reliability between two observers. Bland-Altman analysis demonstrated narrower limits of agreement for the manual para-saggittal approach than for the sagittal approach for both single and two observers. The automated para-sagittal method failed to capture an image in 19% of cases. The mean difference between sagittal and para-sagittal methods was 110. In pregnancies resulting in vaginal delivery, 54% of the variation in time-to-delivery was explained in a model combining parity, epidural and syntocinon use during labour and the sonographic findings of fetal head position and AOP. In the prediction of CS for FTP in the first stage of labour a model which combined maternal factors with the sonographic measurements of AOP and estimated fetal weight was superior to one utilising maternal factors alone (area under the curve 0.80 vs 0.76).
Conclusions: First, the method of measuring AOP with greatest reliability is the manual parasagittal technique and future research should focus on this technique, second, over half of the variation in time to vaginal delivery can be explained by a model that combines maternal factors, pregnancy characteristics and ultrasound findings, and third, the ability of AOP to provide clinically useful prediction CS for FTP in the first stage of labour is limited
Observation of a Turbulence-Induced Large Scale Magnetic Field
An axisymmetric magnetic field is applied to a spherical, turbulent flow of
liquid sodium. An induced magnetic dipole moment is measured which cannot be
generated by the interaction of the axisymmetric mean flow with the applied
field, indicating the presence of a turbulent electromotive force. It is shown
that the induced dipole moment should vanish for any axisymmetric laminar flow.
Also observed is the production of toroidal magnetic field from applied
poloidal magnetic field (the omega-effect). Its potential role in the
production of the induced dipole is discussed.Comment: 5 pages, 4 figures Revisions to accomodate peer-reviewer concerns;
changes to main text including simplification of a proof, Fig. 2 updated, and
minor typos and clarifications; Added refrences. Resubmitted to Phys. Rev.
Let
Entrainment rates and microphysics in POST stratocumulus
The article of record as published may be located at http://dx.doi.org/10.1002/jgrd.50878An aircraft field study (POST; Physics of Stratocumulus Top) was conducted off the
central California coast in July and August 2008 to deal with the known difficulty of
measuring entrainment rates in the radiatively important stratocumulus (Sc) prevalent in that
area. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies Twin Otter
research aircraft flew 15 quasi-Lagrangian flights in unbroken Sc and carried a full
complement of probes including three high-data-rate probes: ultrafast temperature probe,
particulate volume monitor probe, and gust probe. The probes’ colocation near the nose of
the Twin Otter permitted estimation of entrainment fluxes and rates with an in-cloud
resolution of 1m. Results include the following: Application of the conditional sampling
variation of classical mixed layer theory for calculating the entrainment rate into cloud top
for POST flights is shown to be inadequate for most of the Sc. Estimated rates resemble
previous results after theory is modified to take into account both entrainment and
evaporation at cloud top given the strong wind shear and mixing at cloud top. Entrainment
rates show a tendency to decrease for large shear values, and the largest rates are for the
smallest temperature jumps across the inversion. Measurements indirectly suggest that
entrained parcels are primarily cooled by infrared flux divergence rather than cooling from
droplet evaporation, while detrainment at cloud top causes droplet evaporation and cooling
in the entrainment interface layer above cloud top.NSF supported H. Gerber, G. Frick, and S. Malinowski (ATM-0735121, AGS-1020445), D. Khelif (ATM-0734323), and S. Krueger (ATM-0735118). The Office of Naval Research and the Naval Postgraduate School supported in part the deployment of the Twin Otter aircraft
Spiral cracks in drying precipitates
We investigate the formation of spiral crack patterns during the desiccation
of thin layers of precipitates in contact with a substrate. This
symmetry-breaking fracturing mode is found to arise naturally not from torsion
forces, but from a propagating stress front induced by the fold-up of the
fragments. We model their formation mechanism using a coarse-grain model for
fragmentation and successfully reproduce the spiral cracks. Fittings of
experimental and simulation data show that the spirals are logarithmic,
corresponding to constant deviation from a circular crack path. Theoretical
aspects of the logarithmic spirals are discussed. In particular we show that
this occurs generally when the crack speed is proportional to the propagating
speed of stress front.Comment: 4 pages, 5 figures, RevTe
Compact Labelings For Efficient First-Order Model-Checking
We consider graph properties that can be checked from labels, i.e., bit
sequences, of logarithmic length attached to vertices. We prove that there
exists such a labeling for checking a first-order formula with free set
variables in the graphs of every class that is \emph{nicely locally
cwd-decomposable}. This notion generalizes that of a \emph{nicely locally
tree-decomposable} class. The graphs of such classes can be covered by graphs
of bounded \emph{clique-width} with limited overlaps. We also consider such
labelings for \emph{bounded} first-order formulas on graph classes of
\emph{bounded expansion}. Some of these results are extended to counting
queries
Strongly coupled U(1) lattice gauge theory as a microscopic model of Yukawa theory
Dynamical chiral symmetry breaking in a strongly coupled U(1) lattice gauge
model with charged fermions and scalar is investigated by numerical simulation.
Several composite neutral states are observed, in particular a massive fermion.
In the vicinity of the tricritical point of this model we study the effective
Yukawa coupling between this fermion and the Goldstone boson. The perturbative
triviality bound of Yukawa models is nearly saturated. The theory is quite
similar to strongly coupled Yukawa models for sufficiently large coupling
except the occurrence of an additional state -- a gauge ball of mass about half
the mass of the fermion.Comment: 4 page
Energy fluxes in helical magnetohydrodynamics and dynamo action
Renormalized viscosity, renormalized resistivity, and various energy fluxes
are calculated for helical magnetohydrodynamics using perturbative field
theory. The calculation is to first-order in perturbation. Kinetic and magnetic
helicities do not affect the renormalized parameters, but they induce an
inverse cascade of magnetic energy. The sources for the the large-scale
magnetic field have been shown to be (1) energy flux from large-scale velocity
field to large-scale magnetic field arising due to nonhelical interactions, and
(2) inverse energy flux of magnetic energy caused by helical interactions.
Based on our flux results, a premitive model for galactic dynamo has been
constructed. Our calculations yields dynamo time-scale for a typical galaxy to
be of the order of years. Our field-theoretic calculations also reveal
that the flux of magnetic helicity is backward, consistent with the earlier
observations based on absolute equilibrium theory.Comment: REVTEX4; A factor of 2 corrected in helicit
The model of dynamo with small number of modes and magnetic activity of T Tauri stars
The model that describes operation of dynamo in fully convective stars is
presented. It is based on representation of stellar magnetic field as a
superposition of finite number of poloidal and toroidal free damping modes. In
the frame of adopted low of stellar differential rotation we estimated minimal
value of dynamo number D, starting from which generation of cyclic magnetic
field in stars without radiative core is possible. We also derived expression
for period of the cycle. It was found that dynamo cycles of fully convective
stars and stars with thin convective envelopes differ in a qualitative way: 1)
distribution of spots over latitude during the cycle is different in these
stars; 2) the model predicts that spot formation in fully convective stars
should be strongly suppressed at some phases of the cycle.
We have analyzed historical lightcurve of WTTS star V410 Tau and found that
long term activity of the star is not periodic process. Rather one can speak
about quasi cyclic activity with characteristic time of yr and chaotic
component over imposed. We concluded also that redistribution of cool spots
over longitude is the reason of long term variations of V410 Tau brightness. It
means that one can not compare directly results of photometric observations
with predictions of our axially symmetric (for simplicity) model which allows
to investigate time evolution of spot's distribution over latitude. We then
discuss what kind of observations and in which way could be used to check
predictions of the dynamo theory.Comment: 18 pages, 5 figures, accepted to Astron. Let
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