10,994 research outputs found
Physical aspects of computing the flow of a viscous fluid
One of the main themes in fluid dynamics at present and in the future is going to be computational fluid dynamics with the primary focus on the determination of drag, flow separation, vortex flows, and unsteady flows. A computation of the flow of a viscous fluid requires an understanding and consideration of the physical aspects of the flow. This is done by identifying the flow regimes and the scales of fluid motion, and the sources of vorticity. Discussions of flow regimes deal with conditions of incompressibility, transitional and turbulent flows, Navier-Stokes and non-Navier-Stokes regimes, shock waves, and strain fields. Discussions of the scales of fluid motion consider transitional and turbulent flows, thin- and slender-shear layers, triple- and four-deck regions, viscous-inviscid interactions, shock waves, strain rates, and temporal scales. In addition, the significance and generation of vorticity are discussed. These physical aspects mainly guide computations of the flow of a viscous fluid
Computational aerodynamics and artificial intelligence
The general principles of artificial intelligence are reviewed and speculations are made concerning how knowledge based systems can accelerate the process of acquiring new knowledge in aerodynamics, how computational fluid dynamics may use expert systems, and how expert systems may speed the design and development process. In addition, the anatomy of an idealized expert system called AERODYNAMICIST is discussed. Resource requirements for using artificial intelligence in computational fluid dynamics and aerodynamics are examined. Three main conclusions are presented. First, there are two related aspects of computational aerodynamics: reasoning and calculating. Second, a substantial portion of reasoning can be achieved with artificial intelligence. It offers the opportunity of using computers as reasoning machines to set the stage for efficient calculating. Third, expert systems are likely to be new assets of institutions involved in aeronautics for various tasks of computational aerodynamics
Flow in a two-dimensional channel with a rectangular cavity
Flow characteristics in two dimensional channel with rectangular cavit
Potential application of artificial concepts to aerodynamic simulation
The concept of artificial intelligence as it applies to computational fluid dynamics simulation is investigated. How expert systems can be adapted to speed the numerical aerodynamic simulation process is also examined. A proposed expert grid generation system is briefly described which, given flow parameters, configuration geometry, and simulation constraints, uses knowledge about the discretization process to determine grid point coordinates, computational surface information, and zonal interface parameters
Fermi Edge Singularities in the Mesoscopic Regime: I. Anderson Orthogonality Catastrophe
For generic mesoscopic systems like quantum dots or nanoparticles, we study
the Anderson orthogonality catastrophe (AOC) and Fermi edge singularities in
photoabsorption spectra in a series of two papers. In the present paper we
focus on AOC for a finite number of particles in discrete energy levels where,
in contrast to the bulk situation, AOC is not complete. Moreover, fluctuations
characteristic for mesoscopic systems lead to a broad distribution of AOC
ground state overlaps. The fluctuations originate dominantly in the levels
around the Fermi energy, and we derive an analytic expression for the
probability distribution of AOC overlaps in the limit of strong perturbations.
We address the formation of a bound state and its importance for symmetries
between the overlap distributions for attractive and repulsive potentials. Our
results are based on a random matrix model for the chaotic conduction electrons
that are subject to a rank one perturbation corresponding, e.g., to the
localized core hole generated in the photoabsorption process.Comment: 10 pages, 8 figures, submitted to Phys. Rev.
Ballistic Electron Quantum Transport in Presence of a Disordered Background
Effect of a complicated many-body environment is analyzed on the electron
random scattering by a 2D mesoscopic open ballistic structure. A new mechanism
of decoherence is proposed. The temperature of the environment is supposed to
be zero whereas the energy of the incoming particle can be close to or
somewhat above the Fermi surface in the environment. The single-particle
doorway resonance states excited in the structure via external channels are
damped not only because of escape through such channels but also due to the
ulterior population of the long-lived environmental states. Transmission of an
electron with a given incoming through the structure turns out to be
an incoherent sum of the flow formed by the interfering damped doorway
resonances and the retarded flow of the particles re-emitted into the structure
by the environment. Though the number of the particles is conserved in each
individual event of transmission, there exists a probability that some part of
the electron's energy can be absorbed due to environmental many-body effects.
In such a case the electron can disappear from the resonance energy interval
and elude observation at the fixed transmission energy thus resulting
in seeming loss of particles, violation of the time reversal symmetry and, as a
consequence, suppression of the weak localization. The both decoherence and
absorption phenomena are treated within the framework of a unit microscopic
model based on the general theory of the resonance scattering. All the effects
discussed are controlled by the only parameter: the spreading width of the
doorway resonances, that uniquely determines the decoherence rateComment: 7 pages, 1 figure. The published version. A figure has been added;
the list of references has been improved. Some explanatory remarks have been
include
Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots
We have measured the electromagnetic response of micron-size isolated
mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them
to an electromagnetic micro-resonator. Both dissipative and non dissipative
responses exhibit a large magnetic field dependent quantum correction, with a
characteristic flux scale which corresponds to a flux quantum in a dot. The
real (dissipative) magnetoconductance changes sign as a function of frequency
for low enough density of electrons. The signal observed at frequency below the
mean level spacing corresponds to a negative magnetoconductance, which is
opposite to the weak localization seen in connected systems, and becomes
positive at higher frequency. We propose an interpretation of this phenomenon
in relation to fundamental properties of energy level spacing statistics in the
dots.Comment: 4 pages, 4 eps figure
The spectral form factor is not self-averaging
The spectral form factor, k(t), is the Fourier transform of the two level
correlation function C(x), which is the averaged probability for finding two
energy levels spaced x mean level spacings apart. The average is over a piece
of the spectrum of width W in the neighborhood of energy E0. An additional
ensemble average is traditionally carried out, as in random matrix theory.
Recently a theoretical calculation of k(t) for a single system, with an energy
average only, found interesting nonuniversal semiclassical effects at times t
approximately unity in units of {Planck's constant) /(mean level spacing). This
is of great interest if k(t) is self-averaging, i.e, if the properties of a
typical member of the ensemble are the same as the ensemble average properties.
We here argue that this is not always the case, and that for many important
systems an ensemble average is essential to see detailed properties of k(t). In
other systems, notably the Riemann zeta function, it is likely possible to see
the properties by an analysis of the spectrum.Comment: 4 pages, RevTex, no figures, submitted to Phys. Rev. Lett., permanent
e-mail address, [email protected]
Quantum Dots with Disorder and Interactions: A Solvable Large-g Limit
We show that problem of interacting electrons in a quantum dot with chaotic
boundary conditions is solvable in the large-g limit, where g is the
dimensionless conductance of the dot. The critical point of the
theory (whose location and exponent are known exactly) that separates strong
and weak-coupling phases also controls a wider fan-shaped region in the
coupling-1/g plane, just as a quantum critical point controls the fan in at
T>0. The weak-coupling phase is governed by the Universal Hamiltonian and the
strong-coupling phase is a disordered version of the Pomeranchuk transition in
a clean Fermi liquid. Predictions are made in the various regimes for the
Coulomb Blockade peak spacing distributions and Fock-space delocalization
(reflected in the quasiparticle width and ground state wavefunction).Comment: 4 pages, 2 figure
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