Young S. Shin

Modeling and Simulation of Underwater Shock Problems Using a Coupled Lagrangian—Eulerian Analysis Approach

The application of coupled Lagrangian—Eulerian analysis to various types of underwater shock problems was investigated, with the verification and validation of this analysis approach in mind. Analyses were conducted for a simple TNT detonation problem and for the classical problems of an infinite cylindrical shell and a spherical shell loaded by a plane acoustic step wave. The advantages, disadvantages, and limitations of this approach are identified and discussed.

Pseudo Wigner-Ville Time-Frequency Distribution and Its Application to Machinery Condition Monitoring

Machinery operating in a nonstationary mode generates a signature that at each instant of time has a distinct frequency. A Time-frequency domain representation is needed to characterize such a signature. Pseudo Wigner–Ville distribution is ideally suited for portraying a nonstationary signal in the time-frequency domain and is carried out by adapting the fast Fourier transform algorithm. The important parameters affecting the pseudo Wigner–Ville distribution are discussed and sensitivity analyses are also performed. Practical examples of an actual transient signal are used to illustrate its dynamic features jointly in time and frequency.

Surface ship shock modeling and simulation: two-dimensional analysis

The modeling and simulation of the response of a surface ship system to underwater explosion requires an understanding of many different subject areas. These include the process of underwater explosion events, shock wave propagation, explosion gas bubble behavior and bubble-pulse loading, bulk and local cavitation, free surface effect, fluid-structure interaction, and structural dynamics. This paper investigates the effects of fluid-structure interaction and cavitation on the response of a surface ship using USA-NASTRAN-CFA code. First, the one-dimensional Bleich-Sandler model is used to validate the approach, and second, the underwater shock response of a two-dimensional mid-section model of a surface ship is predicted with a surrounding fluid model using a constitutive equation of a bilinear fluid which does not allow transmission of negative pressures.

Damage response of submerged imperfect cylindrical structures to underwater explosion

The effect of the initial geometric imperfections to the damage response of submerged structures is investigated. The type of the submerged structure investigated is the ring-stiffened long circular cylinders submerged in the fluid. The strain hardening mild steel is used in the analysis. The type of the loading is the underwater shock induced by underwater explosions. The modal imperfection concept has been used to simulate the initial geometric imperfections. The numerical analyses were performed to look into the details of damage response of ring-stiffened cylindrical shells. The following models were considered: (i) three-dimensional infinite ring-stiffened cylinder model, and (ii) three-dimensional finite ring-stiffened cylinder model. A finite element hydrocode was used to numerically predict model responses.

Cancellation techniques in underwater scattering of acoustic signals

Advanced materials made of a combination of viscoelastic coating and piezoelectric substances are fast emerging as important acoustics materials that can be used to reduce or eliminate scattered acoustic signals of submerged structures. In this paper, we considered the underlying principles that govern the acoustic performance of viscoelastic and piezoelectric materials. Analytical treatments such as the invariant embedding techniques, potential method, Floquet theory and asymptotics approximation, are employed to derive the mathematical model for predicting the acoustics performance of viscoelastic and piezoelectric materials. Numerical implementations in finite difference methods coupled with boundary integral formulation, and commercial finite elements code, such as ANSYS, are demonstrated for some practical configurations. Results for a few representative canonical examples of the problem, which include two-dimensional acoustic scattering from a fluid-loaded plate embedded with viscoelastic material or piezoelectric elements served as useful benchmarks for future works in this direction.

Explosion Gas Bubbles Near Simple Boundaries

Finite element analyses of explosion gas bubbles show that including the compressibility of the surrounding media leads to appreciable differences in key areas of the bubble’s behavior. In order to more fully understand the behavior of bubbles created by detonations near simple boundaries, analyses incorporating fluid compressibility were conducted at various stand-off distances from simple rigid and constant pressure surface boundaries. The results from these analyses serve to characterize the behavior of the bubbles for the charge type, charge weight, and hydrostatic pressure used in the analyses.

Optical beam jitter control

For several future imaging and communications spacecraft, a challenging area of technology development is the fine acquisition, tracking, and pointing (ATP) control of the spacecraft and its payload. For example, some spacecraft with large aperture(s) in the range of 10~30 m diameter requires a few arc-seconds accuracy, 10~15 nano-radians jitter, and a fast slewing rate to acquire the target. Furthermore these stringent requirements are at risk of great structure and control interactions. This paper we will focus on the control of optical beam jitter. A Laser Jitter Control (LJC) testbed has been constructed to test jitter algorithms. The testbed consists of two fast steering mirrors (FSM), three position sensing modules (PSM), one diode laser, and several beam splitters and mirrors, all on an isolated Newport optical bench. Jitter is injected with one FSM and the other FSM is used to control it. The jitter spectrum, representing the on-orbit spacecraft and beam jitter environment, contains not only narrow band noise due to rotating devices such as gyroscopes and reaction wheels but also broadband noise. The performance of a Wiener Filter-adaptive algorithm with ideal reference signal is established as the baseline for comparison of adaptive control methods in suppressing both broadband and narrowband disturbances. Specifically, the Least Mean Squares (LMS) approach and the Gradient Adaptive Lattice (GAL) approach are investigated during these experiments.

Vibration response of constrained viscoelastically damped plates: Analysis and experiments

Abstract Direct frequency response analyses of the constrained viscoelastically damped flat plates were performed with modifications to account for the frequency-dependent properties of viscoelastic materials (i.e., loss factors and shear modulus). The modal strain energy method was also applied to each plate to compute the same type of responses. Comparisons of the results based on the direct frequency response analysis to the modal strain energy method were made for each plate. Experiments were also performed on the plates to establish the reference data for the comparisons.

A modal analysis of the violin

Abstract The msc/nastran finite element computer program was used to study the modal characteristics of a violin with the Stradivari shape. The violin geometry was modeled using an arcs of circles scheme with patran , a finite element graphics pre/postprocessor program. Belly, back, sound post, bassbar, neck, bridge, tail-piece, strings, rib linings, end and corner blocks are the components of the model. Mode shapes and frequencies were calculated for free top and back plates, the violin box, and the complete violin system, including the strings. The results obtained from the finite element technique were compared to experimental data collected from real violins by other investigators.

Passive Vibration Control Scheme Using Circular Viscoelastic Waveguide Absorbers

A waveguide absorber is a device mounted on a vibrating structure at the selected points to transfer energy from the structure to the device which dampens the energy. The waveguide absorbers reported here are made of viscoelastic material which absorbs vibration energy and dissipates it in the form of heat. The novelty of this approach to damping is the simplicity of application and the effectiveness in the broadband frequency range with relatively less material. In this study, the impedances of the circular viscoelastic waveguide absorbers were evaluated experimentally at different temperatures and the results were compared with those of prediction. The application of the waveguide absorbers in the most effective manner to maximize damping of the structure is also studied. The impedance matching techniques were studies to minimize the vibration amplitude of the structure.