Influence of 2D Steps and Distributed Roughness on Transition on a NACA 63(3)-418
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Wind farm operators observe power production decrease over time. Quantifying performance degradation on individual components is difficult, exacerbating the problem. One potential explanation is accumulation of blade surface roughness, as wind turbines are continuously subjected to environmental hazards. Historically, wind turbine airfoils were designed for lift to be insensitive to roughness by simulating roughness with 2D trip strips. However, roughness was still shown to negatively affect airfoil performance. Experiments have also illustrated that random-distributed roughness is not properly simulated by trip strips. To better understand how real-world roughness affects performance, field measurements of turbine-blade roughness were made and simulated on an airfoil section in a wind tunnel. Of the observed roughness types, 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 two density configurations. The model chord Reynolds number was varied between 0.8 to 4.4×106. Measurements of lift, drag, and pitching moment were completed. Transition location was determined using both infrared thermography and hotfilm anemometry. Results indicate minimal performance loss due to paint-chip roughness. At Rec = 2.4×106, L/Z)max decreased 40% for the dense 140 /xm, sparse-extended 140 /xm, and sparse 200 /xm roughness. This indicates that both density and height are critical performance parameters. Lastly, all but one configuration had Rek,crit within predictions from literature.
author list (cited authors)
Ehrmann, R. S., & White, E. B.