Influence of 2D Steps and Distributed Roughness on Transition on a NACA 63(3)-418

Abstract

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 × 10 6 . 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 × 10 6 , L/Dmax decreased 40% for the dense 140 µm, sparse-extended 140 µm, and sparse 200 µm roughness. This indicates that both density and height are critical performance parameters. Lastly, all but one configuration had Rek,crit within predictions from literature.

Analysis of the Impact of Leading Edge Surface Degradation on Wind Turbine Performance

“…Lastly, transition location was indicated with IR thermography, further details regarding measurement techniques are provided by Ehrmann and White. 13,14 To assess the impact of distributed roughness, numerous different roughness configurations were tested. The roughness was simulated by generating randomly distributed circles in a 152.3 × 152.3 mm 2 area using circles with a normally distributed radius of 1.2 ± 0.15 mm.…”

Transition Detection for Low Speed Wind Tunnel Testing using Infrared Thermography

“…Furthermore, in the case of large metal models the increased model mass causes high material heat transfer rates which have the tendency to overshadow any heat transfer occurring due to flow effects. Models made of metals also create problems with reflections (Zuccher, 2008;Gompertz 2012;Ehrmann, 2014). According to Gompertz (2012) and (Ehrmann, 2014) metal models create "smearing effects" which impede the quality of the thermographs produced.…”

“…Models made of metals also create problems with reflections (Zuccher, 2008;Gompertz 2012;Ehrmann, 2014). According to Gompertz (2012) and (Ehrmann, 2014) metal models create "smearing effects" which impede the quality of the thermographs produced.…”

Aerodynamic Effects of Roughness on Wind Turbine Blade Sections

“…The transition occurring from isolated 3D roughness has been said to be more 'critical' than the 2D case because below the critical roughness Reynolds Number there appears to be no effect on transition but above this value bypass transition occurs (White, 2011). For distributed 3D roughness even less is known about the effect on transition due to difficulties in defining a 'typical' surface roughness (Ehrmann, 2014). One of the main reasons for this is that 3D distributed roughness is a "parameter-rich topic" (White, 2011) which leads to a more experimental approach to this problem.…”

“…Nonetheless the issue is still under ardent study because of the complex flow interactions that rough surfaces initiate and the wide spectrum of aerodynamic variations which can occur based on the roughness type. In relation to wind turbine blades, much focus in recent years has been on reducing the effect of roughness due to material erosion, ice accretion, foreign residues like insects, and coat deterioration (Sagol, 2013;Ehrmann, 2014;Dalili et al, 2009). Solutions to these problems have been attempted through methods to reduce the effect of the roughness elements (nonstick coatings, use of vortex generators, self-cleaning mechanisms) or by designing blade profiles which are less sensitive to roughness (Bak, 2008).…”