Browsing by Author "Waite, Joshua Joseph"

This thesis characterizes the nonlinear behavior of an experimental system that exhibits snap-through buckling behavior. A single-degree-of-freedom snap-through link model is harmonically forced using a Scotch yoke mechanism. In order to establish the sensitivity to initial conditions, experimental basins of attraction are constructed using the stochastic interrogation method. After, frequency sweeps are performed on the system to identify regions of interesting behavior. Then, time series data is collected at specific frequencies of interest to highlight the broad phenomenological behavior of the structural system.

A useful tool when modeling structural systems is numerical analysis. An equation of motion is developed to numerically simulate all experimentally observed results. The numerical results include snap-through boundaries, bifurcation diagrams, full initial condition grid basins of attraction, time-lag embedded basins of attraction, frequency sweeps, and time series of regions of pathological behavior.

The successful, efficient, and safe turbine design requires a thorough understanding of the underlying physical phenomena. This research investigates the physical understanding and parameters highly correlated to flutter, an aeroelastic instability prevalent among low pressure turbine (LPT) blades in both aircraft engines and power turbines. The modern way of determining whether a certain cascade of LPT blades is susceptible to flutter is through time-expensive computational fluid dynamics (CFD) codes. These codes converge to solution satisfying the Eulerian conservation equations subject to the boundary conditions of a nodal domain consisting fluid and solid wall particles. Most detailed CFD codes are accompanied by cryptic turbulence models, meticulous grid constructions, and elegant boundary condition enforcements all with one goal in mind: determine the sign (and therefore stability) of the aerodynamic damping. The main question being asked by the aeroelastician, ``is it positive or negative?'' This type of thought-process eventually gives rise to a black-box effect, leaving physical understanding behind. Therefore, the first part of this research aims to understand and reveal the physics behind LPT flutter in addition to several related topics including acoustic resonance effects. A percentage of this initial numerical investigation is completed using an influence coefficient approach to study the variation the work-per-cycle contributions of neighboring cascade blades to a reference airfoil. The second part of this research introduces new discoveries regarding the relationship between steady aerodynamic loading and negative aerodynamic damping. Using validated CFD codes as computational wind tunnels, a multitude of low-pressure turbine flutter parameters, such as reduced frequency, mode shape, and interblade phase angle, will be scrutinized across various airfoil geometries and steady operating conditions to reach new design guidelines regarding the influence of steady aerodynamic loading and LPT flutter. Many pressing topics influencing LPT flutter including shocks, their nonlinearity, and three-dimensionality are also addressed along the way. The work is concluded by introducing a useful preliminary design tool that can estimate within seconds the entire aerodynamic damping versus nodal diameter curve for a given three-dimensional cascade.