28 research outputs found
Forward-recursive adaptive control.
Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1972 .L88. Source: Dissertation Abstracts International, Volume: 62-13, Section: A. Thesis (Ph.D.)--University of Windsor (Canada), 1972
Experimental investigation of re-wetting of hot horizontal tubes
Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Re-wetting of a hot dry surface is the establishment of
direct contact between the surface and a liquid at a lower
temperature. Re-wetting heat transfer is characterized by a large
increase in heat transfer from the surface and occurs when a
vapor film existing between the dry surface and the liquid is
destabilized. Study of re-wetting heat transfer is very important
in nuclear reactor safety for limiting the extent of core damage
during the early stages of severe accidents after loss of coolant
accidents LOCA and is essential for predicting the rate at which
the coolant cools an overheated core. Surface re-wetting is
established by the formation of a wet patch on the hot dry
surface which then grows in size to cover the entire surface.
The leading edge of the wet patch is called the re-wetting front
and consists of transition and nucleate boiling heat transfer
regions. The aim of this study is to introduce and study two
important variables related to the re-wetting front as it moves
on hot horizontal tubes cooled by water jet impingement. These
variables are: the rebound phenomenon of the re-wetting front
and the width of the re-wetting front. Experimental
observations of this study showed that the re-wetting front
could rebound a small distance just after the formation of the
wet patch due to rapid heat conduction in the solid towards the
wet patch. The rebound distance was found to increase by
decreasing water temperature and the velocity of the jet. The
width of the re-wetting front was found to increase by
increasing water temperature and decreasing the initial surface
temperature. As the solid thermal conductivity increases, this
width was found to increase.mp201
Coherent Tunneling of Atoms from Bose-condensed Gases at Finite Temperatures
Tunneling of atoms between two trapped Bose-condensed gases at finite
temperatures is explored using a many-body linear response tunneling formalism
similar to that used in superconductors. To lowest order, the tunneling
currents can be expressed quite generally in terms of the single-particle
Green's functions of the isolated Bose gases. A coherent first-order tunneling
Josephson current between two atomic Bose-condensates is found, in addition to
coherent and dissipative contributions from second-order
condensate-noncondensate and noncondensate-noncondensate tunneling. Our work is
a generalization of Meier and Zwerger, who recently treated tunneling between
uniform atomic Bose gases. We apply our formalism to the analysis of an
out-coupling experiment induced by light wave fields, using a simple
Bogoliubov-Popov quasiparticle approximation for the trapped Bose gas. For
tunneling into the vacuum, we recover the results of Japha, Choi, Burnett and
Band, who recently pointed out the usefulness of studying the spectrum of
out-coupled atoms. In particular, we show that the small tunneling current of
noncondensate atoms from a trapped Bose gas has a broad spectrum of energies,
with a characteristic structure associated with the Bogoliubov quasiparticle
u^2 and v^2 amplitudes.Comment: 26 pages, 5 figures, minor changes, to appear in PR
Dynamic correlation functions in one-dimensional quasi-condensates
We calculate the static and dynamic single-particle correlation functions in
one-dimensional (1D) trapped Bose gases and discuss experimental measurements
that can directly probe such correlation functions. Using a quantized
hydrodynamic theory for the low energy excitations, we calculate both the
static and dynamic single-particle correlation functions for a 1D Bose gas that
is a phase-fluctuating quasi-condensate. For the static (equal-time)
correlation function, our approximations and results are equivalent to those of
Petrov, Shlyapnikov and Walraven. The Fourier transform of the static
single-particle correlation function gives the momentum distribution, which can
be measured using Doppler-sensitive Bragg scattering experiments on a highly
elongated Bose gas. We show how a two-photon Raman out-coupling experiment can
measure the characteristic features of the dynamic or time-dependent
single-particle correlation function of a 1D Bose quasi-condensate.Comment: 19 pages, 4 figures; submitted to Phys. Rev.
Finite-temperature correlations in the one-dimensional trapped and untrapped Bose gases
We calculate the dynamic single-particle and many-particle correlation
functions at non-zero temperature in one-dimensional trapped repulsive Bose
gases. The decay for increasing distance between the points of these
correlation functions is governed by a scaling exponent that has a universal
expression in terms of observed quantities. This expression is valid in the
weak-interaction Gross-Pitaevskii as well as in the strong-interaction
Girardeau-Tonks limit, but the observed quantities involved depend on the
interaction strength. The confining trap introduces a weak center-of-mass
dependence in the scaling exponent. We also conjecture results for the
density-density correlation function.Comment: 18 pages, Latex, Revtex
Bosonizing one-dimensional cold atomic gases
We present results for the long-distance asymptotics of correlation functions
of mesoscopic one-dimensional systems with periodic and open (Dirichlet)
boundary conditions, as well as at finite temperature in the thermodynamic
limit. The results are obtained using Haldane's harmonic-fluid approach (also
known as ``bosonization''), and are valid for both bosons and fermions, in
weakly and strongly interacting regimes. The harmonic-fluid approach and the
method to compute the correlation functions using conformal transformations are
explained in great detail. As an application relevant to one-dimensional
systems of cold atomic gases, we consider the model of bosons interacting with
a zero-range potential. The Luttinger-liquid parameters are obtained from the
exact solution by solving the Bethe-ansatz equations in finite-size systems.
The range of applicability of the approach is discussed, and the prefactor of
the one-body density matrix of bosons is fixed by finding an appropriate
parametrization of the weak-coupling result. The formula thus obtained is shown
to be accurate, when compared with recent diffusion Montecarlo calculations,
within less than 10%. The experimental implications of these results for Bragg
scattering experiments at low and high momenta are also discussed.Comment: 39 pages + 14 EPS figures; typos corrected, references update
Current status of Melcor 2.2 for fusion safety analyses
MELCOR is an integral code developed by Sandia National Laboratories (SNL) for the US Nuclear Regulatory Commission (USNRC) to perform severe accident analyses of Light Water Reactors (LWR). More recently, MELCOR capabilities are being extended also to analyze non-LWR fission technologies. Within the European MELCOR User Group (EMUG), organized in the framework of the USNRC Cooperative Severe Accident Research Program (CSARP), an activity on the evaluation of the applicability of MELCOR 2.2 for fusion safety analyses has been launched and it has been coordinated by ENEA. The aim of the activity was to identify the physical models to be possibly implemented in MELCOR 2.2 necessary for fusion safety analyses, and to check if those models are already available in MELCOR 1.8.6 fusion version, developed by Idaho National Laboratory (INL). From this activity, a list of modeling needs that emerged from the safety analyses of fusion-related installations has been
identified and described. Then, the importance of the various needs, intended as the priority for model implementation in the MELCOR 2.2 code, has been evaluated according to the technical expert judgment of the authors. In the present paper, the identified modeling needs are discussed. The ultimate goal would be to propose to have a single integrated MELCOR 2.2 code release capable to cover both fission and fusion applications
Momentum distribution and correlation function of quasicondensates in elongated traps
We calculate the spatial correlation function and momentum distribution of a
phase-fluctuating, elongated three-dimensional condensate, in a trap and in
free expansion. We take the inhomogeneous density profile into account
{\it{via}} a local density approximation. We find an almost Lorentzian momentum
distribution, in stark contrast with a Heisenberg-limited Thomas-Fermi
condensate.Comment: 5 pages, 2 figures; final version, references update
Investigation of an optimalising controller
Includes bibliography.This dissertation presents a comprehensive investigation of an optimalising controller and its behaviour when controlling a simulated plant. In addition certain theoretical aspects of optimalising control are presented. Design criteria and details of the circuitry of the experimental controller, built as part of this research project, are given. The necessity to deactivate the controller, for a certain period during each cycle of operation, is shown to be a result of measurement delay in the optimalising loop, and an expression for evaluating the deactive time is derived. Utilising describing function techniques, the steady-state response of the control system is predicted and the results are shown to be in good agreement with the experimental results. The adaptive response of the system is investigated and it is shown that the bandwidth of the adaptive response maybe predicted from steady-state response characteristics. The limitations imposed upon both the steady-state and adaptive response, by measurement delay, are shown . With respect to the theory of optimalising control, a framework for stability analysis is developed and stability criteria for the basic optimalising control loop are derived