Drag reduction in large wind turbines through riblets: Evaluation of different geometries

Wind tunnel experiments were performed with a wind turbine airfoil that was fitted with different riblet geometries. The foil represents the cross section of a full-scale 2.5MW wind turbine blade close to the tip. The test foil was placed in the free stream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Various sizes and geometries of riblets were tested at angles of attack ranging from 0°≤α≤10°, corresponding to a range of lift coefficient of 0.25≤ C_L ≤1.14, and at a Reynolds number of Re=2.2 X 10⁶. Mean drag was measured via a wake survey (momentum deficit) and with a sensitive force balance. Lift was measured directly from the force balance. Tests included the cases of complete and partial riblet coverage on the wing. The results indicate that riblets could provide an overall reduction of skin friction drag, and that the amount of the decrease varied with riblet height and geometry. Partial riblet coverage appears, in some cases, to be more efficient than its complete coverage counterpart. The percentage of drag the riblets reduced varied greatly and in some cases the riblets were even detrimental to the airfoil. The most efficient riblet for a completely covered airfoil was found to be the V-groove shape of 100 μm height. It produced a reduction of roughly 6% in the operational range expected for a turbine airfoil. On the other hand, the most efficient riblet size for partial coverage was also a V-groove shape but had a slightly smaller peak height of 80 μm. This configuration produced a reduction of roughly 4% in the range of angle of attack that is typical for operation in the field. Our testing was limited to one size airfoil. However, comparison of our test data with recent numerical simulations by A-Mayoral and Jimenez suggest that an optimal size riblet configuration can be expressed in normalized form, involving only Reynolds number and the drag coefficient at design angle of attack.