The near wake of a model horizontal-axis wind turbine—II. General features of the three-dimensional flowfield

The three-dimensional near-wake of a model horizontal-axis wind turbine has been measured for three operating conditions: stalled flow over the blades, close to optimum performance, and approaching runaway. The measurements of the mean velocity and turbulence at six axial locations document the formation and development of the near-wake. For the two highest tip speed ratios, the tip vortices are clearly identifiable from the contours of axial velocity and vorticity, and turbulent kinetic energy. At the lowest tip speed ratio, the turbulence level is also high within the blade wakes and these wakes are larger, because of separation in the flow over the blades. The wake structure is simplest for the condition closest to the optimum where the bound vorticity is almost constant with radius. As the tip speed ratio increases, the pitch of the tip vortices decreases and the angular momentum within them increases. This angular momentum reduces the power available from the turbine. The implication is that the structure of the tip vortices must be included in computational models intended to cover the entire operating range of a turbine.