18,170 research outputs found
Barrier island erosion and overwash study -- effect of seawalls. Volume 2
This is the second of a pair of reports documenting the effects of storms on beach systems
including the presence of seawalls. With the aim of simulating the effects of overwash on
barrier islands with seawalls and characterizing their response, a series of eight experiments
was conducted at the Coastal Engineering Laboratory of the University of Florida. The barrier
island was simulated by a 400 feet wide horizontal crest and an initially uniform mildly-sloped
(1:19) beach. The effects of positioning the seawall at two different locations as well as the effects
of various storm surge levels and accompanying overtopping were investigated. Experiments
were conducted with both regular and irregular storm waves. With the seawall located at the
slope break between the crest and the sloping beach of the barrier island, and the crest of the
seawall just submerged in sand, the effects on the sediment transport process were found to be
minimal. For the same position of the seawall but with the crest of the seawall raised above the
surrounding ground level, overtopping caused washover of sand indicating substantial transport
in suspension. Increased levels of overtopping tended to accentuate bed profile changes but
supress bar formation (as did irregular waves). Positioning the seawall at the Mean Sea Level
shoreline caused significant scour both immediately landward as well as immediately seaward
of the seawall. A prominent scour trough developed further seaward. The longshore bar was
highly three-dimensional. It appears that seawalls need to be located adequately landward of the
shoreline to discharge their function effectively without adverse effect to the beach. In addition,
concerns for safety warrant the presence of an adequate buffer-zone between the seawall and
the upland property. (61 pp.
Effective diffusion constant in a two dimensional medium of charged point scatterers
We obtain exact results for the effective diffusion constant of a two
dimensional Langevin tracer particle in the force field generated by charged
point scatterers with quenched positions. We show that if the point scatterers
have a screened Coulomb (Yukawa) potential and are uniformly and independently
distributed then the effective diffusion constant obeys the
Volgel-Fulcher-Tammann law where it vanishes. Exact results are also obtained
for pure Coulomb scatterers frozen in an equilibrium configuration of the same
temperature as that of the tracer.Comment: 9 pages IOP LaTex, no figure
Renormalization of Drift and Diffusivity in Random Gradient Flows
We investigate the relationship between the effective diffusivity and
effective drift of a particle moving in a random medium. The velocity of the
particle combines a white noise diffusion process with a local drift term that
depends linearly on the gradient of a gaussian random field with homogeneous
statistics. The theoretical analysis is confirmed by numerical simulation. For
the purely isotropic case the simulation, which measures the effective drift
directly in a constant gradient background field, confirms the result
previously obtained theoretically, that the effective diffusivity and effective
drift are renormalized by the same factor from their local values. For this
isotropic case we provide an intuitive explanation, based on a {\it spatial}
average of local drift, for the renormalization of the effective drift
parameter relative to its local value. We also investigate situations in which
the isotropy is broken by the tensorial relationship of the local drift to the
gradient of the random field. We find that the numerical simulation confirms a
relatively simple renormalization group calculation for the effective
diffusivity and drift tensors.Comment: Latex 16 pages, 5 figures ep
Continuum Derrida Approach to Drift and Diffusivity in Random Media
By means of rather general arguments, based on an approach due to Derrida
that makes use of samples of finite size, we analyse the effective diffusivity
and drift tensors in certain types of random medium in which the motion of the
particles is controlled by molecular diffusion and a local flow field with
known statistical properties. The power of the Derrida method is that it uses
the equilibrium probability distribution, that exists for each {\em finite}
sample, to compute asymptotic behaviour at large times in the {\em infinite}
medium. In certain cases, where this equilibrium situation is associated with a
vanishing microcurrent, our results demonstrate the equality of the
renormalization processes for the effective drift and diffusivity tensors. This
establishes, for those cases, a Ward identity previously verified only to
two-loop order in perturbation theory in certain models. The technique can be
applied also to media in which the diffusivity exhibits spatial fluctuations.
We derive a simple relationship between the effective diffusivity in this case
and that for an associated gradient drift problem that provides an interesting
constraint on previously conjectured results.Comment: 18 pages, Latex, DAMTP-96-8
Symmetry Relations for Trajectories of a Brownian Motor
A Brownian Motor is a nanoscale or molecular device that combines the effects
of thermal noise, spatial or temporal asymmetry, and directionless input energy
to drive directed motion. Because of the input energy, Brownian motors function
away from thermodynamic equilibrium and concepts such as linear response
theory, fluctuation dissipation relations, and detailed balance do not apply.
The {\em generalized} fluctuation-dissipation relation, however, states that
even under strongly thermodynamically non-equilibrium conditions the ratio of
the probability of a transition to the probability of the time-reverse of that
transition is the exponent of the change in the internal energy of the system
due to the transition. Here, we derive an extension of the generalized
fluctuation dissipation theorem for a Brownian motor for the ratio between the
probability for the motor to take a forward step and the probability to take a
backward step
Perturbation theory for the effective diffusion constant in a medium of random scatterer
We develop perturbation theory and physically motivated resummations of the
perturbation theory for the problem of a tracer particle diffusing in a random
media. The random media contains point scatterers of density uniformly
distributed through out the material. The tracer is a Langevin particle
subjected to the quenched random force generated by the scatterers. Via our
perturbative analysis we determine when the random potential can be
approximated by a Gaussian random potential. We also develop a self-similar
renormalisation group approach based on thinning out the scatterers, this
scheme is similar to that used with success for diffusion in Gaussian random
potentials and agrees with known exact results. To assess the accuracy of this
approximation scheme its predictions are confronted with results obtained by
numerical simulation.Comment: 22 pages, 6 figures, IOP (J. Phys. A. style
Solution of large scale nuclear structure problems by wave function factorization
Low-lying shell model states may be approximated accurately by a sum over
products of proton and neutron states. The optimal factors are determined by a
variational principle and result from the solution of rather low-dimensional
eigenvalue problems. Application of this method to sd-shell nuclei, pf-shell
nuclei, and to no-core shell model problems shows that very accurate
approximations to the exact solutions may be obtained. Their energies, quantum
numbers and overlaps with exact eigenstates converge exponentially fast as the
number of retained factors is increased.Comment: 12 pages, 12 figures (from 15 eps files) include
Overscreening in 1D lattice Coulomb gas model of ionic liquids
Overscreening in the charge distribution of ionic liquids at electrified
interfaces is shown to proceed from purely electrostatic and steric
interactions in an exactly soluble one dimensional lattice Coulomb gas model.
Being not a mean-field effect, our results suggest that even in higher
dimensional systems the overscreening could be accounted for by a more accurate
treatment of the basic lattice Coulomb gas model, that goes beyond the mean
field level of approximation, without any additional interactions.Comment: 4 pages 5 .eps figure
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