Projects Descriptions
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Substructured, Meshless and Parametric Modeling of Vibroacoustic Systems
    Sponsor: NASA LaRC

Aerospace structures are often subjected to a broad spectrum of mechanical and/or aerodynamic excitations and, therefore, there is a real need for techniques which can be used for modeling, with high fidelity and adequate spatial and spectral resolutions, of vibroacoustic systems over the entire frequency spectrum. A dynamic system typically exhibits distinctively different response characteristics as frequency increases. In recognizing the complicated behavior, distinct analysis methods are used at different frequency ranges, usually classified as low, mid, and high frequency ranges. A substructure-based modeling technique that is applicable at all frequencies, including the critical mid-frequency range, is developed for the modeling of complex dynamic systems. This method does not require meshing as is traditionally used in discretization methods, computationally very efficient, and allows for rational modeling of manufacturing variability.
Hybrid Element Method for Composite Structures Subjected to Boundary Layer Loading
    Sponsor: NASA LaRC

The modeling of composite structures becomes more and more important since many new vehicle designs incorporate increased amount of composite structural components due to weight specific advantages of composites. This project is directed towards the development of techniques that will allow the prediction of noise in the interior of an enclosure such as aircraft due to the transmission of turbulent boundary layer loading in the presence of composite structural components. An innovative Hybrid Element Method (HEM) solution tool for mid and high frequency analyses, which utilizes elements of DEA together with conventional low frequency FEM tools and high frequency EFEM tools, will provide a unified framework that is applicable for the solution of full frequency spectrum vibroacoustic prediction of nonuniform aerospace structures including metallic/composite configurations, accurately and efficiently.
Failure Initiation Predictors for Reliability-Based Design of Composite Structures
    Sponsor: AFOSR

This project is concerned with the development of a novel failure initiation and progressive failure analysis (PFA) modeling method for advanced composite structures, utilizing a fundamental physics based multiscale mechanics model embedded in a non-linear finite element code. First, experimental works with a refined strain analysis using digital correlation technique have been performed to understand the failure initiation and the interaction of various failure mechanisms for composite laminate structures under compression. Based on the experimental observation, micromechanics finite element model has been developed to predict the nonlinear lamina level deformation and failure response. Concurrently, PFA methodology combining physics-based failure prediction models such as Schapery theory and the discrete cohesive zone method (DCZM) has been developed for modeling laminated composite to account for all possible failure mechanisms and their interactions. In addition, probabilistic analysis capability has been implemented into the PFA methodology to account for material variability and manufacturing inconsistencies.
Hybrid Element Method for Mid-Frequency Vibroacoustic Analysis
    Sponsor: NASA LaRC

An innovative, accurate and efficient Hybrid Element Method (HEM) is developed for mid frequency vibroacoustic analysis of non-uniform aerospace structures. The development is based on the concept that, by using transcendental functions based on the exact (or near exact) solutions of free wave equations and statistical phase function as interpolation functions, the mid frequency problems can be resolved using finite element models. When combined with low (FEM) and high frequency (EFEM) analysis tools, this is expected to offer a unified framework for the full frequency spectrum vibroacoustic analysis of aerospace systems accurately and efficiently using the same finite element database.
Identification and Reduction of Turbomachinery Noise Sources
    Sponsor: NASA GRC

Communities near airports are often exposed to high noise levels due to low flying aircraft in the takeoff or landing phase of flight. Since propulsion noise is a major contributor to the overall noise level, the identification of propulsion noise of turbofan engines plays an important role in the design of low-noise aircrafts. However, the noise generation mechanisms of a typical turbofan engine are very complicated and it is not practical, if not impossible, to identify these noise sources efficiently and accurately using numerical or experimental techniques alone. Nearfield acoustical holography (NAH) refers to a process by which all aspects of the sound field in a three-dimensional domain can be reconstructed based on sound pressure measurements in the nearfield on a two-dimensional surface. In this project, a generalized acoustical holography (GAH) system that is capable of handling aeroacoustic sources under stead-state and transient conditions is developed by extending inverse boundary element techniques. Major accomplishments of the project include the development of direct and indirect boundary element methods for generation of transfer matrix that relates aeroacoustic sources (monopoles, dipoles and quadrupoles) to sound pressure field measured using surface mounted microphones, application of innovative filtering techniques of measurement data through the use of de-noising techniques, and the development of extensive regularization methods for the identification of the sources based on measured sound pressure field, and a graphical user interface to facilitate the ease-of-use of the software system. The efficacy of the development is verified using numerically synthesized and experimental data.
Vehicle Interior Noise Prediction Using Energy Finite Element Method
    Sponsor: NASA LaRC

The project is directed towards the development and validattion a numerical technique based on Energy Finite Element Method (EFEM) for the analysis of high frequency vibroacoustic problems. Major accomplishments of the project include the development of a new method for interior noise prediction based on EFEM, development of an extensive library of structural and noise control elements, development of the joint element, development and incorporation of the energy boundary element method for the modeling of acoustic space, development of a hybrid method for the accurate modeling of local excitation, efficient solution methods, development of method for the automatic identification of point, line and area junctions at geometric and material discontinuities, and development of a graphical user interface to facilitate the ease-of-use of the software.
Identification of Turbomachinery Noise Sources Using Acoustical Holography
    Sponsor: NASA GRC
The noise generation mechanisms of a typical turbofan engine are complicated, which makes it a significant challenge to identify the sources. Currently, turbomachinery noise is often predicted using semi-empirically derived correlations and scaling procedures, many of which are extremely elaborate and incorporate a great number of aerodynamic, aeromechanical, and geometric parameters. Various numerical techniques for predicting turbomachinery noise have also been developed in recent years. However, all these techniques require knowledge of the sound sources. The identification of these sound sources is especially difficult in the presence of many potential sources that interact, which is exactly the case with turbomachinery noise. Nearfield acoustical holography (a process by which all aspects of the sound field in a three-dimensional domain can be reconstructed based on nearfield sound pressure measurements), conceptually allows the identification of complex noise sources that are otherwise difficult to characterize. The project is directed towards the development of improved nearfield acoustical holography techniques for the identification and ranking of turbomachinery noise sources.