1,924 research outputs found
A research program in active control/aeroelasticity in the JIAFS at NASA Langley Research Center
A control law synthesis methodology for multifunctional active control system to satisfy root-mean-square load and response constraints as well as to meet stability robustness requirements at plant input and output was developed. Modern control theory, singular value analysis and optimization techniques were utilized. All stability and response derivative expressions were derived analytically for sensitivity study. The software is incorporated as an update to the AB/LAD general control design software package PADLOCS
Analytical method for determining the stability of linear retarded systems with two delays
The stability is considered of the solution differential-difference equations of the retarded type with constant coefficients and two constant time delays. A method that makes use of analytical expressions to determine stability boundaries, and the stability of the system, is derived. The method was applied to a system represented by a second-order differential equation with constant coefficients and time delays in the velocity and displacement terms. The results obtained is in agreement with those obtained by other investigators
Stability of neutral equations with constant time delays
A method was developed for determining the stability of a scalar neutral equation with constant coefficients and constant time delays. A neutral equation is basically a differential equation in which the highest derivative appears both with and without a time delay. Time delays may appear also in the lower derivatives or the independent variable itself. The method is easily implemented, and an illustrative example is presented
Nanoscale fluid flows in the vicinity of patterned surfaces
Molecular dynamics simulations of dense and rarefied fluids comprising small
chain molecules in chemically patterned nano-channels predict a novel switching
from Poiseuille to plug flow along the channel. We also demonstrate behavior
akin to the lotus effect for a nanodrop on a chemically patterned substrate.
Our results show that one can control and exploit the behavior of fluids at the
nanoscale using chemical patterning.Comment: Phys. Rev. Lett. in pres
Non-spherical shapes of capsules within a fourth-order curvature model
We minimize a discrete version of the fourth-order curvature based Landau
free energy by extending Brakke's Surface Evolver. This model predicts
spherical as well as non-spherical shapes with dimples, bumps and ridges to be
the energy minimizers. Our results suggest that the buckling and faceting
transitions, usually associated with crystalline matter, can also be an
intrinsic property of non-crystalline membranes.Comment: 6 pages, 4 figures (LaTeX macros EPJ), accepted for publication in
EPJ
Self-assembly mechanism in colloids: perspectives from Statistical Physics
Motivated by recent experimental findings in chemical synthesis of colloidal
particles, we draw an analogy between self-assembly processes occurring in
biological systems (e.g. protein folding) and a new exciting possibility in the
field of material science. We consider a self-assembly process whose elementary
building blocks are decorated patchy colloids of various types, that
spontaneously drive the system toward a unique and predetermined targeted
macroscopic structure.
To this aim, we discuss a simple theoretical model -- the Kern-Frenkel model
-- describing a fluid of colloidal spherical particles with a pre-defined
number and distribution of solvophobic and solvophilic regions on their
surface. The solvophobic and solvophilic regions are described via a
short-range square-well and a hard-sphere potentials, respectively.
Integral equation and perturbation theories are presented to discuss
structural and thermodynamical properties, with particular emphasis on the
computation of the fluid-fluid (or gas-liquid) transition in the
temperature-density plane.
The model allows the description of both one and two attractive caps, as a
function of the fraction of covered attractive surface, thus interpolating
between a square-well and a hard-sphere fluid, upon changing the coverage.
By comparison with Monte Carlo simulations, we assess the pros and the cons
of both integral equation and perturbation theories in the present context of
patchy colloids, where the computational effort for numerical simulations is
rather demanding.Comment: 14 pages, 7 figures, Special issue for the SigmaPhi2011 conferenc
Oxidation of graphene on metals
We use low-energy electron microscopy to investigate how graphene is removed
from Ru(0001) and Ir(111) by reaction with oxygen. We find two mechanisms on
Ru(0001). At short times, oxygen reacts with carbon monomers on the surrounding
Ru surface, decreasing their concentration below the equilibrium value. This
undersaturation causes a flux of carbon from graphene to the monomer gas. In
this initial mechanism, graphene is etched at a rate that is given precisely by
the same non-linear dependence on carbon monomer concentration that governs
growth. Thus, during both growth and etching, carbon attaches and detaches to
graphene as clusters of several carbon atoms. At later times, etching
accelerates. We present evidence that this process involves intercalated
oxygen, which destabilizes graphene. On Ir, this mechanism creates observable
holes. It also occurs mostly quickly near wrinkles in the graphene islands,
depends on the orientation of the graphene with respect to the Ir substrate,
and, in contrast to the first mechanism, can increase the density of carbon
monomers. We also observe that both layers of bilayer graphene islands on Ir
etch together, not sequentially.Comment: 15 pages, 10 figures. Manuscript revised to improve discussion,
following referee comments. Accepted for publication in Journal of Physical
Chemistry C, Feb. 11, 201
Quasiparticle interfacial level alignment of highly hybridized frontier levels: HO on TiO(110)
Knowledge of the frontier levels' alignment prior to photo-irradiation is
necessary to achieve a complete quantitative description of HO
photocatalysis on TiO(110). Although HO on rutile TiO(110) has been
thoroughly studied both experimentally and theoretically, a quantitative value
for the energy of the highest HO occupied levels is still lacking. For
experiment, this is due to the HO levels being obscured by hybridization
with TiO(110) levels in the difference spectra obtained via ultraviolet
photoemission spectroscopy (UPS). For theory, this is due to inherent
difficulties in properly describing many-body effects at the
HO-TiO(110) interface. Using the projected density of states (DOS) from
state-of-the-art quasiparticle (QP) , we disentangle the adsorbate and
surface contributions to the complex UPS spectra of HO on TiO(110). We
perform this separation as a function of HO coverage and dissociation on
stoichiometric and reduced surfaces. Due to hybridization with the TiO(110)
surface, the HO 3a and 1b levels are broadened into several peaks
between 5 and 1 eV below the TiO(110) valence band maximum (VBM). These
peaks have both intermolecular and interfacial bonding and antibonding
character. We find the highest occupied levels of HO adsorbed intact and
dissociated on stoichiometric TiO(110) are 1.1 and 0.9 eV below the VBM. We
also find a similar energy of 1.1 eV for the highest occupied levels of HO
when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In
both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than
those estimated from UPS difference spectra, which are inconclusive in this
energy region. Finally, we apply self-consistent QP (scQP1) to obtain
the ionization potential of the HO-TiO(110) interface.Comment: 12 pages, 12 figures, 1 tabl
Generic properties of a quasi-one dimensional classical Wigner crystal
We studied the structural, dynamical properties and melting of a
quasi-one-dimensional system of charged particles, interacting through a
screened Coulomb potential. The ground state energy was calculated and,
depending on the density and the screening length, the system crystallizes in a
number of chains. As a function of the density (or the confining potential),
the ground state configurations and the structural transitions between them
were analyzed both by analytical and Monte Carlo calculations. The system
exhibits a rich phase diagram at zero temperature with continuous and
discontinuous structural transitions. We calculated the normal modes of the
Wigner crystal and the magneto-phonons when an external constant magnetic field
is applied. At finite temperature the melting of the system was studied via
Monte Carlo simulations using the (MLC). The
melting temperature as a function of the density was obtained for different
screening parameters. Reentrant melting as a function of the density was found
as well as evidence of directional dependent melting. The single chain regime
exhibits anomalous melting temperatures according to the MLC and as a check we
study the pair correlation function at different densities and different
temperatures, formulating a different criterion. Possible connection with
recent theoretical and experimental results are discussed and experiments are
proposed.Comment: 13 pages text, 21 picture
Ninth and Tenth Order Virial Coefficients for Hard Spheres in D Dimensions
We evaluate the virial coefficients B_k for k<=10 for hard spheres in
dimensions D=2,...,8. Virial coefficients with k even are found to be negative
when D>=5. This provides strong evidence that the leading singularity for the
virial series lies away from the positive real axis when D>=5. Further analysis
provides evidence that negative virial coefficients will be seen for some k>10
for D=4, and there is a distinct possibility that negative virial coefficients
will also eventually occur for D=3.Comment: 33 pages, 12 figure
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