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 nea r wake of a small horizontal-axis wind turbine over a range of tip spe ed ratios. The primary aim was to document the formation and developme nt of the three-dimensional near-wake; this was done at six axial loca tions within two chord lengths of the blades. This paper describes the experimental arrangements, the measurement techniques based on hot-wi re anemometry, and the results which relate directly to the simple wak e models that lead, for instance, to the Betz limit and are used in tr aditional blade element theory. Later papers will describe the complex , three-dimensional flow field, the properties of the tip vortices, an d the process by which the hub ''vortices'' may diffuse to form a cyli ndrical vortex sheet. For the conditions giving the largest power coef ficient, the bound circulation of the blade is approximately constant with radius and the velocity distribution immediately behind a blade i s similar to that behind an aerofoil with the same circulation. This g ood agreement with aerofoil behaviour extends to the highest tip speed ratio measured. At the lowest tip speed ratio, the turbulence level i n the wake is very high, suggesting separation from the blades which a re operating at high angles of attack. It is also shown that the three -dimensionality does not contribute significantly to the balance of an gular momentum in the wake. There is evidence that increasing amounts of angular momentum reside in the tip vortices as the tip speed ratio increases. (C) 1997 Elsevier Science Ltd.