Clyde L. Scandrett

Scattering and active acoustic control from a submerged spherical shell

This paper is concerned with the scattering from a submerged (heavy fluid) bilaminate spherical shell composed of an outer layer of steel, and an inner layer of radially polarized piezoelectric material. The methodology used includes separation formulas for the stresses and displacements, which in turn are used (coupled with spherical harmonics) to reduce the governing equations to linear systems of ordinary differential equations. This technique uses the full equations of elasticity rather than any of the various thin-shell approximations in determining the axisymmetric scattering from a shell, normal modes of vibration for the shell, as well as voltages necessary for annihilation of a scattered pressure due to insonification of the shell by an incident plane wave.

Minority carrier lifetime variations associated with misfit dislocation networks in heteroepitaxial GaInP

Variations in minority carrier transport properties associated with networks of misfit dislocations have been measured using a unique optical technique in p-type GaInP grown on Ge. The diffusion length L of minority carriers has been measured with a spatial resolution of 0.4 µm in regions showing alternating light and dark luminescence bands. Periodic variations of ±4% from a mean diffusion length of 3.9 µm were measured and found to be anti-correlated to intensity fluctuations. A model based on the coupling between luminescence intensity and minority carrier lifetime allows for the extraction of spatial variation of both radiative and non-radiative lifetimes. For this high quality material, with relatively low concentrations of non-radiative recombination centers, the results indicate variations in radiative recombination lifetime associated with dopant fluctuations.

The propagation of time harmonic Rayleigh–Lamb waves in a bimaterial plate

The time harmonic elastodynamic response of two semi-infinite elastic plates of dissimilar material properties perfectly bonded along their lateral faces is studied. The wave field in either half-plate can be written as a superposition of the so-called Rayleigh–Lamb eigenmodes of an infinite plate. The interaction of a time harmonic incident wave with the interface results in reflected and transmitted fields that contain contributions from all of the real, imaginary, and complex eigenmodes of an infinite plate. Attention is focused on the distribution of energy among the various reflected and transmitted eigenmodes over a range of frequencies. The fundamental symmetric and the fundamental antisymmetric Lamb modes are each used as input excitations. Such excitations can be approximately realized in experiments. It is assumed that the solution of such a canonical problem will facilitate the solution of problems with complicated time-dependent sources.

Acoustic interactions in arrays of spherical elastic shells

The acoustical performance of a submerged linear array of spherical transducers is examined by combining the T‐matrix method of solving for multiple acoustic interactions among separate bodies with a model for transducers as thin spherical elastic shells. This approach solves the fully coupled problem of the response of the array to internal forcing. The results show that the assumptions giving rise to the Chebyshev criteria for optimal arrays of point sources appear to apply well even for large spheres at low frequencies. However, at frequencies near or above the lowest resonant frequency, the directional pattern may be degraded, depending on the material of the shells.

Assessment of a new radiation damping model for structural acoustic interactions

The equations that govern the scattering of acoustic waves from a baffled membrane strip couple the fluid pressure and the membrane displacement in an intimate way. The equations cannot be decoupled and consequently analytical solutions cannot be found. Approximate methods are often used to decouple the system. A new approximate method is presented, which can be classified as an ‘‘impedance approximation,’’ in that it relates the membrane displacement to a differential operator applied to the fluid pressure at the membrane surface. The differential operator arises from applying a radiation condition, which is strictly valid far away from the structure, to the pressure on the membrane’s surface. This new technique is compared to the plane‐wave approximation method and to a finite difference solution of the fully coupled system. It is found that the new approximation method improves upon the plane‐wave approximation method, particularly near the in vacuo eigenfrequencies of the membrane.

Three-dimensional transport imaging for the spatially resolved determination of carrier diffusion length in bulk materials.

A contact-free optical technique is developed to enable a spatially resolved measurement of minority carrier diffusion length and the associated mobility-lifetime (μτ) product in bulk semiconductor materials. A scanning electron microscope is used in combination with an internal optical microscope and imaging charge-coupled device (CCD) to image the bulk luminescence from minority carrier recombination associated with one-dimensional excess carrier generation. Using a Greens function to model steady-state minority carrier diffusion in a three-dimensional half space, non-linear least squares analysis is then applied to extract values of carrier diffusion length and surface recombination velocity. The approach enables measurement of spatial variations in the μτ product with a high degree of spatial resolution.

Comparison of several iterative techniques in the solution of symmetric banded equations on a two-pipe cyber 205

The effectiveness of several iterative techniques for solving matrix equations resulting from finite-difference approximations to self-adjoint parabolic and elliptic partial differential equations is investigated: the strongly implicit procedure (sip), the incomplete Cholesky conjugate-gradient method (iccg), the vectorized iccg (viccg), the modified iccg (miccg), the diagonally scaled conjugate-gradient method (dscg), the polynomial preconditioned conjugate-gradient method. (polcg), and the polynomial form of iccg (piccg). The comparison is made on a vector machine (two-pipe Cyber 205) where vectorization of the code is done primarily by the vector compiler available. It is found that of the methods studied, polcg and miccg appear to require the least amount of CPU time. An advantage of miccg over polcg is that it is less sensitive to increasing matrix size. Its disadvantages are that it requires an iteration parameter, has a greater setup time, and needs more storage than polcg.

Acoustic behavior of magnetorheological fluids in magnetic fields

Acoustic metamaterials are being considered for periodic structures where specific microscopic material properties can be tailored to alter macroscopic acoustic fields. One type of acoustic metamaterial being considered is an active fluid known as magnetorheological (MR) fluids. MR fluids contain magnetic particles dispersed within a host fluid where its viscoelastic behavior is controllable by varying the magnetic field intensity. A series of acoustic experiments has recently been conducted at the National High Magnetic Field Laboratory in Tallahassee, Florida. The acoustic sound speed of MR fluids was measured as functions of applied magnetic field strength, normal and orthogonal field orientations, and acoustic frequency. This presentation will discuss MR fluids, measurement methodology, and preliminary results. [Work supported by NAVSEA Division Newport ILIR.]

Scattering reduction of an acoustically hard cylinder covered with layered pentamode metamaterials.

Transformational acoustics offers the theoretical possibility of cloaking obstacles within fluids, provided metamaterials having continuously varying bulk moduli and densities can be found or constructed. Realistically, materials with the proper, continuously varying anisotropies do not presently exist. However, discretely layered cloaks having constant material parameters within each layer may be a viable alternative in practice. The present work considers a range of cloaks, from those comprised of fluid layers that are isotropic in bulk moduli with anisotropic density (inertial cloaks) to those having anisotropic bulk moduli and isotropic density (pentamode cloaks). In this paper an analytical solution is obtained for the case of plane wave scattering from a submerged rigid cylinder covered with a multilayered cylindrical cloak composed of discrete anisotropic fluid layers. An investigation of the parameter space defining such cloaks is undertaken with the goal of minimizing the far-field scattered pressure, using layer constituent anisotropic properties (density and bulk modulus) constrained to lie within reasonable ranges relative to those of water.

Acoustic scattering from layered pentamode materials.

While receiving less attention in the literature than in the field of electromagnetic scattering, theoretical efforts to define and create acoustic skins capable of reducing scattered energy from obstacles by way of mimicking coordinate transformations through use of meta‐materials have begun. The present work extends recent analysis of Norris by considering a variety of acoustic skins, from those comprised of fluid layers which are isotropic in bulk moduli with anisotropic density to those having anisotropic bulk moduli and isotropic density. In all but pure inertial types, fluid layers comprising the skins are pentamode materials governed by a special scalar acoustic equation for pseudo‐pressure derived by Norris. In most cases presented, material properties of the fluid/pentamode layers are based upon target values specified by continuously varying properties resulting from theoretical coordinate transformations geared to minimize scattered pressure limited by realistic goals. The present work analyzes...