12,422 research outputs found
Conductances between confined rough walls
Two- and three-dimensional creeping flows and diffusion transport through constricted and possibly
rough surfaces are studied. Asymptotic expansions of conductances are derived as functions of the
constriction local geometry. The validity range of the proposed theoretical approximations is
explored through a comparison either with available exact results for specific two-dimensional
aperture fields or with direct numerical computations for general three-dimensional geometries. The
large validity range of the analytical expressions proposed for the hydraulic conductivity (and to a
lesser extent for the electrical conductivity) opens up interesting perspectives for the simulation of
flows in highly complicated geometries with a large number of constrictions
Basic studies in microwave remote sensing
Scattering models were developed in support of microwave remote sensing of earth terrains with particular emphasis on model applications to airborne Synthetic Aperture Radar measurements of forest. Practically useful surface scattering models based on a solution of a pair of integral equations including multiple scattering effects were developed. Comparisons of these models with controlled scattering measurements from statistically known random surfaces indicate that they are valid over a wide range of frequencies. Scattering models treating a forest environment as a two and three layered media were also developed. Extensive testing and comparisons were carried out with the two layered model. Further studies with the three layered model are being carried out. A volume scattering model valid for dense media such as a snow layer was also developed that shows the appropriate trend dependence with the volume fraction of scatterers
Surfaces roughness effects on the transmission of Gaussian beams by anisotropic parallel plates
Influence of the plate surfaces roughness in precise ellipsometry experiments
is studied. The realistic case of a Gaussian laser beam crossing a uniaxial
platelet is considered. Expression for the transmittance is determined using
the first order perturbation theory. In this frame, it is shown that
interference takes place between the specular transmitted beam and the
scattered field. This effect is due to the angular distribution of the Gaussian
beam and is of first order in the roughness over wavelength ratio. As an
application, a numerical simulation of the effects of quartz roughness surfaces
at normal incidence is provided. The interference term is found to be strongly
connected to the random nature of the surface roughness.Comment: 18 pages, Journal of Physics D: Applied Physics, volume 36, issue 21,
pages 2697 - 270
Trapping and displacement of liquid collars and plugs in rough-walled tubes
A liquid film wetting the interior of a long circular cylinder redistributes
under the action of surface tension to form annular collars or occlusive plugs.
These equilibrium structures are invariant under axial translation within a
perfectly smooth uniform tube and therefore can be displaced axially by very
weak external forcing. We consider how this degeneracy is disrupted when the
tube wall is rough, and determine threshold conditions under which collars or
plugs resist displacement under forcing. Wall roughness is modelled as a
non-axisymmetric Gaussian random field of prescribed correlation length and
small variance, mimicking some of the geometric irregularities inherent in
applications such as lung airways. The thin film coating this surface is
modelled using lubrication theory. When the roughness is weak, we show how the
locations of equilibrium collars and plugs can be identified in terms of the
azimuthally averaged tube radius; we derive conditions specifying equilibrium
collar locations under an externally imposed shear flow, and plug locations
under an imposed pressure gradient. We use these results to determine the
probability of external forcing being sufficient to displace a collar or plug
from a rough-walled tube, when the tube roughness is defined only in
statistical terms
On the use of the finite element method for the modeling of acoustic scattering from one-dimensional rough fluid-poroelastic interfaces
textA poroelastic finite element formulation originally derived for modeling porous absorbing material in air is adapted to the problem of acoustic scattering from a poroelastic seafloor with a one-dimensional randomly rough interface. The developed formulation is verified through calculation of the plane wave reflection coefficient for the case of a flat surface and comparison with the well known analytical solution. The scattering strengths are then obtained for two different sets of material properties and roughness parameters using a Monte Carlo approach. These numerical results are compared with those given by three analytic scattering models---perturbation theory, the Kirchhoff approximation, and the small-slope approximation---and from those calculated using two finite element formulations where the sediment is modeled as an acoustic fluid.Mechanical Engineerin
Supported Membranes on Chemically Structured and Rough Surfaces
We present a general linear response description of membrane adhesion at
rough or chemically structured surfaces. Our method accounts for non-local Van
der Waals effects and contains the more approximate (and local) Deryagin
approach in a simple limit. Specializing to supported membranes we consider the
effects of substrate structure on the membrane adhesion energy and
configuration. Adhesion is usually less favorable for rough substrates and the
membrane shape tends to follow that of the surface contours. Chemical
patterning, however, favors adhesion with the membrane configuration being out
of phase with the surface structure. Finally, considering a surface indented
with `V'-shaped trenches, we show that our approach is in fair agreement with
an exact numerical solution.Comment: 21 pages, 7 ps figures, submitted to PR
Correlation functions near Modulated and Rough Surfaces
In a system with long-ranged correlations, the behavior of correlation
functions is sensitive to the presence of a boundary. We show that surface
deformations strongly modify this behavior as compared to a flat surface. The
modified near surface correlations can be measured by scattering probes. To
determine these correlations, we develop a perturbative calculation in the
deformations in height from a flat surface. Detailed results are given for a
regularly patterned surface, as well as for a self-affinely rough surface with
roughness exponent . By combining this perturbative calculation in
height deformations with the field-theoretic renormalization group approach, we
also estimate the values of critical exponents governing the behavior of the
decay of correlation functions near a self-affinely rough surface. We find that
for the interacting theory, a large enough can lead to novel surface
critical behavior. We also provide scaling relations between roughness induced
critical exponents for thermodynamic surface quantities.Comment: 31 pages, 2 figure
Modelling scattering of electromagnetic waves in layered media: An up-to-date perspective
This paper addresses the subject of electromagnetic wave scattering in layered media, thus covering the recent progress achieved with different approaches. Existing theories and models are analyzed, classified, and summarized on the basis of their characteristics. Emphasis is placed on both theoretical and practical application. Finally, patterns and trends in the current literature are identified and critically discussed
The persistence of logarithmic solutions in turbulent boundary layer systems
The present work studies the prevalence of logarithmic solutions in the near wall region of turbulent boundary layers. Local solutions for flows subject to such diverse effects as compressibility, wall transpiration, heat transfer, roughness, separation, shock waves, unsteadiness, non-Newtonian fluids or a combination of these factors are discussed. The work also analyzes eleven different propositions by several authors for the near wall description of the mean velocity profile for the incompressible zero-pressure-gradient turbulent boundary layer. The asymptotic structure of the flow is discussed from the point of view of double limit processes. Cases of interest include attached and separated flows for the velocity and temperature fields.Indisponível
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