Aerodynamic modelling of wind turbine blade loads during extreme deflection events

Abstract

Long, flexible wind turbine blades experience significant deflections during operation. Historically, the blade element momentum model in Bladed assumed that the blade is split into a set of radial aerodynamic sections which are aligned perpendicular to the pitch axis, and that any rotation of the sections due to deflection, pre-bend or pre-sweep in the blade can be ignored. For modern-day flexible rotor blades, under extreme loading conditions, this assumption no longer holds. Including this rotation when calculating inflow velocities and angle of attack, and when resolving the resultant forces at the element, causes a significant change in axial force (Fz) along the pitch axis, and therefore the blade out of plane bending moment (My). This in turn drives a change in overturning moments on the hub (which is driven by differences in bending moments between blades). An example 160m rotor shows an increase of 68% in hub overturning moment during an extreme gust event when aerofoil rotation due to deflection is included. This is verified through comparison to the alternative free vortex wake rotor model in Bladed. DNV GL Bladed includes the rotation of aerofoils by default at version 4.8.