Wind farm operators observe production deficits as machines age. Quantifying deterioration on individual components is difficult, but one potential explanation is accumulation of blade surface roughness. Historically, wind turbine airfoils were designed for lift to be insensitive to roughness by simulating roughness with trip strips. However, roughness was still shown to negatively affect performance. Furthermore, experiments illustrated distributed roughness is not properly simulated by trip strips. To understand how real-world roughness affects performance, field measurements of turbine-blade roughness were made and simulated on a NACA 63 3-418 airfoil in a wind tunnel. Insect roughness and paint chips were characterized and recreated as distributed roughness and a forward-facing step. Distributed roughness was tested in three heights and five density configurations. The model chord Reynolds number was varied between 0.8 to 4.8 × 10 6. Measurements of lift, drag, pitching moment, and boundary-layer transition were completed. Results indicate minimal effect from paint-chip roughness. As distributed roughness height and density increase, lift-curve slope, maximum lift, and lift-to-drag ratio decrease. As Reynolds number increases, bypass transition occurs earlier. The critical roughness Reynolds number varies between 178 to 318, within the historical range. Little sensitivity to pressure gradient is observed. At a chord Reynolds number of 3.2 × 10 6 , the maximum lift-to-drag ratio decreases 40% for 140 µm roughness, corresponding to a 2.3% loss in annual energy production. Simulated performance loss compares well to measured performance loss on an in-service wind turbine.

Wind farm operators observe power production decay over time, with the exact cause unknown and difficult to quantify. A likely explanation is blade surface roughness, as wind turbines are continuously subjected to environmental hazards. Difficulty arises in understanding and quantifying performance degradation. Historically, wind turbine airfoil families were designed for the lift to be insensitive to roughness by simulating roughness with 2D trip strips. Despite this, roughness is still shown to negatively affect airfoil lift performance. Experiments have also illustrated that random-distributed roughness is not properly simulated by trip strips. Therefore, to better understand how real roughness effects performance, field measurements of turbine-blade roughness were made and simulated on an airfoil section in a wind tunnel. This data will serve to validate and calibrate a one-equation, computational roughness amplification model that interacts with the Langtry-Menter transition model. The observed roughness contains 2D steps, heavy 2D erosion, pitting, insects, and repairs. Of these observations, 2D steps from paint chips were characterized and recreated for this particular wind tunnel entry. The model was tested at chord Reynolds numbers up to 3.6 × 10 6 . Measurements of lift, drag, and pitching moment were made with and without roughness contamination. Transition location was acquired with infrared thermography and a hotfilm array. The paint roughness yields a consistent increase in drag compared to the clean configuration. Numerical simulations are only compared to the clean configuration and match well to lift, drag, and transition for Rec = 1.6 × 10 6 . However, drag is overpredicted at Rec = 3.2 × 10 6 .

Over time it has been reported wind turbine power output can diminish below manufacturers promised levels. This is clearly undesirable from an operator standpoint, and can also put pressure on turbine companies to make up the difference. A likely explanation for the discrepancy in power output is the contamination of the leading edge due to environmental conditions creating surfaces much coarser than intended. To examine the effects of airfoil leading edge roughness, a comprehensive study has been performed both experimentally and computationally on a NACA 633 − 418 airfoil. A description of the experimental setup and test matrix are provided, along with an outline of the computational roughness amplification model used to simulate rough configurations. The experimental investigation serves to provide insight into the changes in measurable airfoil properties such as lift, drag, and boundary layer transition location. The computational effort is aimed at using the experimental results to calibrate a roughness model that has been implemented in an unsteady RANS solver. Furthermore, a blade element momentum code was used to assess the impact on the performance of a turbine as whole due to discrepancies in clean vs. soiled airfoil characteristics. The results have implications in predicting the power loss due to leading edge surface roughness, and can help to establish an upper bound on admissible surface contamination levels.

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