The near wake of a model horizontal-axis wind turbine - Part 3: Propertiesof the tip and hub vortices

This paper concludes a series on the formation and development of the near- wake of a model horizontal-axis wind. The three-dimensional mean velocity a nd turbulence fields were obtained at six axial locations within two chord lengths of the blades for three operating conditions: stalled flow over the blades, close to optimum performance, and approaching runaway. Here we con centrate on the tip and hub vortices. For the second and third conditions, the hub vortex quickly becomes a sheet of vorticity spread over the cylindr ical centrebody. It is suggested that the 'trailing vorticity' at thr: hub is formed by the skewing of the circumferential vorticity in the upstream b oundary Layer as it encounters the blades, so that the 'bound' vorticity of the blades simply passes through the centrebody surface; The tip vortices, which were shown in Part 2 to contain increasing amounts of angular moment um as the tip speed ratio increases, are associated with large variations i n all velocity components and high levels of turbulence. The pitch of the h elical tip vortices decreases with increasing tip speed ratio. (C) 2000 Els evier Science Ltd. All rights reserved.