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No description available.

Experimental spatial dynamics modeling is a new approach to dynamics modeling using high-spatial-density experimental data from a scanning laser Doppler vibrometer (LDV). This instrument measures the surface velocity of vibrated structures. Time-signal data from the LDV is statistically modeled with multiple linear regression for harmonically excited structures. A weighted least-squares discrete finite element formulation is developed to solve for the complex-valued continuous 3-D velocity response field from sampled velocity data. The formulation is derived from the steady-state solution of the differential equation with spatial and temporal components of harmonic structural dynamic response. Linear, quadratic, cubic, and cubic B-spline basis functions are used to form isoparametric finite elements in the dynamic response model. Velocity measurements acquired from multiple positions are transformed into a single model that minimizes the least-squares error between the experimental data and the field equations in the 3-D shell element model. A multiple point nonlinear registration algorithm is developed to determine position and orientation of the LDV relative to the test structure. Polygonal shape models are successfully integrated with the experimental spatial dynamic response models via polygon ray intersection. Finite element shape models are generated from simple flat surfaces or extracted from existing finite element models of 3-D structures. By postprocessing the model solution, many dynamic properties including rotations, full-field strains and stresses, and acoustic prediction are derived from the dynamic response representation. Visualization software was developed for animation of the 3-D spatial dynamic response models with superimposed color to represent the postprocessed results. The interactive graphics allow presentation and investigation of the experimental spatial dynamics. To examine the method, an analytical test model is defined to simulate the surface velocity response of a structure with both in-plane and out-of-plane harmonic vibration. Random and uniformly spaced measurements of the simulated dynamic system are acquired from multiple locations. Applications of experimental spatial dynamics modeling, postprocessing, and visualization are also demonstrated with five different test structures. Through mesh refinement, increase in order of the basis functions, and additional sampling, the finite element models are converged to statistically qualified solutions. / Ph. D.

This dissertation is concerned with the evaluation of a new statistically sound reconstruction methodology for continuous 3-D dynamic response fields of harmonically excited structures in steady-state vibration. This results in an experimental process which reconstructs the response field from a set of 3-D projections based on Laser-Doppler-Vibrometer (LDV) localized instantaneous velocity measurements. Included along with an estimate of the 3-D velocity field, are its statistical characteristics and the inferential tools required to test the quality of the estimation. This dissertation documents in detail the development and evaluation of the proposed reconstruction methodology and its relevant subprocesses which inc1ude the formulation of a deterministic laser-structure kinematic model, and regression models that afford statistical inference for the time-domain and spatial-domain structural dynamics, as well as for the projection recombination process. / Ph. D.

A second-order system of differential equations containing a multifrequency parametric excitation and weak quadratic and cubic nonlinearities is investigated. The method of multiple scales is used to carry out a general analysis, and three resonance conditions are considered in detail. First, the case in which the sum of two excitation frequencies is near two times a natural frequency, λ_s + λ_t <u>~</u>2Ï _q, is examined. Second, the influence of an internal resonance, Ï <sub>q</sub =<u>~</u>3Ï r, on the previous case is studied. Finally, the effect of the internal resonance w_r<u>~</u>3w_q on the resonance λ_s + λ_t <u>~</u>2Ï _q is investigated. Results are presented as plots of response amplitudes as functions of a detuning parameter, excitation amplitude, and, for the first case, a measure of the relative values of λ_s + λ_t. / Master of Science

Proper mathematical models of structures are beneficial for designers and analysts. The accuracy of the results is essential. Therefore, verification and/or correction of the models is vital. This can be done by utilizing experimental results or other analytical solutions. There are different methods of generating the accurate mathematical models. These methods range from completely analytically derived models, completely experimentally derived models, to a combination of the two. These model generation procedures are called System Identification. Today a popular method is to create an analytical model as accurately as possible and then improve this model using experimental results. This thesis provides a review of System Identification methods as applied to vibrating structures. One simple method and three more complex methods, chosen from current engineering literature, are implemented on the computer. These methods offer the capability to correct a discrete (for example, finite element based) model through the use of experimental measurements. The validity of the methods is checked on a two degree of freedom problem, an eight degree of freedom example frequently used in the literature, and with experimentally derived vibration results of a free-free beam. / Master of Science

Free jets are naturally unstable. As a result the jet which is initially laminar becomes turbulent. During this transition process, large-scale structures are formed and fluid is "induced" to join the jet from the surroundings. This induction of fluid is defined as entrainment and depends greatly upon the jet geometry. In particular, an elliptical jet has been found to entrain at a higher rate than its circular counterpart. Numerical simulations of elliptical jets have been conducted on NRL’s parallel CM-5 and CM-200 supercomputers using the Flux Corrected Transport algorithm and initialized with the results of a linear stability analysis with the objective of investigating the entrainment process and explaining the dependence of the entrainment on jet geometry. Through the medium of scientific visualization, mechanisms responsible for entrainment have been identified and associated with the results of a linear stability analysis to suggest passive means by which entrainment can be enhanced. In particular, it was found that increasing the aspect ratio, thinning the shear layer, and non-uniformly distributing the shear layer serve to increase entrainment. / Master of Science

The need for a validation technique for computational fluid dynamics (CFD) codes in STOVL applications has led to research efforts to apply infrared thermal imaging techniques to visualize gaseous flow fields. Specifically, a heated, free-jet test facility was constructed. The gaseous flow field of the jet exhaust was characterized using an infrared imaging technique in the 2 to 5.6μm wavelength band as well as conventional pitot tube and thermocouple methods. These infrared images are compared to computer-generated images using the equations of radiative exchange based on the temperature distribution in the jet exhaust measured with the thermocouple traverses. Temperature and velocity measurement techniques, infrared imaging, and the computer model of the infrared imaging technique are presented and discussed. From the study, it is concluded that infrared imaging techniques coupled with the radiative exchange equations applied to CFD models are a valid method to qualitatively verify CFD codes used in STOVL applications. / Master of Science

Suspension flows in varied settings and at different concentrations of particles are studied theoretically using various modeling techniques. Particulate suspension flows are dispersion of particles in a continuous medium and their properties are a consequence of the interplay among hydrodynamic, buoyancy, interparticle and Brownian forces. The applicability of continuum modeling techniques to suspension flows at different particle concentration was assessed by studying systems at different time and length scales. The first two studies involve the use of modeling techniques that are valid in systems where the forces between particles are negligible, which is the case in dilute suspension flows. In the first study, the growth and progradation of deltaic geologic bodies from the sedimentation of particles from dilute turbidity currents is modeled using the shallow water equations or vertically averaged equations of motions coupled with a particle conservation equation. The shallow water model provides a basis for extracting grain size and depositional history information from seismic data. Next, the Navier-Stokes equations of motion and the convection-diffusion equation are used to model suspension flow in a biomedical application involving the flow and reaction of drug laden nanovectors in arteries. Results from this study are then used prescribe the best design parameters for optimal nanovector uptake at the desired sites within an artery. The third study involves the use of macroscopic two phase models to describe concentrated suspension flows where interparticle hydrodynamic forces cannot be neglected. The isotropic form of both the diffusion-flux and the suspension balance models are solved for a buoyant bidisperse pressure-driven flow system. The model predictions are found to compare fairly well with experimental results obtained previously in our laboratory. Finally, the power of discrete type models in connecting macroscopic observations to structural details is demonstrated by studying a system of aggregating colloidal particles via Brownian dynamics. The results from the simulations match experimental shear rheology and also provide a structural explanation for the observed macroscopic behavior of aging. / text

Abstract not available