Understanding 3D flows around rotating blades – how to infer the angle of attack in the field?

Estimating and measuring the aerodynamic performance of wind turbine blades can be complex, because the local wind encountering the rotating blade is the vectorial combination of the unsteady and turbulent wind speed and the rotational speed of the blade, modified by the induction through the rotor. This means, in the frame of reference of the rotating blade, that the inflow gradually changes as it approaches the blade, hence no reference wind speed and direction can be uniquely defined. And yet, to improve aerodynamic models of rotor blades, their aerodynamic performance needs to be compared with 2D non-rotating measurements or simulations, for which the inflow conditions are well defined. An experimental method is therefore required to compare aerodynamic measurements, such as pressure distributions, from a rotating blade to fixed 2D sections. In this work, we present such a method, which utilises pressure measurements from a newly-developed measurement system that can easily be attached to a wind turbine blade. It includes an array of absolute pressure sensors, which allow the pressure distribution to be obtained using corrections from an inertial measurement unit. The inflow conditions are inferred with a hybrid model combining potential flow theory and differential pressure measurements at the leading edge. In this presentation, after introducing the measurement system and the experimental method, its effectiveness will be demonstrated using measurement data from an operating wind turbine.