The National Rotor Testbed (NRT) includes research to quantify the degree to which the blade design load distribution influences the rotor near- and mid-wake velocity deficits and fluid structures. Wind-plant performance and the effects of turbine–turbine interaction have been identified as the primary research topics for the NRT blades. The wake deficit and fluid structures are important, as they directly relate to wind-plant performance and dynamic loads resulting from turbine–turbine interaction. Investigations into the relationships between blade design and rotor wake is a necessary step in specification of the NRT blades.
Wake deficit at two diameters downstream of the rotor plane.
Two blades with the same integrated loads, but different load distributions, have been defined. The first blade was designed to produce a maximum power coefficient via a theoretical optimum distribution of axial induction. The second blade was designed to exhibit a modified distribution of axial induction, lower outboard and higher inboard, while still operating at the same rotor thrust coefficient as the first blade. These two blade designs are referred to as ‘Max CP Design’ and ‘Lower Tip Loading Design.’
The figure shows the wake-velocity deficit for the two designs at two diameters downstream of the rotor; the wake deficits were predicted by a relaxed-wake vortex method using steady, uniform inflow. These results show that the lower tip loading design produces a narrower wake than the maximum CP design, but also produces almost 20% higher peak velocity deficit. These predictions confirm that the blade load distribution has an important impact on the development of the near wake.
The next stage of this work will compare many blade design parametric variations to determine the detailed blade-loading sensitivity to the predicted near- and mid-wake deficits. A parallel effort is underway to compare predictions of actuator line large-eddy simulation (AL-LES) with predictions of the vortex method. The effect of sheared and turbulent inflow conditions on the predicted wake deficits of several blade designs will be assessed as part of the AL-LES effort. The results of this work will provide a rigorous definition of the motivation for NRT design specifications to produce the most useful rotor wake physics for experimental campaigns.