Michael A. Sprague

Spectral elements and field separation for an acoustic fluid subject to cavitation

Spectral elements based on Legendre polynomials are used to improve an existing finite-element method for simulating a highly nonlinear field phenomenon: fluid cavitation in an underwater-shock environment. Further improvement is provided by separation of the total field into its equilibrium, incident, and scattered components. These enhancements promise to make the finite-element method suitable for practical, three-dimensional engineering computations.

Response of empty and fluid-filled, submerged spherical shells to plane and spherical, step-exponential acoustic waves

The title problem is solved through extension of a method previously formulated for plane step-wave excitation, which employs generalized Fourier series augmented by partial closure of those series at early time. The extension encompasses both plane and spherical incident waves with step-exponential pressure profiles. The effects of incident-wave curvature and profile decay rate on response behavior are examined. A method previously developed for assessing the discrepancy between calculated and measured response histories is employed to evaluate the convergence of the truncated series solutions. Also studied is the performance of doubly-asymptotic approximations. Finally, the efficacy of modified Cesaro summation for improving the convergence of series solutions is examined. The documented computer program that produced the numerical results appearing in this paper, SPHSHK/MODSUM, may be down-loaded from the Web site http://saviac.xservices.com.

Computational Treatments of Cavitation Effects in Near-Free-Surface Underwater Shock Analysis

Fluid cavitation constitutes an expensive computational nuisance in underwater-shock response calculations for structures at or just below the free surface. In order to avoid the use of a large array of cavitating acoustic finite elements (CAFE), various wet-surface approximations have been proposed. This paper examines the performance of two such approximations by comparing results produced by them for 1-D canonical problems with corresponding results produced by more rigorous CAFE computations. It is found that the fundamental limitation of wet-surface approximations is their inability to capture fluid-accretion effects. As an alternative, truncated CAFE fluid meshes with plane-wave radiation boundaries are shown to give good results. In fact, a single layer of CAFE is found to be comparable in accuracy to the better of the wet-surface approximations. The paper concludes with an examination of variations in CAFE modeling.

Photomask in-plane distortion induced during e-beam patterning

As feature sizes decrease and the demand for throughput increases, the semiconductor industry must concentrate on pattern positioning accuracy and process efficiency. Thermomechanical distortions induced in the photomask during fabrication may act to constrain the desired range of operating conditions to meet the manufacturing requirements for pattern placement accuracy and throughput. 3D finite element heat transfer and structural models have been developed to determine the global in-plane distortions induced in the photomask during e-beam patterning. Results obtained from these models show that the thermal-induced distortions, caused by global heating, are significant. Whereas, distortions due to the mechanical loading, caused by resist in situ stress relief, are minimal and can be neglected.

X-ray mask distortions during e-beam patterning

Abstract The pattern placement accuracy of an x-ray lithography mask is affected by thermomechanical distortion induced during the electron beam patterning process. Three-dimensional finite element models have been developed to simulate the response of the membrane and resist stack and identify the in-plane distortions due to the e-beam writing. Thermal displacements (due to the e-beam heating) and mechanical displacements (due to the resist in situ stress relief) were calculated for the ARPA-NIST standad x-ray mask.

A residual-potential boundary for time-dependent, infinite-domain problems in computational acoustics

A theoretically exact computational boundary is introduced, that is based on modal residual potentials for the spherical geometry. The boundary produces a set of first-order, uncoupled ordinary differential equations for nodal boundary responses, and a set of uncoupled time-stepping equations for modal boundary responses. The two sets are coupled through nodal-modal transformation based on the orthogonal surface functions for the spherical boundary. Numerical results generated with the boundary are presented for a step-wave-excited, elastic, spherical shell submerged in an infinite acoustic medium. Extension of the method to other separable geometries for partial differential equations defined in unbounded domains is mentioned.