On the skin friction drag reduction in large wind turbines using sharp V-grooved riblets

摘要

Skin friction drag reduction through the use of riblets has been a topic of intensive research during the last decades. Main efforts have been placed on both numerical (mainly DNS) and experimental (wind tunnel and fluid channel) approaches. In spite of the valuable efforts to date, the fundamental mechanisms that induce drag reduction are not well established and, in particular, the potential benefits associated with the use of riblets in wind turbines are unclear. In this research, wind tunnel experiments were performed to quantify the drag reduction in a wind turbine airfoil using V-groove riblet structures. A full-scale 2.5MW wind turbine airfoil section (typical of the 75-80% blade span), was placed in the freestream flow of the wind tunnel at the Saint Anthony Falls Laboratory (SAFL), University of Minnesota. Different sizes of V-groove riblets were tested at different angles of attack at full scale Reynolds number. Force sensors were used to measure lift and drag. Momentum deficit was also measured in the wake of the airfoil to determine the net drag. Flat plate experiments were also performed to determine the distribution of the skin friction coefficient in a developing boundary layer. The experimental measurements will be used to develop and test the performance of near-wall boundary conditions for the velocity field that can account for the effects of riblet roughness in the context of RANS and hybrid RANS/LES models. The numerical simulations along with the laboratory experiments will be used in the future to determine the best riblet configuration and guide the application and testing of riblets in a 2.5MW wind turbine under real-life wind conditions.