25,227 research outputs found
A Piecewise Linear State Variable Technique for Real Time Propulsion System Simulation
The emphasis on increased aircraft and propulsion control system integration and piloted simulation has created a need for higher fidelity real time dynamic propulsion models. A real time propulsion system modeling technique which satisfies this need and which provides the capabilities needed to evaluate propulsion system performance and aircraft system interaction on manned flight simulators was developed and demonstrated using flight simulator facilities at NASA Ames. A piecewise linear state variable technique is used. This technique provides the system accuracy, stability and transient response required for integrated aircraft and propulsion control system studies. The real time dynamic model includes the detail and flexibility required for the evaluation of critical control parameters and propulsion component limits over a limited flight envelope. The model contains approximately 7.0 K bytes of in-line computational code and 14.7 K of block data. It has an 8.9 ms cycle time on a Xerox Sigma 9 computer. A Pegasus-Harrier propulsion system was used as a baseline for developing the mathematical modeling and simulation technique. A hydromechanical and water injection control system was also simulated. The model was programmed for interfacing with a Harrier aircraft simulation at NASA Ames. Descriptions of the real time methodology and model capabilities are presented
Depletion potentials near geometrically structured substrates
Using the recently developed so-called White Bear version of Rosenfeld's
Fundamental Measure Theory we calculate the depletion potentials between a
hard-sphere colloidal particle in a solvent of small hard spheres and simple
models of geometrically structured substrates: a right-angled wedge or edge. In
the wedge geometry, there is a strong attraction beyond the corresponding one
near a planar wall that significantly influences the structure of colloidal
suspensions in wedges. In accordance with an experimental study, for the edge
geometry we find a free energy barrier of the order of several which
repels a big colloidal particle from the edge.Comment: 7 pages, 7 figure
Ab initio calculations of reactions with light nuclei
An {\em ab initio} (i.e., from first principles) theoretical framework
capable of providing a unified description of the structure and low-energy
reaction properties of light nuclei is desirable to further our understanding
of the fundamental interactions among nucleons, and provide accurate
predictions of crucial reaction rates for nuclear astrophysics, fusion-energy
research, and other applications. In this contribution we review {\em ab
initio} calculations for nucleon and deuterium scattering on light nuclei
starting from chiral two- and three-body Hamiltonians, obtained within the
framework of the {\em ab initio} no-core shell model with continuum. This is a
unified approach to nuclear bound and scattering states, in which
square-integrable energy eigenstates of the -nucleon system are coupled to
target-plus-projectile wave functions in the spirit of the resonating
group method to obtain an efficient description of the many-body nuclear
dynamics both at short and medium distances and at long ranges.Comment: 9 pages, 5 figures, proceedings of the 21st International Conference
on Few-Body Problems in Physic
A real time Pegasus propulsion system model for VSTOL piloted simulation evaluation
A real time propulsion system modeling technique suitable for use in man-in-the-loop simulator studies was developd. This technique provides the system accuracy, stability, and transient response required for integrated aircraft and propulsion control system studies. A Pegasus-Harrier propulsion system was selected as a baseline for developing mathematical modeling and simulation techniques for VSTOL. Initially, static and dynamic propulsion system characteristics were modeled in detail to form a nonlinear aerothermodynamic digital computer simulation of a Pegasus engine. From this high fidelity simulation, a real time propulsion model was formulated by applying a piece-wise linear state variable methodology. A hydromechanical and water injection control system was also simulated. The real time dynamic model includes the detail and flexibility required for the evaluation of critical control parameters and propulsion component limits over a limited flight envelope. The model was programmed for interfacing with a Harrier aircraft simulation. Typical propulsion system simulation results are presented
Repulsive Casimir Pistons
Casimir pistons are models in which finite Casimir forces can be calculated
without any suspect renormalizations. It has been suggested that such forces
are always attractive. We present three scenarios in which that is not true.
Two of these depend on mixing two types of boundary conditions. The other,
however, is a simple type of quantum graph in which the sign of the force
depends upon the number of edges.Comment: 4 pages, 2 figures; RevTeX. Minor additions and correction
Ab Initio study of neutron drops with chiral Hamiltonians
We report ab initio calculations for neutron drops in a 10 MeV external
harmonic-oscillator trap using chiral nucleon-nucleon plus three-nucleon
interactions. We present total binding energies, internal energies, radii and
odd-even energy differences for neutron numbers N = 2 - 18 using the no-core
shell model with and without importance truncation. Furthermore, we present
total binding energies for N = 8, 16, 20, 28, 40, 50 obtained in a
coupled-cluster approach. Comparisons with Green's Function Monte Carlo
results, where available, using Argonne v8' with three-nucleon interactions
reveal important dependences on the chosen Hamiltonian.Comment: 7 pages, 5 figure
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