Abstract
Abstract. The root flow of wind turbine blades is subjected to complex physical mechanisms that influence significantly the rotor aerodynamic performance. Spanwise flows, the Himmelskamp effect, and the formation of the root vortex are examples of interrelated aerodynamic phenomena that take place in the blade root region. In this study we address those phenomena by means of particle image velocimetry (PIV) measurements and Reynolds-averaged Navier–Stokes (RANS) simulations. The numerical results obtained in this study are in very good agreement with the experiments and unveil the details of the intricate root flow. The Himmelskamp effect is shown to delay the stall onset and to enhance the lift force coefficient Cl even at moderate angles of attack. This improvement in the aerodynamic performance occurs in spite of the negative influence of the mentioned effect on the suction peak of the involved blade sections. The results also show that the vortex emanating from the spanwise position of maximum chord length rotates in the opposite direction to the root vortex, which affects the wake evolution. Furthermore, the aerodynamic losses in the root region are demonstrated to take place much more gradually than at the tip.
Highlights
The aerodynamic design of wind turbine blades is subjected to important levels of uncertainty
3.1 Main characteristics of the flow field over the blades The detailed particle image velocimetry (PIV) and numerical results provide a good insight into the main flow characteristics over the blade root
The use of PIV measurements and Reynolds-averaged Navier–Stokes (RANS) simulations enabled the analysis of the flow in the root region of a wind turbine blade operating under design conditions with axisymmetric inflow
Summary
The aerodynamic design of wind turbine blades is subjected to important levels of uncertainty. As a matter of fact, transient operational states can pose a challenge to the wind turbine designer and seemingly simple cases involving steady operation under axisymmetric, uniform inflow conditions (Leishman, 2002; Schepers, 2012) This is especially true for the tip and root regions of the blades, where the flow is three-dimensional and strongly influenced by the trailing vortices (Micallef, 2012). The Himmelskamp effect, known as stall delay or rotational augmentation, has been studied by many authors both experimentally (Schreck and Robinson, 2002; Sicot et al, 2008; Ronsten, 1992) and numerically (Guntur and Sørensen, 2014; Herráez et al, 2014; Schreck et al, 2007), it still remains far from being well understood and characterized It mainly affects the blade root region and is known to be closely related to the existence of spanwise flows in the boundary layer.
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