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Experimental Investigation of Wind Turbine Wakes and Their Interaction

Pierella, Fabio
Doctoral thesis
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PhD (Locked)
URI
http://hdl.handle.net/11250/276360
Date
2014
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  • Institutt for energi og prosessteknikk [3337]
Abstract
In a wind farm, the interaction between downstream turbines and the wake of the

machines positioned upstream is a complex problem, which influences the overall

energy production of the turbine cluster and the lifetime of the machines. Despite

the intensive investigation performed in the latest years, the physics of wake-rotor

interaction is, to some extent, not yet understood. In addition, the accuracy of wind

turbine simulation computational models was proven by many studies to be not

satisfactory.

The thesis aims to improve the knowledge on the rotor wake dynamics and on rotorwake

interaction issues, by means of wind tunnel investigation of the performance of

model turbines in single and tandem arrangement and of porous discs. Moreover, the

collected experimental data on wake-rotor interaction represent a solid test case for

simulation benchmarking purpose.

The experiments were run on two three-bladed model wind turbines. Despite having a

slightly different support structure, both turbines had the same blade set, based on the

S826 airfoil from NREL, and rotor diameter,D = 0.9m. The turbines were operated in a

low turbulence uniformflow, with a free stream velocity of U∞ = 10 m/s, both in single

and tandem arrangement at two different downstream separations, namely 3 and 5

rotor diameters. The tip chord local Reynolds number for the upstream turbine, which

was running at the optimum tip speed ratio of λ = 6throughout all the experiments,

was approximately Retip

c ≈ 105, which was demonstrated to guarantee an acceptable

rotor scaling.

The non-symmetries observed in the structure of wind turbine wakes were investigated

via a dedicated experiment where one of the model turbines was equipped with two

symmetrical support towers. The results highlighted that the wake asymmetries are a

consequence of an asymmetry in cross-stream momentum transport, induced by the

tower wake, which forces the rotor wake to sink downward.

The analysis of the performance and of the wake of a turbine tandem in a low turbulence

flow showed that the power production of the downstream turbine is almost

insensitive to the downstream distance between the turbines, while it strongly depends

on the rotational speed of the downstream rotor. When the downstream turbine is operated at higher-than-optimum tip speed ratios, the wake of the turbine tandem recovers

faster. The efficiency of the downstream turbine, on the other hand, is only marginally

lower than the efficiency of the upstream turbine, meaning that most of the power loss

is to be ascribed to the reduction in mean velocity rather than to inefficiencies of the

rotor.

An assessment of the most common models for turbine simulation was performed

via a "Blind Test" challenge, where experimenters were invited to submit computer

simulations reproducing the behavior of the two in-line turbines by the only knowledge

of the boundary conditions at which the test was run.

The comparison of different models to the experimental results highlighted a big scatter

among the thrust and power predictions of the different computations. The tandem

mean wake was generally not accurately reproduced. For thrust and power prediction

purposes, a fully resolved approach simulating the whole blade and turbine surface

gave the best results. An actuator line rotor model coupled with a LES solver outputted

the best mean and turbulent wake predictions. The experimental-numerical comparison

campaign represents an important database which can become a reference

for modellers who need to choose the appropriate numerical model to solve a wake

interaction problem.

The near wake of static wind turbine wake simulators was also analyzed, as a function

of their geometry. Experiments on a porous disc with a diameter of D = 0.8 m, run for

a diameter based Reynolds number of ReD = 5· 105, showed that the modification of

the planar arrangement of the bars composing the mesh can strongly affect the mean

wake structure and induce a non-axisymmetric near wake.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2014:200

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