307 research outputs found
Transformation of a shoaling undular bore
We consider the propagation of a shallow-water undular bore over a gentle monotonic bottom slope connecting two regions of constant depth, in the framework of the variable-coefficient Korteweg-de Vries equation. We show that, when the undular bore advances in the direction of decreasing depth, its interaction with the slowly varying topography results, apart from an adiabatic deformation of the bore itself, in the generation of a sequence of isolated solitons - an expanding large-amplitude modulated solitary wavetrain propagating ahead of the bore. Using nonlinear modulation theory we construct an asymptotic solution describing the formation and evolution of this solitary wavetrain. Our analytical solution is supported by direct numerical simulations. The presented analysis can be extended to other systems describing the propagation of undular bores (dispersive shock waves) in weakly non-uniform environments
Simulation of Internal Undular Bores Propagating over a Slowly Varying Region
Internal undular bores have been observed in many parts of the world. Studies have shown that many marine structures face danger and risk of destruction caused by internal undular bores due to the amount of energy it carries. This paper looks at the transformation of internal undular bore in two-layer fluid flow under the influence of variable topography. Thus, the surface of the bottom is considered to be slowly varying. The appropriate mathematical model is the variable-coefficient extended Korteweg-de Vries equation. We are particularly interested in looking at the transformation of KdV-type and table-top undular bore over the variable topography region. The governing equation is solved numerically using the method of lines, where the spatial derivatives are first discretised using finite difference approximation so that the partial differential equation becomes a system of ordinary differential equations which is then solved by 4th order Runge-Kutta method. Our numerical results show that the evolution of internal undular bore over different types of varying depths regions leads to a number of adiabatic and non-adiabatic effects. When the depth decreases slowly, a solitary wavetrain is observed at the front of the transformed internal undular bore. On the other hand, when the depth increases slowly, we observe the generation of step-like wave and weakly nonlinear trailing wavetrain, the occurrence of multi-phase behaviour, the generation of transformed undular bore of negative polarity and diminishing transformed undular bore depending on the nature of the topography after the variable topography
A chamber experiment for the feasibility study of an artificial plasma reflector for OTH radar applications
The feasibility of using two intersecting beams for plasma generation in the upper atmosphere as an over-the-horizon radar reflector was investigated. A cube was filled with dry air to a pressure corresponding to the simulated altitude, and two components of a split microwave beam were fed into the cube at right angles. Plasma layers were generated where the two beams intersected. Three critical issues were addressed: (1) reflectivity of the generated plasma layers; (2) propagation of high power microwave pulses; and (3) lifetime of the plasma
Optical Propagation and Communication
Contains summary of research and reports on four research projects.National Science Foundation (Grant ECS81-20637)U.S. Navy - Office of Naval Research (Contract N00014-81-K-0662)Maryland Procurement Office (Contract MDA904-84-C-6037)U.S. Army Research Office - Durham (Contract DAAG29-80-K-0022)U.S. Army Research Office - Durham (Contract DAAG29-84-K-0095)U.S. Navy - Office of Naval Research (Contract N00014-80-C-0941
Optical Propagation and Communication
Contains research summary and reports on four research projects.National Science Foundation (Grant ECS81-20637)National Science Foundation (Grant ECS85-09143)Maryland Procurement Office (Contract MDA904-84-C-6037)National Science Foundation (Grant ECS84-15580)U.S. Army Research Office - Durham (Contract DAAG29-84-K-0095)U.S. Navy - Office of Naval Research (Contract NO0014-80-C-0941
Local structure evolution in polycrystalline ZnMgO () studied by Raman and by synchrotron x-ray pair distribution analysis
The local structures of ZnMgO alloys have been studied by Raman
spectroscopy and by synchrotron x-ray pair distribution function (PDF)
analysis. Within the solid solution range () of
ZnMgO, the wurtzite framework is maintained with Mg homogeneously
distributed throughout the wurtzite lattice. The Raman line
of ZnMgO displays systematic changes in response to the evolution
of the crystal lattice upon the Mg-substitution. The red-shift and broadening
of the mode are explained by the expansion of hexagonal
-dimensions, and compositional disorder of Zn/Mg, respectively. Synchrotron
x-ray PDF analyses of ZnMgO reveal that the Mg atoms have a
slightly reduced wurtzite parameter and more regular tetrahedral bond
distances than the Zn atoms. For both Zn and Mg, the internal tetrahedral
geometries are independent of the alloy composition.Comment: 10 pages, 12 figures RevTe
Optical Propagation and Communication
Contains research objectives and reports on four research projects.National Science Foundation (Grant ECS 85-09143)Maryland Procurement Office (Contract MDA 904-84-C-6037)National Science Foundation (Grant ECS 84-15580)U.S. Army Research Office - Durham (Contract DAAG29-84-K-0095)U.S. Navy - Office of Naval Research (Contract N00014-80-C-0941
Optical Propagation and Communication
Contains research summary and reports on four research projects.Maryland Procurement Office (Contract MDA 904-87-C-4044)National Science Foundation (Grant ECS 87-18970)U.S. Army Research Office (Contract DAAL03-87-K-0117)U.S. Navy - Office of Naval Research (Contract N0001 4-80-C-0941)U.S. Air Force - Office of Scientific Research (Contract F49620-87-C-0043
Optical Propagation and Communication
Contains research objectives and reports on six research projects.National Science Foundation (Grant ECS 85-09143)Maryland Procurement Office (Contract MDA 904-84-C-6037)Maryland Procurement Office (Contract MDA 904-87-C-4044)National Science Foundation (Grant ECS 84-15580)National Science Foundation (Grant INT-86-14329)U.S. Navy - Office of Naval Research (Contract N00014-87-G-0198)U.S. Army Research Office - Durham (Contract DAAG29-84-K-0095)U.S. Army Research Office - Durham (Contract DAALO3-87-K-0117)U.S. Navy - Office of Naval Research (Contract N00014-80-C-0941_U.S. Air Force - Office of Scientific Research (Contract F49620-87-C-0043
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