Rotor wake turbulence: An experimental study of a wind turbine wake
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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.