26,177 research outputs found
Controllability and stabilizability of distributed bilinear systems: Recent results and open problems
This paper describes recent results for controlling and stabilizing control systems of the form ú(t) = Au(t) + p(t) B(u(t)) where A is the infinitesimal generator C∞ semigroup
on a Banach space X, B' map from X into X, and p(t) is a real valued control. Application to a vibrating beam problem is given for illusstration of the theory
Controllability for Distributed Bilinear Systems
This paper studies controllability of systems of the form where is the infinitesimal generator of a semigroup of bounded linear operators on a Banach space , is a map, and is a control. The paper (i) gives conditions for elements of to be accessible from a given initial state and (ii) shows that controllability to a full neighborhood in of is impossible for . Examples of hyperbolic partial differential equations are provided
Current research in cavitating fluid films
A review of the current research of cavitation in fluid films is presented. Phenomena and experimental observations include gaseous cavitation, vapor cavitation, and gas entrainment. Cavitation in flooded, starved, and dynamically loaded journal bearings, as well as squeeze films are reviewed. Observations of cavitation damage in bearings and the possibility of cavitation between parallel plates with microasperities were discussed. The transcavity fluid transport process, meniscus motion and geometry or form of the film during rupture, and reformation were summarized. Performance effects were related to heat transfer models in the cavitated region and hysteresis influence on rotor dynamics coefficients. A number of cavitation algorithms was presented together with solution procedures using the finite difference and finite element methods. Although Newtonian fluids were assumed in most of the discussions, the effect of non-Newtonian fluids on cavitation was also discussed
QCD sum rules in the effective heavy quark theory
We derive sum rules for the leptonic decay constant of a heavy-light meson in the effective heavy quark theory. We show that the summation of logarithms in the heavy quark mass by the renormalization group technique enhances considerably radiative corrections. Our result for the decay constant in the static limit agrees well with recent lattice calculations. Finite quark mass corrections are estimated
Anisotropic diffusion limited aggregation in three dimensions : universality and nonuniversality
We explore the macroscopic consequences of lattice anisotropy for diffusion limited aggregation (DLA) in three dimensions. Simple cubic and bcc lattice growths are shown to approach universal asymptotic states in a coherent fashion, and the approach is accelerated by the use of noise reduction. These states are strikingly anisotropic dendrites with a rich hierarchy of structure. For growth on an fcc lattice, our data suggest at least two stable fixed points of anisotropy, one matching the bcc case. Hexagonal growths, favoring six planar and two polar directions, appear to approach a line of asymptotic states with continuously tunable polar anisotropy. The more planar of these growths visually resembles real snowflake morphologies. Our simulations use a new and dimension-independent implementation of the DLA model. The algorithm maintains a hierarchy of sphere coverings of the growth, supporting efficient random walks onto the growth by spherical moves. Anisotropy was introduced by restricting growth to certain preferred directions
QCD Sum Rules Calculation of Heavy Semileptonic Decay
We set up sum rules for heavy lambda decays in a full QCD calculation which
in the heavy quark mass limit incorporates the symmetries of heavy quark
effective theory. For the semileptonic \La_c decay we obtain a reasonable
agreement with experiment. For the \La_b semileptonic decay we find at the
zero recoil point a violation of the heavy quark symmetry of about 20%.Comment: Revised version. Title changed. 11 pages (RevTex), 4 PS figure
Perturbative Part of the Non-Singlet Structure Function F_2 in the Large-N_F Limit
We have calculated Wilson coefficients and anomalous dimensions
for the non-singlet part of the structure function F_2 in the large-N_F limit.
Our result agrees with exact two and three loop calculations and gives the
leading N_F dependence of the perturbative non-singlet Wilson coefficients to
all orders in .Comment: 11 pages, including one figur
Dynamic mechanical response of polymer networks
The dynamic-mechanical response of flexible polymer networks is studied in
the framework of tube model, in the limit of small affine deformations, using
the approach based on Rayleighian dissipation function. The dynamic complex
modulus G* is calculated from the analysis of a network strand relaxation to
the new equilibrium conformation around the distorted primitive path. Chain
equilibration is achieved via a sliding motion of polymer segments along the
tube, eliminating the inhomogeneity of the polymer density caused by the
deformation. The characteristic relaxation time of this motion separates
the low-frequency limit of the complex modulus from the high-frequency one,
where the main role is played by chain entanglements, analogous to the rubber
plateau in melts. The dependence of storage and loss moduli, G' and G'', on
crosslink and entanglement densities gives an interpolation between polymer
melts and crosslinked networks. We discuss the experimental implications of the
rather short relaxation time and the slow square-root variation of the moduli
and the loss factor tan at higher frequencies.Comment: Journal of Chemical Physics (Oct-2000); Lates, 4 EPS figures include
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