Search

Degree of freedom model reduction and frequency windowing via interpolation are combined for application to acoustic finite element analysis. Projection operators are employed for the model reduction or condensation process. Interpolation is then introduced over a user defined frequency window, which can have real and imaginary boundaries and be quite large. Hermitian (which require derivative information) interpolation functions as well as standard Lagrangian functions, which can be linear, quadratic or cubic, have been used to construct the interpolation windows. For 1D windows, the location of interior evaluation points can be varied to increase accuracy. Excellent results are obtained for the example problems, even for large degree of freedom (DOF) reduction and large frequency windows containing numerous resonance (eigenvalues). The solution for the reduced models within the interpolation windows accurately indicate the location of the resonance. The program FWR (Frequency Window Reduction) is written in Fortran 90 and has the ability to read in third party generated finite element models., Prepared in collaboration with Geo-Centers, Inc., Fort Washington, MD.

The acoustic response of a simple thick plate with embedded actuators is investigated in this work, and its performance optimized. The actuators themselves are not modeled. The plate is subjected to an incident wave and the actuators (pistons) do not necessarily cover the entire plate. A standard minimization method as well as a genetic algorithm are used to minimize the acoustic intensity of the far field at both selected field points and the entire far field. A closed form solution is derived for the case of the actuators being driven at the same velocity amplitude, while a genetic algorithm is employed to fwd optimum solutions for the actuators being driven at different velocity amplitudes. The results indicate that the closed form solution, which can include actuators functioning below 100% capacity, produces excellent solutions when the plate is mostly covered with actuators. However, for plates that are only partially covered with actuators or if some of the actuators are not firing or well below 100%, the closed form approach can produce very poor results. The genetic algorithm generate viable solutions in all cases, but shows less spectacular improvements in the reduction of the far field intensities., Prepared in cooperation with GEO-Centers Inc., Fort Washington, MD.

This report is a summary of our recent efforts here at NRL in developing a methodology for predicting the response of thick composite materials subjected to multidimensional shock loadings. A focus of this work has been the development of a 1D, 2D, and 3D evolving damage theory applicable to highly transient loading environments such as underwater shock and impact. This approach is phenomenological in nature and has achieved a degree of success. Comparisons to 2D composite plates impact experiments have been very encouraging. in this work, we assume a local material damage and effective resiting area perspective, in which the evolving damage is considered to be a vector quantity. The iD theory includes explicit dispersion/viscoelastic effects as well., Prepared in cooperation with Geo-Centers, Inc., Fort Washington, MD 20744.

Smoothed particle hydrodynamics (SPH) has the potential to be an important method for structural analysis. However, the SPH method is currently plagued by tension instability. In this work, a new unconventional approach is introduced, in which the stresses are calculated at points other than the SPH nodes, to address this difficulty. This algorithm is implemented into a one dimensional (1D) program called SPH1D. In addition, objective stress rate calculations are discussed and specialized to 1D applications. The results from applying the unconventional approach to a simple bar are very encouraging. The bar holds together in tension and compares quite well to a finite element analysis., Prepared in collaboration with Geo-Centers Inc., Fort Washington, MD.

The transient analysis of coupled fluid-structure systems is a very challenging and important area of particular concern to the naval community. However, the general state of affairs in this class of problems is far from fully satisfactory. In this paper, a brief discussion concerning first transient structural analysis and then fluid-structure interaction is undertaken. Following this, the nonlinear finite element method (FEM) program ABAQUS is used to model the structure, while the fluid realm is discretized with boundary elements (BEM) employing the retarded potential (RP) formulation. The RP method has not been extensively applied in the past to such classes of problems due primarily to storage requirements. However, the RP approach does offer important advantages over both the use of FEM for the fluid or the-widely used Doubly Asymptotic Approximation (DAA). which is also a boundary element method. Discussion concerning time stepping (including automatic considerations), modeling and applications, accuracy, storage, stability, and the overall efficiency of this coupled technique in a MICRO-VAX environment is included. We conclude that the RP method is quite viable for the fluid and that the coupling to ABAQUS to form ABAQUS-RP is a very promising tool for transient., Prepared in collaboration with Geo-Centers, Inc., Fort Washington, MD.