Showing papers by "Naval Surface Warfare Center published in 1985"

Total-energy local-spin-density approach to structural and electronic properties of ferromagnetic iron.

Abstract: Obtention des proprietes structurales du fer bcc a partir de calculs de structure electronique d'energie totale comme fonction de la constante de reseau et grâce a l'utilisation de la methode d'onde plane augmentee, polarisee par un potentiel entier et a l'approximation de densite de spin local. Presentation des structures de bandes electroniques, de la densite des etats et de la surface de Fermi a la constante de reseau d'equilibre calculee, comparaison aux donnees de de Haas-van Alphen

Sonar cross sections of bodies partially insonified by finite sound beams

Abstract: An object that is partially insonified by a collimated sound beam may have a scattering cross section sometimes much larger than when the object is totally covered by the incident beam. We quantitatively study this partial insonification problem here, under the classical method of physical optics. The importance of this study stems from the fact that partial coverage of the target by the beam is the situation most likely to occur in many cases of practical importance. We consider several basic target shapes partially insonified by finite beams. These shapes include the spherical, the infinite and finite cylinder, the flat plate, and the capped sphere. High-frequency approximations of the resulting integrals, obtained by means of the saddle-point method, show the relative importance of the scattering centers located at the beam's specular reflection points, or at the edges of the spots that the beam shines on the scatterers. The physical-optics method is extended to obtain formulas for the bistatic cross sections of partially insonified objects. The results are numerically evaluated and graphically displayed in many pertinent instances and compared to the predictions of approaches, such as the Fresnel-zones method and the Geometrical Theory of Diffraction (GTD). The predictions of the physical-optics method have all the advantages and deficiencies of this method and, with very minor modifications, hold equally well for the partial illumination of objects by beams of electromagnetic radiation.

Deconvolution of Impulse Response from Time-Limited Input and Output: Theory and Experiment

Abstract: Since it is impossible to generate and propagate an impulse, often a system is excited by a narrow time-domain pulse. The output is recorded and then a numerical deconvolution is often done to extract the impulse response of the object. Classically, the fast Fourier transform (FFT) technique has been applied with much success to the above deconvolution problem. However, when the signal-to-noise ratio becomes small, sometimes one encounters instability with the FFT approach. In this paper, the method of conjugate gradient is applied to the deconvolution problem. The problem is solved entirely in the time domain. The method converges for any initial guess in a finite number of steps. Also, for the application of the conjugate gradient method, the time samples need not be uniform, like FFT. Since, in this case, one is solving the operator equation directly, by passing the autocorrelation matrix computation, storage required is 5N as opposed to N2. Computed impulse response utilizing this technique has been presented for measured incident and scattered fields.

Characterization of the corrosion behavior of zinc-aluminum thermal spray coatings

Abstract: Evaluation du comportement a la corrosion d'un revetement de zinc-15% en poids aluminium et d'un revetement de pseudo-alliage zinc-aluminium de meme composition, dans un environnement marin

Power-Handling Capabilities of Circular Dielectric Waveguide at Millimeter Wavelengths

Abstract: The power-handling capabilities of a circular dielectric waveguide propagating the single HE/sub 11/ mode are discussed. Simple calculations illustrate the limits imposed by dielectric heating and dielectric breakdown. Examples are presented for polystyrene and polytetrafluoroethylene (PTFE) guides at 70 GHz. The results suggest that maximum power levels for circular dielectric waveguide in the millimeter-wave specturm will be of the orders of 10 to 100 W and that dielectric heating is the limiting phenomenon.