19,569 research outputs found
Ignition Transients of Large Segmented Solid Rocket Boosters
A model is described which provides a means for analyzing the complexities of ignition transients and pressure peaks of large, high performance, segmented solid rocket boosters. The method accounts for: (1) temporal and spatial development of the flow field set up by the head end igniter discharge, (2) ignition and flame spreading coupled to chamber flow, (3) the steep velocity, pressure, and temperature gradients that occur during the early phases of ignition, and (4) the interactions that produce ignition spikes (i.e., compression of chamber gases during pressurization, erosive burning, and mass added effect of igniter discharge). The technique differs from earlier models in that the flow interactions between the slots and main chamber are accounted for, and the original computer program for monolithic motors is improved. The procedures were used to predict the ignition transients of the current design for the space shuttle booster
On convective instability of a rotatiing fluid with a horizontal temperature contrast
A simple expression of the criterion for the onset of symmetrical therm.al convection of a rotating fluid subject to a horizontal temperature contrast is obtained. It is shown that both the rotation and a positive static stability act to inhibit convection...
Fano interference effect on the transition spectrum of single electron transistors
We theoretically study the intraband transition spectrum of single electron
transistors (SETs) composed of individual self-assembled quantum dots. The
polarization of SETs is obtained by using the nonequilibrium Green's function
technique and the Anderson model with three energy levels. Owing to
nonradiative coupling between two excited states through the continuum of
electrodes, the Fano interference effect significantly influences the peak
position and intensity of infrared wavelength single-photon spectrum.Comment: 4 pages, 5 figure
Tunnelling current and emission spectrum of a single electron transistor under optical pumping
Theoretical studies of the tunnelling current and emission spectrum of a
single electron transistor (SET) under optical pumping are presented. The
calculation is performed via Keldysh Green's function method within the
Anderson model with two energy levels. It is found that holes in the quantum
dot (QD) created by optical pumping lead to new channels for the electron
tunnelling from emitter to collector. As a consequence, an electron can tunnel
through the QD via additional channels, characterized by the exciton, trion and
biexciton states. It is found that the tunnelling current as a function of the
gate voltage displays a series of sharp peaks and the spacing between these
peaks can be used to determine the exciton binding energy as well as the
electron-electron Coulomb repulsion energy. In addition, we show that the
single-photon emission associated with the electron-hole recombination in the
exciton complexes formed in the QD can be controlled both electrically and
optically.Comment: 24 pages, 10 figure
The starting transient of solid propellant rocket motors with high internal gas velocities
A comprehensive analytical model which considers time and space development of the flow field in solid propellant rocket motors with high volumetric loading density is described. The gas dynamics in the motor chamber is governed by a set of hyperbolic partial differential equations, that are coupled with the ignition and flame spreading events, and with the axial variation of mass addition. The flame spreading rate is calculated by successive heating-to-ignition along the propellant surface. Experimental diagnostic studies have been performed with a rectangular window motor (50 cm grain length, 5 cm burning perimeter and 1 cm hydraulic port diameter), using a controllable head-end gaseous igniter. Tests were conducted with AP composite propellant at port-to-throat area ratios of 2.0, 1.5, 1.2, and 1.06, and head-end pressures from 35 to 70 atm. Calculated pressure transients and flame spreading rates are in very good agreement with those measured in the experimental system
Bifurcation in electrostatic resistive drift wave turbulence
The Hasegawa-Wakatani equations, coupling plasma density and electrostatic
potential through an approximation to the physics of parallel electron motions,
are a simple model that describes resistive drift wave turbulence. We present
numerical analyses of bifurcation phenomena in the model that provide new
insights into the interactions between turbulence and zonal flows in the
tokamak plasma edge region. The simulation results show a regime where, after
an initial transient, drift wave turbulence is suppressed through zonal flow
generation. As a parameter controlling the strength of the turbulence is tuned,
this zonal flow dominated state is rapidly destroyed and a turbulence-dominated
state re-emerges. The transition is explained in terms of the Kelvin-Helmholtz
stability of zonal flows. This is the first observation of an upshift of
turbulence onset in the resistive drift wave system, which is analogous to the
well-known Dimits shift in turbulence driven by ion temperature gradients.Comment: 21 pages, 11 figure
Gravitons and Lightcone Fluctuations II: Correlation Functions
A model of a fluctuating lightcone due to a bath of gravitons is further
investigated. The flight times of photons between a source and a detector may
be either longer or shorter than the light propagation time in the background
classical spacetime, and will form a Gaussian distribution centered around the
classical flight time. However, a pair of photons emitted in rapid succession
will tend to have correlated flight times. We derive and discuss a correlation
function which describes this effect. This enables us to understand more fully
the operational significance of a fluctuating lightcone. Our results may be
combined with observational data on pulsar timing to place some constraints on
the quantum state of cosmological gravitons.Comment: 16 pages and two figures, uses eps
Interacting Fock space characterization of probability measures
In this paper we characterize the probability measures, on Rd, with
square summable support, in terms of their associated preservation operators
and the commutators of the annihilation and creation operators
Moments and commutators of probability measures
Let a(0), a(-) and a(+) be the preservation, annihilation, and creation operators of a probability measure mu on R-d, respectively. The operators a(0) and [a(-), a(+)] are proven to uniquely determine the moments of mu. We discuss the question: "What conditions must two families of operators satisfy, in order to ensure the existence of a probability measure, having finite moments of any order, so that, its associated preservation operators and commutators between the annihilation and creation operators are the given families of operators?" For the case d = 1, a satisfactory answer to this question is obtained as a simple condition in terms of the Szego-Jacobi parameters. For the multidimensional case, we give some necessary conditions for the answer to this question. We also give a table with the associated preservation and commutator between the annihilation and creation operators, for some of the classic probability measures on R
Semiclassical Gravity Theory and Quantum Fluctuations
We discuss the limits of validity of the semiclassical theory of gravity in
which a classical metric is coupled to the expectation value of the stress
tensor. It is argued that this theory is a good approximation only when the
fluctuations in the stress tensor are small. We calculate a dimensionless
measure of these fluctuations for a scalar field on a flat background in
particular cases, including squeezed states and the Casimir vacuum state. It is
found that the fluctuations are small for states which are close to a coherent
state, which describes classical behavior, but tend to be large otherwise. We
find in all cases studied that the energy density fluctuations are large
whenever the local energy density is negative. This is taken to mean that the
gravitational field of a system with negative energy density, such as the
Casimir vacuum, is not described by a fixed classical metric but is undergoing
large metric fluctuations. We propose an operational scheme by which one can
describe a fluctuating gravitational field in terms of the statistical behavior
of test particles. For this purpose we obtain an equation of the form of the
Langevin equation used to describe Brownian motion.Comment: In REVTEX. 20pp + 4 figures(not included, available upon request)
TUTP-93-
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