5,501 research outputs found
FDTD Simulation Techniques for Simulation of Very Large 2D and 3D Domains Applied to Radar Propagation over the Ocean
abstract: A domain decomposition method for analyzing very large FDTD domains, hundreds of thousands of wavelengths long, is demonstrated by application to the problem of radar scattering in the maritime environment. Success depends on the elimination of artificial scattering from the “sky” boundary and is ensured by an ultra-high-performance absorbing termination which eliminates this reflection at angles of incidence as shallow as 0.03 degrees off grazing. The two-dimensional (2D) problem is used to detail the features of the method. The results are cross-validated by comparison to a parabolic equation (PE) method and surface integral equation method on a 1.7km sea surface problem, and to a PE method on propagation through an inhomogeneous atmosphere in a 4km-long space, both at X-band. Additional comparisons are made against boundary integral equation and PE methods from the literature in a 3.6km space containing an inhomogeneous atmosphere above a flat sea at S-band. The applicability of the method to the three-dimensional (3D) problem is shown via comparison of a 2D solution to the 3D solution of a corridor of sea. As a technical proof of the scalability of the problem with computational power, a 5m-wide, 2m-tall, 1050m-long 3D corridor containing 321.8 billion FDTD cells has been simulated at X-band. A plane wave spectrum analysis of the (X-band) scattered fields produced by a 5m-wide, 225m-long realistic 3D sea surface, and the 2D analog surface obtained by extruding a 2D sea along the width of the corridor, reveals the existence of out-of-plane 3D phenomena missed by the traditional 2D analysis. The realistic sea introduces random strong flashes and nulls in addition to a significant amount of cross-polarized field. Spatial integration using a dispersion-corrected Green function is used to reconstruct the scattered fields outside of the computational FDTD space which would impinge on a 3D target at the end of the corridor. The proposed final approach is a hybrid method where 2D FDTD carries the signal for the first tens of kilometers and the last kilometer is analyzed in 3D.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201
Local orientations of fluctuating fluid interfaces
Thermal fluctuations cause the local normal vectors of fluid interfaces to
deviate from the vertical direction defined by the flat mean interface
position. This leads to a nonzero mean value of the corresponding polar tilt
angle which renders a characterization of the thermal state of an interface.
Based on the concept of an effective interface Hamiltonian we determine the
variances of the local interface position and of its lateral derivatives. This
leads to the probability distribution functions for the metric of the interface
and for the tilt angle which allows us to calculate its mean value and its mean
square deviation. We compare the temperature dependences of these quantities as
predicted by the simple capillary wave model, by an improved phenomenological
model, and by the microscopic effective interface Hamiltonian derived from
density functional theory. The mean tilt angle discriminates clearly between
these theoretical approaches and emphasizes the importance of the variation of
the surface tension at small wave lengths. Also the tilt angle two-point
correlation function is determined which renders an additional structural
characterization of interfacial fluctuations. Various experimental accesses to
measure the local orientational fluctuations are discussed.Comment: 29 pages, 12 figure
Effect of disorder geometry on the critical force in disordered elastic systems
We address the effect of disorder geometry on the critical force in
disordered elastic systems. We focus on the model system of a long-range
elastic line driven in a random landscape. In the collective pinning regime, we
compute the critical force perturbatively. Not only our expression for the
critical force confirms previous results on its scaling with respect to the
microscopic disorder parameters, it also provides its precise dependence on the
disorder geometry (represented by the disorder two-point correlation function).
Our results are successfully compared to the results of numerical simulations
for random field and random bond disorders.Comment: 18 pages, 7 figure
Average stresses and force fluctuations in non-cohesive granular materials
A lattice model is presented for investigating the fluctuations in static
granular materials under gravitationally induced stress. The model is similar
in spirit to the scalar q-model of Coppersmith et al., but ensures balance of
all components of forces and torques at each site. The geometric randomness in
real granular materials is modeled by choosing random variables at each site,
consistent with the assumption of cohesionless grains. Configurations of the
model can be generated rapidly, allowing the statistical study of relatively
large systems. For a 2D system with rough walls, the model generates
configurations consistent with continuum theories for the average stresses
(unlike the q-model) without requiring the assumption of a constitutive
relation. For a 2D system with periodic boundary conditions, the model
generates single-grain force distributions similar to those obtained from the
q-model with a singular distribution of q's.Comment: 18 pages, 10 figures. Uses aps,epsfig,graphicx,floats,revte
The Tribology of Sliding Elastic Media
The tribology of a sliding elastic continuum in contact with a disordered
substrate is investigated analytically and numerically via a bead-spring model.
The deterministic dynamics of this system exhibits a depinning transition at a
finite driving force, with complex spatial-temporal dynamics including
stick-slip events of all sizes. These behaviors can be understood completely by
mapping the system to the well-known problem of a directed-path in {\em
higher-dimensional } random media.Comment: Uuencode file: 4 pages, small changes in the previous versio
Towards a statistical theory of solid dry friction
Wearless dry friction of an elastic block of weight N, driven by an external
force F over a rigid substrate, is investigated. The slider and substrate
surfaces are both microscopically rough, interacting via a repulsive potential
that depends on the local overlap. The model reproduces Amontons's laws which
state that the friction force is proportional to the normal loading force N and
independent of the nominal surface area. In this model, the dynamic friction
force decays for large velocities and approaches a finite static friction for
small velocities if the surface profiles are self-affine on small length
scales.Comment: Latex, 10 pages. Jounal reference adde
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