Date of Award

2017

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Fernando Ponta

Committee Member 1

Jeffery Allen

Committee Member 2

Leonard Bohmann

Committee Member 3

Lucia Gauchia

Abstract

Greater penetration of wind energy demands better utilization of available wind. This has led to a formidable increase in the rotor diameter over the past few years. Bigger rotors call for lighter, more flexible blades to reduce loads and improve fatigue life. As a result, future blades will deform substantially more than the relatively stiff blades of the past. More efficient use of wind power also calls for incorporating advanced active and passive control strategies and increasing the range of velocities over which wind energy is captured. Hence an improvement in the quality of numerical simulations capable of capturing the effects of these deformations is key to innovations in windturbine technology. The code on which this research aims to improve upon is called the Dynamic Rotor Deformation - Blade Element Momentum model (DRD-BEM) introduced by Ponta et al. It combines an advanced structural model with an aerodynamic model implemented in a parallel HPC supercomputer platform. The structural part simulates the response of heterogeneous composite blades, based on a variation of the dimensional reduction technique proposed by Hodges and Yu. This approach reduces the 3-Dimensional complexity of the blade section into a stiffness matrix for an equivalent beam, substantially reducing the computational effort required to model the structural dynamics at every time step. The aerodynamic model is based on an advanced implementation of the Blade Element Momentum (BEM) theory, where all velocities and forces are re-projected into the deformed configuration at that instant. This ensures that the effects of all the complex modes of rotor deformation and subsequent rotation of airfoil sections are accounted for while computing the aerodynamic forces. As a result of the out of plane attitudes of the rotor sections introduced by blade deformations and various control strategies the hitherto small radial component of aerodynamic forces in the hub must now be taken into account. In this research we present a way to extend the capabilities of DRD-BEM by taking into consideration the 3-Dimensional effects of these forces on rotor interference. In this method, called the 3-D DRD-BEM, the coordinate system where the momentum balance is performed in BEM theory is moved, from the hub, to the instantaneous position and alignment of the blade section in its deformed configuration. Another aspect that becomes important as blades become more flexible and control strategies become more complex, is the high axial induction factor regime of turbine operation. This becomes more evident in the instaneous blade section coordinate system of the 3-D DRD-BEM. In most implementations of BEM, this flow regime is modeled using empirical relations based on experimental data with no consensus on which empirical relation to use. This research uses CFD solutions to develop an improved actuator disk model and revisit the above mentioned experiments for a more accurate representation of these operational states.

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