Vibro-acoustic optimization of panel with curvilinear stiffeners
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With the development of manufacturing techniques such as the Electron Beam Free Form Fabrication (EBF3), a metal deposition technique which deposits metal in complex shapes on a metallic base plate, it has become easy to manufacture complex shapes such as panels with curvilinear stiffeners. Designing and optimizing stiffened panels with predefined structural and acoustic response is the focus of this dissertation. Researchers have dealt with sizing optimization of panels with straight/curvilinear stiffeners for many years and it has been proven that in some cases the mass of a panel with curvilinear stiffeners is lesser than the mass of a panel with straight stiffeners for a complex loading such as bi-axial compression with shear and transverse pressure. The research work in this dissertation addresses the sizing as well as placement optimization of panel with straight and curvilinear stiffeners for desired structural as well as acoustic response. For acoustic optimization, point-excited stiffened panels are designed for minimal sound radiation given the constraint on total mass of the structure. To reduce the computational expense of structural-acoustic optimization, a new methodology for the objective function evaluation is also proposed and optimal design for minimum radiated acoustic power is discussed. The developed framework, named EBF3PanelOpt for structural acoustic optimization of point excited stiffened panels is extended to multi point excitation to capture the realistic excitations such as turbulent boundary layer (TBL) pressure fluctuations. The Corcos model of representing TBL is used to capture correlation of TBL pressure excitation. Validation of the approach using Corcos TBL model, implemented in EBF3PanelOpt is performed using fast multi-pole boundary element method in FastBEM and a conventional boundary element code, HELM3D. The optimal designs are obtained for a panel with two and four stiffeners, respectively. The minimization of both, the mass and the acoustic response during structural-acoustic optimization is conflicting in nature. Therefore, a multi-objective design optimization using non-dominated sorting genetic algorithm-II (NSGA-II) is performed. The Pareto optimal designs, obtained using multi-objective design optimization approach has reduced the acoustic response significantly with a minor mass penalty of the structure when compared to a baseline design while meeting all the constraints such as buckling eigenvalue, von Mises, and crippling stresses. A multi-objective design optimization framework is also developed for design optimization of diffuse field excited panels with straight and curvilinear stiffeners. The Pareto optimal designs of panel with six stiffeners are obtained using developed framework and a comparative study is performed with a baseline design with six straight stiffeners. The developed framework is also extended to perform multi-objective design optimization of point excited complex structures such as curved panels with straight or curvilinear stiffeners. A fast multi-pole boundary element method is used to calculate the acoustic response of the curved panel with stiffeners and design optimization results of a curved panel with two and four stiffeners are discussed. Experiments are also performed at Sound and Acoustic Load Transmission (SALT) facility of Langley Research Center to measure the sound radiation and transmission loss for two panels with straight and curvilinear stiffeners, respectively. The stiffened test panels with six stiffeners have been designed using multi-objective design optimization framework for TBL excitation.
- Doctoral Dissertations