The near wake of a model horizontal-axis wind turbine—I. Experimental arrangements and initial results

This paper is the first of a series describing measurements in the near wake of a small horizontal-axis wind turbine over a range of tip speed ratios. The primary aim was to document the formation and development of the three-dimensional near-wake; this was done at six axial locations within two chord lengths of the blades. This paper describes the experimental arrangements, the measurement techniques based on hot-wire anemometry, and the results which relate directly to the simple wake models that lead, for instance, to the Betz limit and are used in traditional blade element theory. Later papers will describe the complex, three-dimensional flow field, the properties of the tip vortices, and the process by which the hub “vortices” may diffuse to form a cylindrical vortex sheet. For the conditions giving the largest power coefficient, the bound circulation of the blade is approximately constant with radius and the velocity distribution immediately behind a blade is similar to that behind an aerofoil with the same circulation. This good agreement with aerofoil behaviour extends to the highest tip speed ratio measured. At the lowest tip speed ratio, the turbulence level in the wake is very high, suggesting separation from the blades which are operating at high angles of attack. It is also shown that the three-dimensionality does not contribute significantly to the balance of angular momentum in the wake. There is evidence that increasing amounts of angular momentum reside in the tip vortices as the tip speed ratio increases.