Acoustic Behavior of Near Periodic Elastic Structures

1995 ◽  
Author(s):  
Douglas M. Photiadis
Keyword(s):  
Cross Section ◽  
Wave Packets ◽  
Acoustic Scattering ◽  
Direct Interaction ◽  
Bloch Wave ◽  
Acoustic Properties ◽  
Theoretical Development ◽  
Naval Research ◽  
Bragg Scattering ◽  
Elastic Structures

Abstract Near periodic arrays of discontinuities have been predicted to have a significant impact on the acoustic properties of elastic structures. The discontinuities in the elastic properties of the structure produce a characteristic signature in the acoustic scattering cross section of the structure via two distinct mechanisms; a direct interaction producing acoustic Bragg scattering, and an indirect interaction wherein the discontinuities fundamentally alter the free waves of the structure. The locally propagating states of the pseudo-periodic system are Floquet or Bloch wave packets and the locations of the highlights in the cross section may be determined simply from the Bloch wavenumber via a phase matching argument. Predicting the resulting scattering levels requires an understanding of the propagation of the Bloch wave packets in the finite, pseudo-periodic structure. In the case of a thin ribbed cylindrical shell or plate this scattering mechanism can arise from flexural waves, and recent experimental results obtained at Naval Research Laboratory have demonstrated the importance of both this mechanism and Bragg scattering on the acoustic far field over a broad frequency range. In this paper, these results and the underlying theoretical development will be discussed.

Analysis of Threshold-Induced Bias Inherent in Acoustic Scattering Cross-Section Estimates of Individual Fish

1975 ◽  
Vol 32 (12) ◽  
pp. 2547-2551 ◽  
Author(s):  
R. T. Weimer ◽  
J. E. Ehrenberg
Keyword(s):  
Cross Section ◽  
Stock Assessment ◽  
Acoustic Scattering ◽  
Operating Conditions ◽  
Scattering Cross Section ◽  
Fish Stock ◽  
Small Fish ◽  
Scattering Cross ◽  
Electronic Circuitry ◽  
Fish Stock Assessment

During acoustic fish stock assessment surveys, it is often desirable to measure the distribution of the acoustic scattering cross-section of single fish. One of the problems in such measurements is that a threshold in the electronic circuitry discriminates against small fish. This effect is analyzed in detail, and an expression is derived for the threshold-induced bias in the mean scattering cross-section estimate. Results are plotted for a typical set of operating conditions.

Acoustic Scattering Cross Sections of Smart Obstacles: A Case Study

2011 ◽  
Vol 10 (3) ◽  
pp. 672-694
Author(s):  
Lorella Fatone ◽  
Maria Cristina Recchioni ◽  
Francesco Zirilli
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Space Shuttle ◽  
Scattering Problem ◽  
Plane Waves ◽  
Acoustic Scattering ◽  
Scattering Cross Section ◽  
Scattering Cross ◽  
Scattering Cross Sections

AbstractAcoustic scattering cross sections of smart furtive obstacles are studied and discussed. A smart furtive obstacle is an obstacle that, when hit by an incoming field, avoids detection through the use of a pressure current acting on its boundary. A highly parallelizable algorithm for computing the acoustic scattering cross section of smart obstacles is developed. As a case study, this algorithm is applied to the (acoustic) scattering cross section of a “smart” (furtive) simplified version of the NASA space shuttle when hit by incoming time-harmonic plane waves, the wavelengths of which are small compared to the characteristic dimensions of the shuttle. The solution to this numerically challenging scattering problem requires the solution of systems of linear equations with many unknowns and equations. Due to the sparsity of these systems of equations, they can be stored and solved using affordable computing resources. A cross section analysis of the simplified NASA space shuttle highlights three findings: i) the smart furtive obstacle reduces the magnitude of its cross section compared to the cross section of a corresponding “passive” obstacle; ii) several wave propagation directions fail to satisfactorily respond to the smart strategy of the obstacle; iii) satisfactory furtive effects along all directions may only be obtained by using a pressure current of considerable magnitude. Numerical experiments and virtual reality applications can be found at the website: http://www.ceri.uniromal.it/ceri/zirilli/w7.

THE SPECTRUM OF NEUTRONS FROM THE IRRADIATION OF URANIUM BY 14-MEV NEUTRONS

1961 ◽  
Vol 39 (7) ◽  
pp. 957-966 ◽  
Author(s):  
R. L. Clarke
Keyword(s):  
Inelastic Scattering ◽  
Cross Section ◽  
Neutron Beam ◽  
Strong Evidence ◽  
Direct Interaction ◽  
Natural Uranium ◽  
Incident Neutron ◽  
Absolute Yield ◽  
Time Of Flight Method ◽  
Incident Neutron Beam

The spectrum and absolute yield of neutrons resulting from 14-Mev neutrons incident on natural uranium was observed at 90°± 16 to the incident neutron beam by a time-of-flight method. Further observations of the spectrum from 9 to 13.5 Mev were made for scattering angles at 30° and 60°. In addition to the expected neutrons resulting from evaporation and fission, strong evidence is seen for direct interaction inelastic scattering. The (n,n′) cross section is estimated to be 150 ± 70 millibarns.

scholarly journals Theory underpinning multislice simulations with plasmon energy losses

Microscopy ◽  
2020 ◽  
Vol 69 (3) ◽  
pp. 173-175
Author(s):  
B G Mendis
Keyword(s):  
Monte Carlo ◽  
Cross Section ◽  
Monte Carlo Methods ◽  
Bloch Wave ◽  
Incident Electron ◽  
Scattering Cross Section ◽  
Energy Losses ◽  
Plasmon Energy ◽  
Scattering Cross ◽  
Wave Nature

Abstract The theoretical conditions for small-angle inelastic scattering where the incident electron can effectively be treated as a particle moving in a uniform potential is examined. The motivation for this work is the recent development of a multislice method that combines plasmon energy losses with elastic scattering using Monte Carlo methods. Since plasmon excitation is delocalized, it was assumed that the Bloch wave nature of the incident electron in the crystal does not affect the scattering cross-section. It is shown here that for a delocalized excitation the mixed dynamic form factor term of the scattering cross-section is zero and the scattered intensities follow a Poisson distribution. These features are characteristic of particle-like scattering and validate the use of Monte Carlo methods to model plasmon losses in multislice simulations.

Simulation and Quantification of High-Resolution Z-Contrast Imaging of Semiconductor Interfaces

1989 ◽  
Vol 159 ◽  
Author(s):  
D. E. Jesson ◽  
S. J. Pennycook ◽  
M. F. Chisholm
Keyword(s):  
High Resolution ◽  
Cross Section ◽  
Image Interpretation ◽  
Bloch Wave ◽  
Small Computer ◽  
High Angle ◽  
Contrast Imaging ◽  
Resolution Function ◽  
Semiconductor Interfaces ◽  
Z Contrast

ABSTRACTIncoherent characteristics of Z-contrast STEM images are explained using a Bloch wave approach. To a good approximation, the image is given by the columnar high-angle cross-section multiplied by the s-state intensity at the projected atom sites, convoluted with an appropriate resolution function. Consequently, image interpretation can be performed intuitively and quantitative simulation can be implemented on a small computer. The feasibility of ‘column-by-column’ compositional mapping is discussed.

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