Various aerodynamic load computation approaches for undertaking the aeroelastic analysis of wind turbines has been developed. Blade Element Momentum, for instance, is applied frequently owing to its fast and simple nature. This method provides the loads along the blade span, torque and the amount of power generated by the turbine from the wind speed, aerofoil attributes and blade geometry. Wind turbine blades are designed with aerofoil cross sections; therefore, the Blade Element Method demands aerofoil characteristics as a function of the angle of attack in order to calculate the forces.
Real time flow over the turbine are in axial and radial directions. However, only axial flow is taken into consideration when using 2D aerofoil characteristics while radial flow is normally neglected. This creates high discrepancies of the computed aerofoil characteristics and forces of the CFD or Experiment analysis with the values calculated from the Blade Element Momentum code implementing the 2D aerofoil attributes. This unpleasant agreement is due to stall delay and is evident in most stall-regulated turbines.
Professor E.Y.K. Ng and his PhD student Ijaz Fazil at Nanyang Technological University in Singapore discussed the aerofoil characteristics as well as its various extrapolation methods for an array of angles of attack. They conducted Unsteady RANS (URANS) CFD analysis on two-bladed NREL Phase VI wind turbine and the measurements were compared with the experimental outcomes of the NREL/NASA test for confirmation. Their research work is now published in Energy.
The NREL phase VI rotor adopted for in their study was a two blade stall regulated wind turbine and the blades were tapered and twisted. The rotational speed was 72rpm for all wind speeds.. They conducted the URANS analysis for different inlet wind speeds by implementing the sliding mesh approach. The authors measured pressure at 18 radial locations including the five radial locations considered in experimental measurements.
There was a rise in the lift coefficient along the blade span when compared to the 2-dimensional aerofoil attributes for the same angle of attack owing to the span wise flow. This is the stall delay. This effect if effective at inboard sections and it reduces gradually towards the blades’ tips. Apparently, there is no clear explanation for stall delay but the authors believed that when stall occurred, separated flow on the suction side of the blade, appeared to rotate along with the blade, and experienced centrifugal force. The centrifugal force caused the separated flow to move in a radial manner towards the tip. This span wise flow also permitted Coriolis force to act towards the trailing edge, and therefore, resulted in stall delay.
The research team also discussed in their paper the reasons for the over prediction of the Blade Element Momentum analysis on using existing stall delay correction models. The Blade Element Momentum with or without the stall delay models couldn’t predict the correction distribution of aerofoil characteristics along the span of the blade in order to contemplate the impact of the stall delay.
The study proposed the use of the 3D aerofoil characteristics which is computed using Inverse BEM method in the Blade Element Momentum to take care of the stall delay. A good agreement with the aerofoil characteristics distribution along the span of the blade was realized in the current method.
Ijaz Fazil Syed Ahmed Kabir, E.Y.K. Ng. Insight into stall delay and computation of 3D sectional aerofoil characteristics of NREL phase VI wind turbine using inverse BEM and improvement in BEM analysis accounting for stall delay effect. Energy, volume 120 (2017), pages 518-536.Go To Energy