1,731 research outputs found
Coupled rotor-body equations of motion hover flight
A set of linearized equations of motion to predict the linearized dynamic response of a single rotor helicopter in a hover trim condition to cyclic pitch control inputs is described. The equations of motion assume four fuselage degrees of freedom: lateral and longitudinal translation, roll angle, pitch angle: four rotor degrees of freedom: flapping (lateral and longitudinal tilt of the tip path plane), lagging (lateral and longitudinal displacement of the rotor plane center of mass); and dynamic inflow (harmonic components). These ten degrees of freedom correspond to a system with eighteen dynamic states. In addition to examination of the full system dynamics, the computer code supplied with this report permits the examination of various reduced order models. The code is presented in a specific form such that the dynamic response of a helicopter in flight can be investigated. With minor modifications to the code the dynamics of a rotor mounted on a flexible support can also be studied
Design and numerical evaluation of full-authority flight control systems for conventional and thruster-augmented helicopters employed in NOE operations
The development and methodology is presented for development of full-authority implicit model-following and explicit model-following optimal controllers for use on helicopters operating in the Nap-of-the Earth (NOE) environment. Pole placement, input-output frequency response, and step input response were used to evaluate handling qualities performance. The pilot was equipped with velocity-command inputs. A mathematical/computational trajectory optimization method was employed to evaluate the ability of each controller to fly NOE maneuvers. The method determines the optimal swashplate and thruster input histories from the helicopter's dynamics and the prescribed geometry and desired flying qualities of the maneuver. Three maneuvers were investigated for both the implicit and explicit controllers with and without auxiliary propulsion installed: pop-up/dash/descent, bob-up at 40 knots, and glideslope. The explicit controller proved to be superior to the implicit controller in performance and ease of design
Eulerian simulation of the fluid dynamics of helicopter brownout
A computational model is presented that can be used to simulate the development of the dust cloud
that can be entrained into the air when a helicopter is operated close to the ground in desert or dusty
conditions. The physics of this problem, and the associated pathological condition known as ‘brownout’
where the pilot loses situational awareness as a result of his vision being occluded by dust suspended in the
flow around the helicopter, is acknowledged to be very complex. The approach advocated here involves
an approximation to the full dynamics of the coupled particulate-air system. Away from the ground, the
model assumes that the suspended particles remain in near equilibrium under the action of aerodynamic
forces. Close to the ground, this model is replaced by an algebraic sublayer model for the saltation and
entrainment process. The origin of the model in the statistical mechanics of a distribution of particles
governed by aerodynamic forces allows the validity of the method to be evaluated in context by comparing
the physical properties of the suspended particulates to the local properties of the flow field surrounding
the helicopter. The model applies in the Eulerian frame of reference of most conventional Computational
Fluid Dynamics codes and has been coupled with Brown’s Vorticity Transport Model. Verification of the
predictions of the coupled model against experimental data for particulate entrainment and transport in
the flow around a model rotor are encouraging. An application of the coupled model to analyzing the
differences in the geometry and extent of the dust clouds that are produced by single main rotor and
tandem-rotor configurations as they decelerate to land has shown that the location of the ground vortex
and the size of any regions of recirculatory flow, should they exist, play a primary role in governing the
extent of the dust cloud that is created by the helicopter
Tomographic Separation of Composite Spectra. VIII. The Physical Properties of the Massive Compact Binary in the Triple Star System HD 36486 (delta Orionis A)
Double-lined spectroscopic orbital elements have recently been found for the
central binary in the massive triple, delta Orionis A based on radial
velocities from cross-correlation techniques applied to IUE high dispersion
spectra and He I 6678 spectra obtained at Kitt Peak. The primary and secondary
velocity amplitudes were found to be 94.9 +/- 0.6 km/s and 186 +/- 9 km/s
respectively. Tomographic reconstructions of the primary and secondary stars'
spectra confirm the O9.5 II classification of the primary and indicate a B0.5
III type for the secondary. The widths of the UV cross-correlation functions
are used to estimate the projected rotational velocities, Vsin i = 157 +/- 6
km/s and 138 +/- 16 km/s for the primary and secondary, respectively implying
that both stars rotate faster than their orbital motion. We used the
spectroscopic results to make a constrained fit of the Hipparcos light curve of
this eclipsing binary, and the model fits limit the inclination to the range
between 67 and 77 degrees. The i = 67 degrees solution, which corresponds to a
near Roche-filling configuration, results in a primary mass of 11.2 solar
masses and a secondary mass of 5.6 solar masses, both of which are
substantially below the expected masses for stars of their luminosity. This
binary may have experienced a mass ratio reversal caused by Case A Roche lobe
overflow, or the system may have suffered extensive mass loss through a binary
interaction, perhaps during a common envelope phase, in which most of the
primary's mass was lost from the system rather than transferred to the
secondary.Comment: 27 pages, 15 figures in press, the Astrophysical Journal, February 1,
200
Theoretical study of O adlayers on Ru(0001)
Recent experiments performed at high pressures indicate that ruthenium can
support unusually high concentrations of oxygen at the surface. To investigate
the structure and stability of high coverage oxygen structures, we performed
density functional theory calculations, within the generalized gradient
approximation, for O adlayers on Ru(0001) from low coverage up to a full
monolayer. We achieve quantitative agreement with previous low energy electron
diffraction intensity analyses for the (2x2) and (2x1) phases and predict that
an O adlayer with a (1x1) periodicity and coverage of 1 monolayer can form on
Ru(0001), where the O adatoms occupy hcp-hollow sites.Comment: RevTeX, 6 pages, 4 figure
Thermochemistry of Alane Complexes for Hydrogen Storage: A Theoretical and Experimental Comparison
Knowledge of the relative stabilities of alane (AlH3) complexes with electron
donors is essential for identifying hydrogen storage materials for vehicular
applications that can be regenerated by off-board methods; however, almost no
thermodynamic data are available to make this assessment. To fill this gap, we
employed the G4(MP2) method to determine heats of formation, entropies, and
Gibbs free energies of formation for thirty-eight alane complexes with NH3-nRn
(R = Me, Et; n = 0-3), pyridine, pyrazine, triethylenediamine (TEDA),
quinuclidine, OH2-nRn (R = Me, Et; n = 0-2), dioxane, and tetrahydrofuran
(THF). Monomer, bis, and selected dimer complex geometries were considered.
Using these data, we computed the thermodynamics of the key formation and
dehydrogenation reactions that would occur during hydrogen delivery and alane
regeneration, from which trends in complex stability were identified. These
predictions were tested by synthesizing six amine-alane complexes involving
trimethylamine, triethylamine, dimethylethylamine, TEDA, quinuclidine, and
hexamine, and obtaining upper limits of delta G for their formation from
metallic aluminum. Combining these computational and experimental results, we
establish a criterion for complex stability relevant to hydrogen storage that
can be used to assess potential ligands prior to attempting synthesis of the
alane complex. Based on this, we conclude that only a subset of the tertiary
amine complexes considered and none of the ether complexes can be successfully
formed by direct reaction with aluminum and regenerated in an alane-based
hydrogen storage system.Comment: Accepted by the Journal of Physical Chemistry
Electron affinities of the first- and second- row atoms: benchmark ab initio and density functional calculations
A benchmark ab initio and density functional (DFT) study has been carried out
on the electron affinities of the first- and second-row atoms. The ab initio
study involves basis sets of and quality, extrapolations to
the 1-particle basis set limit, and a combination of the CCSD(T), CCSDT, and
full CI electron correlation methods. Scalar relativistic and spin-orbit
coupling effects were taken into account. On average, the best ab initio
results agree to better than 0.001 eV with the most recent experimental
results. Correcting for imperfections in the CCSD(T) method improves the mean
absolute error by an order of magnitude, while for accurate results on the
second-row atoms inclusion of relativistic corrections is essential. The latter
are significantly overestimated at the SCF level; for accurate spin-orbit
splitting constants of second-row atoms inclusion of (2s,2p) correlation is
essential. In the DFT calculations it is found that results for the 1st-row
atoms are very sensitive to the exchange functional, while those for second-row
atoms are rather more sensitive to the correlation functional. While the LYP
correlation functional works best for first-row atoms, its PW91 counterpart
appears to be preferable for second-row atoms. Among ``pure DFT'' (nonhybrid)
functionals, G96PW91 (Gill 1996 exchange combined with Perdew-Wang 1991
correlation) puts in the best overall performance. The best results overall are
obtained with the 1-parameter hybrid modified Perdew-Wang (mPW1) exchange
functionals of Adamo and Barone [J. Chem. Phys. {\bf 108}, 664 (1998)], with
mPW1LYP yielding the best results for first-row, and mPW1PW91 for second-row
atoms. Indications exist that a hybrid of the type mPW1LYP +
mPW1PW91 yields better results than either of the constituent functionals.Comment: Phys. Rev. A, in press (revised version, review of issues concerning
DFT and electron affinities added
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The reaction between silylene and ammonia: some gas-phase kinetic and quantum chemical studies
Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by 193 nm laser flash photolysis of silacyclopent-3-ene, have been carried out in the presence of ammonia, NH3. Second order kinetics were observed. The reaction was studied in the gas phase at 10 Torr total pressure in SF6 bath gas at each of the three temperatures, 299, 340 and 400 K. The second order rate constants (laser pulse energy of 60 mJ/pulse) fitted the Arrhenius equation:
log(k/cm3 molecule-1 s-1) = (-10.37 ± 0.17) + (0.36 ± 1.12 kJ mol-1)/RTln10
Experiments at other pressures showed that these rate constants were unaffected by pressure in the range 10-100 Torr, but showed small decreases in value at 3 and 1 Torr. There was also a weak intensity dependence, with rate constants decreasing at laser pulse energies of 30 mJ/pulse. Ab initio calculations at the G3 level of theory, show that SiH2 + NH3 should form an initial adduct (donor-acceptor complex), but that energy barriers are too great for further reaction of the adduct. This implies that SiH2 + NH3 should be a pressure dependent association reaction. The experimental data are inconsistent with this and we conclude that SiH2 decays are better explained by reaction of SiH2 with the amino radical, NH2, formed by photodissociation of NH3 at 193 nm. The mechanism of this previously unstudied reaction is discussed
Surface reconstruction induced geometries of Si clusters
We discuss a generalization of the surface reconstruction arguments for the
structure of intermediate size Si clusters, which leads to model geometries for
the sizes 33, 39 (two isomers), 45 (two isomers), 49 (two isomers), 57 and 61
(two isomers). The common feature in all these models is a structure that
closely resembles the most stable reconstruction of Si surfaces, surrounding a
core of bulk-like tetrahedrally bonded atoms. We investigate the energetics and
the electronic structure of these models through first-principles density
functional theory calculations. These models may be useful in understanding
experimental results on the reactivity of Si clusters and their shape as
inferred from mobility measurements.Comment: 9 figures (available from the author upon request) Submitted to Phys.
Rev.
Live Recombinant Salmonella Typhi Vaccines Constructed to Investigate the Role of rpoS in Eliciting Immunity to a Heterologous Antigen
We hypothesized that the immunogenicity of live Salmonella enterica serovar Typhi vaccines expressing heterologous antigens depends, at least in part, on its rpoS status. As part of our project to develop a recombinant attenuated S. Typhi vaccine (RASTyV) to prevent pneumococcal diseases in infants and children, we constructed three RASTyV strains synthesizing the Streptococcus pneumoniae surface protein PspA to test this hypothesis. Each vector strain carried ten engineered mutations designed to optimize safety and immunogenicity. Two S. Typhi vector strains (χ9639 and χ9640) were derived from the rpoS mutant strain Ty2 and one (χ9633) from the RpoS+ strain ISP1820. In χ9640, the nonfunctional rpoS gene was replaced with the functional rpoS gene from ISP1820. Plasmid pYA4088, encoding a secreted form of PspA, was moved into the three vector strains. The resulting RASTyV strains were evaluated for safety in vitro and for immunogenicity in mice. All three RASTyV strains were similar to the live attenuated typhoid vaccine Ty21a in their ability to survive in human blood and human monocytes. They were more sensitive to complement and were less able to survive and persist in sewage and surface water than their wild-type counterparts. Adult mice intranasally immunized with any of the RASTyV strains developed immune responses against PspA and Salmonella antigens. The RpoS+ vaccines induced a balanced Th1/Th2 immune response while the RpoS− strain χ9639(pYA4088) induced a strong Th2 immune response. Immunization with any RASTyV provided protection against S. pneumoniae challenge; the RpoS+ strain χ9640(pYA4088) provided significantly greater protection than the ISP1820 derivative, χ9633(pYA4088). In the pre-clinical setting, these strains exhibited a desirable balance between safety and immunogenicity and are currently being evaluated in a Phase 1 clinical trial to determine which of the three RASTyVs has the optimal safety and immunogenicity profile in human hosts
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