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Current industry accepted methods for predicting structural blast damage to pre-engineered metal buildings (PEMBs) subject to vapor cloud explosions are quick and cost-effective to perform. However, they also tend to be overly conservative; while costs may be saved on analysis, the results may prompt unnecessary mitigation. This paper investigates the effectiveness of using more advanced computational techniques with increasing levels of refinement and sophistication to produce more realistic estimates of structural damage and corresponding human injury. The goal is to reduce the amount of over-conservatism in results and avoid the degree of costly mitigation measures that may be unnecessary. In this study, a typical PEMB is selected and assessed for selected loads using a variety of approaches of increasing analytical sophistication. The extent of building damage is determined by each method and the results are compared to demonstrate the benefit of each analytical method. The use of refined analytical methods is shown to estimate significantly less damage in the PEMB for the loads considered.

Vapor cloud explosions (VCEs) can cause significant damage to nearby buildings, facilities and infrastructure with potential loss of life and significant business interruption, so the accuracy of predicting blast loads on facility buildings is critical in estimating these losses. Closely spaced buildings and process equipment outside of the congested region of a VCE provide a complicated flow field for an expanding blast wave. Their presence can channel and shield the blast, resulting in significant effects on the blast load magnitude and waveform shape. Currently, the most common way to estimate applied blast pressures resulting from VCE's is to use simplified methods that account for the total energy from the stoichiometric portion of the vapor cloud, fuel reactivity, and level of congestion and confinement, such as the TNO Multi-energy, equivalent TNT, CAM, and BST methods. These simplified tools assume an unobstructed line-of-site condition, which can overestimate and/or underestimate blast loads. This paper illustrates the use of a fast-running Computational Fluid Dynamics (CFD) approach that can account for channeling and shielding effects without having to use a turbulent combustion model. This approach provides a convenient tool for designers and process safety planners to more accurately quantify the hazard from postulated VCE hazards that include site-specific channeling and shielding effects. The accuracy of the approach is demonstrated via comparisons of CFD simulations to experimentally measured waveforms. Computed pressure and impulse are also compared to the BST predictions for unobstructed and obstructed sites.

Non-linear, explicit, finite element models were generated to predict the response of aircraft impacting into concrete runways and soil surfaces or reinforced concrete, steel lined shelters. Previously, simplified methods were used to represent the forces resulting from an aircraft crash. Models of C-130 and C-141 aircraft were developed for a Defence Threat Reduction Agency sponsored safety assessment. The purpose of the calculations was to senerate the internal aircraft and structural environments for the safety assessment as a result of the aircraft impacts into shelters and onto soil and runway surfaces. “Riera-type” loadings were also used to determine the general capacity and failure modes of the impacted structures. Finite element C-130 and C-141 models were then used to impact the shelters with the same aircraft weight and velocity. Three explicit calculations were run to compare the shelter responses. One case did not fail the shelter and two cases - one C-I30 and the C-141 did. Details ...

This talk examines an industrial, air-coupled tonpilz transducer used for acoustic ranging. The study is prompted by a situation common to long-standing industrial products: passive/active material supplies decline or disappear and replacements must be integrated into old designs. The conventional approach relies on 1D analyses and experimental prototypes. Today, new material integration is facilitated by 2D and 3D time-domain finite element models. These are used here to investigate overall modal characteristics, foam matching layer response, and device mounting issues.

We describe 2D/3D model studies of resonance and radiation characteristics of diced matching layers for ultrasound transducers. Calculations are done with PZFlex, a time-domain, finite element, electromechanical code. Continuous, thin film, quarter-wave matching layers have, of course, been used routinely at optical interfaces for most of this century. A similar approach is often vital to achieving the acoustic performance required of ultrasound imaging transducers. However, the ultrasound problem is complicated by lateral propagation in the layer and crosstalk between transducer elements. This necessitates dicing the continuous layer into discrete resonators on the piezoelectric element(s), whence, crosstalk is minimized, but sometimes at the expense of anomalous local modes and compromised radiation patterns. To better understand multi-dimensional diced matching layer dynamics, a single, solid piezoceramic element and a multi-element composite are modeled. We examine beam pressure and mode shapes and include comparisons with experimental composite data and a coupled-mode design curve.

The design, construction, performance, and comparison to FEM simulation of a broadband 375 kHz, 100 element, composite based, 115 mm radius curved receive array is described. This array was designed for use in a forward-looking, mine-hunting application with 60, 1.5/spl deg/ beams scanning a 900 sector. A 150/spl deg/ sector can be scanned with beamwidths widening beyond the center 90/spl deg/. The bandwidth was desired to be 250 kHz, centered at 375 kHz. This array was demonstrated using an imaging system constructed by UDI-Fugro as part of a TTCP collaboration. The array was designed by Ultrex Corporation supported by FEM modeling at Weidlinger Associates and the University of Strathclyde. The array was constructed by Materials Systems Inc. (MSI) using their proprietary injection-molded composite technology and was tested by UDI-Fugro. Materials characterization was conducted at the Royal Military College of Canada. The details of the final array design are described including the final composite microstructure, choice of composite filler, matching layer material and dicing pattern, and kerf filler. Array parameters achieved include 330 kHz center frequency, 245 kHz bandwidth, element acceptance angle from 62/spl deg/ at 250 kHz to 48/spl deg/ at 500 kHz, and sensitivity of -193 dB re 1V/uPa. FEM simulations showed excellent correspondence in predicting element electrical impedance.

Large-scale simulations of ultrasonic waves in heterogeneous tissue models are useful in biomedical R&D for imaging and therapeutics. The scale of bioacoustic models is hundreds of wavelengths. Typical 2D wave solvers are not practical at this scale, and 3D is out of the question, because of numerical errors and/or computer limits. To achieve much higher performance we use the periodic pseudospectral (PS) method, where spatial derivatives are calculated from FFTs over Cartesian grids. With a 4th order explicit time integrator, the PS method yields the necessary accuracy and efficiency. However, the domain must be periodic. We show how to circumvent this intrinsic limitation with Berenger's perfectly matched layer (PML) on the boundaries. High accuracy, computational efficiency, and parallelism are demonstrated and a large-scale bioacoustic model is calculated. Generalizations of the method are described, including attenuation and nonlinearity.

Finite element modeling is being adopted in the design of ultrasonic transducers and imaging arrays. Impetus is accelerated product design cycles and the need to push the technology. Existing designs are being optimized and new concepts are being explored. This recent acceptance follows the convergence of improvements on many fronts: necessary computer resources are more accessible, lean, specialized algorithms replacing general-purpose approaches, and better material characterization The basics of the finite element method (REM) for the coupled piezoelectric-acoustic problem are reviewed. We contrast different FEM formulations and discuss the implications of each: time-domain versus frequency domain, implicit versus explicit algorithms, linear versus nonlinear. Beyond discussions of the theoretical underpinnings of numerical methods, the paper also examines other modeling ingredients such as discretization, material attenuation, boundary conditions, farfield extrapolation, and electric circuits. Particular emphasis is placed on material characterization, and this is discussed through an actual "modelbuild-test" validation sequence, undertaken recently. Some applications are also discussed. Keywords: Arrays, Attenuation, Finite Element Method, Imaging, Piezoelectric, Transducer, Ultrasound