16,844 research outputs found
Shaping of molecular weight distribution by iterative learning probability density function control strategies
A mathematical model is developed for the molecular weight distribution (MWD) of free-radical styrene polymerization in a simulated semi-batch reactor system. The generation function technique and moment method are employed to establish the MWD model in the form of Schultz-Zimmdistribution. Both static and dynamic models are described in detail. In order to achieve the closed-loop MWD shaping by output probability density function (PDF) control, the dynamic MWD model is further developed by a linear B-spline approximation. Based on the general form of the B-spline MWD model, iterative learning PDF control strategies have been investigated in order to improve the MWD control performance. Discussions on the simulation studies show the advantages and limitations of the methodology
Properties of nuclei in the nobelium region studied within the covariant, Skyrme, and Gogny energy density functionals
We calculate properties of the ground and excited states of nuclei in the
nobelium region for proton and neutron numbers of 92 <= Z <= 104 and 144 <= N
<= 156, respectively. We use three different energy-density-functional (EDF)
approaches, based on covariant, Skyrme, and Gogny functionals, each within two
different parameter sets. A comparative analysis of the results obtained for
odd-even mass staggerings, quasiparticle spectra, and moments of inertia allows
us to identify single-particle and shell effects that are characteristic to
these different models and to illustrate possible systematic uncertainties
related to using the EDF modellingComment: 43 LaTeX pages, 14 figures, accepted in Nuclear Physics A, Special
Issue on Superheavy Element
Calculation of composition distribution of ultrafine ion-H2O-H2SO4 clusters using a modified binary ion nucleation theory
Thomson's ion nucleation theory was modified to include the effects of curvature dependence of the microscopic surface tension of field dependent, nonlinear, dielectric properties of the liquid; and of sulfuric acid hydrate formation in binary mixtures of water and sulfuric acid vapors. The modified theory leads to a broadening of the ion cluster spectrum, and shifts it towards larger numbers of H2O and H2SO4 molecules. Whether there is more shifting towards larger numbers of H2O or H2SO4 molecules depends on the relative humidity and relative acidity of the mixture. Usually, a broadening of the spectrum is accompanied by a lowering of the mean cluster intensity. For fixed values of relative humidity and relative acidity, a similar broadening pattern is observed when the temperature is lowered. These features of the modified theory illustrate that a trace of sulfuric acid can facilitate the formation of ultrafine, stable, prenucleation ion clusters as well as the growth of the prenucleation ion clusters towards the critical saddle point conditions, even with low values of relative humidity and relative acidity
Design of Copolymeric Materials
We devise a method for designing materials that will have some desired
structural characteristics. We apply it to multiblock copolymers that have two
different types of monomers, A and B. We show how to determine what sequence of
A's and B's should be synthesised in order to give a particular structure and
morphology. %For example in a melt of such %polymers, one may wish to engineer
a body-centered %cubic structure. Using this method in conjunction with the
theory of microphase separation developed by Leibler, we show it is possible to
efficiently search for a desired morphology. The method is quite general and
can be extended to design isolated heteropolymers, such as proteins, with
desired structural characteristics. We show that by making certain
approximations to the exact algorithm, a method recently proposed by
Shakhnovich and Gutin is obtained. The problems with this method are discussed
and we propose an improved approximate algorithm that is computationally
efficient.Comment: 15 pages latex 2.09 and psfig, 1 postscript figure
Numerical modeling study of the momentum deposition of small amplitude gravity waves in the thermosphere
We study the momentum deposition in the thermosphere from the dissipation of
small amplitude gravity waves (GWs) within a wave packet using a fully
nonlinear two-dimensional compressible numerical model. The model solves the
nonlinear propagation and dissipation of a GW packet from the stratosphere
into the thermosphere with realistic molecular viscosity and thermal
diffusivity for various Prandtl numbers. The numerical simulations are
performed for GW packets with initial vertical wavelengths (λ<sub><i>z</i></sub>)
ranging from 5 to 50 km. We show that λ<sub><i>z</i></sub> decreases in
time as a GW packet dissipates in the thermosphere, in agreement with the
ray trace results of Vadas and Fritts (2005) (VF05). We also find good
agreement for the peak height of the momentum flux (<i>z</i><sub>diss</sub>) between our
simulations and VF05 for GWs with initial λ<sub><i>z</i></sub> ≤ 2π <i>H</i> in
an isothermal, windless background, where <i>H</i> is the density scale height. We
also confirm that <i>z</i><sub>diss</sub> increases with increasing Prandtl number. We
include eddy diffusion in the model, and find that the momentum deposition
occurs at lower altitudes and has two separate peaks for GW packets with
small initial λ<sub><i>z</i></sub>. We also simulate GW packets in a
non-isothermal atmosphere. The net λ<sub><i>z</i></sub> profile is a competition
between its decrease from viscosity and its increase from the increasing
background temperature. We find that the wave packet disperses more in the
non-isothermal atmosphere, and causes changes to the momentum flux and
λ<sub><i>z</i></sub> spectra at both early and late times for GW packets with
initial λ<sub><i>z</i></sub> ≥ 10 km. These effects are caused by the
increase in <i>T</i> in the thermosphere, and the decrease in <i>T</i> near the mesopause
Consumer Willingness to Pay for Improved Attributes of Fresh Vegetables: A Comparison Between Atlanta and Berlin
Crop Production/Industries, Food Consumption/Nutrition/Food Safety,
Slave particle approach to the finite temperature properties of ultracold Bose gases in optical lattices
By using slave particle (slave boson and slave fermion) technique on the
Bose-Hubbard model, we study the finite temperature properties of ultracold
Bose gases in optical lattices. The phase diagrams at finite temperature are
depicted by including different types of slave particles and the effect of the
finite types of slave particles is estimated. The superfluid density is
evaluated using the Landau second order phase transition theory. The atom
density, excitation spectrum and dispersion curve are also computed at various
temperatures, and how the Mott-insulator evolves as the temperature increases
is demonstrated. For most quantities to be calculated, we find that there are
no qualitatively differences in using the slave boson or the slave fermion
approaches. However, when studying the stability of the mean field state, we
find that in contrast to the slave fermion approach, the slave boson mean field
state is not stable. Although the slave boson mean field theory gives a
qualitatively correct phase boundary, it corresponds to a local maximum of
Landau free energy and can not describe the second order phase transition
because the coefficient of the fourth order term is always negative in
the free energy expansion.Comment: 27 pages, 8 figures, final version for publicatio
- …