Degree Type

Dissertation

Date of Award

2016

Degree Name

Doctor of Philosophy

Department

Aerospace Engineering

Major

Aerospace Engineering

First Advisor

Anupam Sharma

Abstract

This dissertation serves to summarize my research into wind farm and wind turbine aerodynamics.

Included in this thesis is a summary of the methods I use as well as the four research problems that I investigated.

Motivation is provided for my research as well as an overview of the computational

methods that I use. These methods include analytical methods such as blade element

momentum (BEM) theory and the vortex lattice method as well as computational fluid dynamic methods like

the Reynolds averaged Navier-Stokes (RANS) equations and large eddy simulation (LES). These methods are used

to investigate wind turbine and wind farm aerodynamics. In particular, I use these methods to confront the

various forms of loss that wind turbines and wind farms experience. They include the losses that individual turbines

experience due to swirl, induction, and viscosity as well as the loss that wind farms experience due to turbine-wake interaction.

Horizontal axis wind turbines (HAWTs) suffer

from aerodynamic ineffciencies in the blade root region (near the hub) due to several non-aerodynamic

constraints. Aerodynamic interactions between turbines in a wind farm also lead to signifcant loss of wind

farm efficiency. A new dual-rotor wind turbine (DRWT) concept is proposed that aims at mitigating these two

losses. A DRWT is designed that uses an existing turbine rotor for the main rotor, while the secondary rotor

is designed using a high lift-to-drag ratio airfoil. Reynolds Averaged Navier-Stokes computational fluid

dynamics simulations are used to optimize the design. Large eddy simulations confirm the increase energy

capture potential of the DRWT. Wake comparisons however do not show enhanced entrainment of axial momentum.

I extend the prescribed wake vortex lattice method (VLM) to

perform aerodynamic analysis and optimization of dual-rotor wind turbines.

The additional vortex system introduced

by the secondary rotor of a DRWT is modeled while taking into account the

singularities that occur when the trailing vortices from the secondary

(upstream) rotor interact with the bound vortices of the main (downstream)

rotor. Pseduo-steady assumption is invoked and averaging over multiple

relative rotor positions is performed to account for the primary and

secondary rotors operating at different rotational velocities. This

implementation of the VLM is first validated against experiments and blade

element momentum theory results for a conventional, single rotor turbine. The

solver is then verified against RANS CFD

results for two DRWTs. Parametric sweeps are performed using the proposed VLM

algorithm to optimize a DRWT design. The problem with the algorithm at high

loading conditions is highlighted and a solution is proposed that uses RANS

CFD results to calibrate the VLM model.

In addition to wake losses, aerodynamic interaction between

turbines in wind farms leads to surface flow convergence . This phenomenon has been

observed in field tests with surface flux stations. A hypothesis is

proposed to explain this surface flow convergence phenomenon - incomplete

pressure recovery behind a turbine leading to successive pressure drops in

tightly-spaced turbine arrays leads to drop in overall pressure deep inside

a wind plant; this low-pressure acts as an attractor leading to flow

convergence. Numerical investigations of the phenomenon of surface flow

convergence are carried out that support this hypothesis. An actuator disk

model to represent wind turbines in an LES

CFD solver is used to simulate hypothetical wind plants. The flow

convergence phenomenon reflects as change in flow velocity direction and is

more prominent near the ground than at turbine hub height.

Numerical simulations of wind plant aerodynamics are conducted with various

approximations to investigate and explain the flow convergence phenomenon.

Copyright Owner

Aaron John Rosenberg

Language

en

File Format

application/pdf

File Size

100 pages

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