Rotor wake turbulence: An experimental study of a wind turbine wake
Doctoral thesis
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http://hdl.handle.net/11250/2382264Utgivelsesdato
2016Metadata
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Sammendrag
An experimental investigation of the first five diameters of the wake of a 0.9
meter in diameter model wind turbine with three blades has been undertaken.
The blades are twisted and tapered and uses a NREL S826 profile
along the full length of the blade. The test environment is a closed loop
wind tunnel with a cross-section of 1.8 by 2.7 meters, which produces a
uniform flow with 0.24% turbulence intensity. All measurements are performed
at the turbines design condition, at which R = 6 and Retip 105.
This measurement campaign is an extension of the experimental work undertaken
by the author for the 2011 blindtest workshop arranged at NTNU
by NOWITECH and NORCOWE where the numerical community was invited
to predict the development of the wake. The results of the blindtest
were reported by Krogstad and Eriksen (2013).
High-speed measurements are obtained with a four wire hot-wire probe operated
at constant temperature which can resolve all three components of
the velocity vector. An existing data reduction scheme proposed by Maciel
and Gleyzes (2000) has been modified to work over the wide range of velocities
and flow angles encountered in a wind turbine wake. In addition to
the velocity vector, the rotor position was measured simultaneously. This
allowed for conditional averaging of the acquired data, which made it possible
to reveal periodic coherent structures in the flow. The investigation
has also looked at conventional time averaged statistics and frequency and
wave-number spectra. The results from these different methods of analysis
have been used to estimate terms in the energy budgets of the mean, periodic
and turbulent motions in the flow.
The analysis reveals how the wake develops from a flow dominated by periodic coherent structures to one where purely turbulent motions governs
production of turbulent kinetic energy and transport of momentum into the
wake. The coherent motions do initially contain a significant portion of the
time averaged turbulent kinetic energy near the edge of the wake and are
found to be important in both production and radial transport of turbulent
kinetic energy. The vortices do naturally dominate the spectral content of
the initial wake. Investigations of wave-number spectra has revealed that
while the energy containing range gradually moves towards larger scales
as the periodic coherent structures decay, a significant inertial sub-range
emerges, parts of which can be described as isotropic. After the collapse of
the tip vortex system the mean dissipation and production has been found
to balance and the evolution of the turbulent kinetic energy level is governed
by radial diffusion.