Aerodynamics of Airfoils with Leading Edge Roughness

Abstract

Leading edge roughness is an important challenge for the wind energy industry as blades are permanently exposed to the damaging impacts of the environment. The contamination due to impact of insects, ice accretion and dust; as well as the erosion of the blade due to rain, debris and abrasive airborne particles increase roughness upon the surface. Leading edge roughness leads to the deterioration of aerodynamic performance, including early transition from laminar to turbulent flow, increased drag, and reduced lift. These performance losses directly translate into a decreased annual energy production (AEP) and increased levelized costs of energy (LCoE). The aim of this study is to enhance the understanding of airfoil aerodynamics with LER, specifically focusing on the usage and impact of tripping tapes as a standard representation of LER in wind tunnel campaignsb by deviating proven zigzag tapes in non-standard setups. The research objectives include investigating critical roughness height, analyzing the impact of different tripping tape setups, studying drag generation in the boundary layer, and exploring the effects of real erosion.
Contrary to expected reference values, the critical height was found to be traceable with the Roughness Reynolds number for zigzag tape usage as Rek = 125, opposing to expected critical Reynolds number Rek = 200 in current literature. Most interesting application parameter after
critical condition, often related to height, is found as tape location. Drag and lift performance penalties behave as linear function of tape location for pre-stall drag bucket and linear lift region. In stall, the performance penalty is heavily related to locations closer to the leading edge with increased importance of tape height. Additionally, real roughness of reference studies has been more closely resembled by tripping tapes closest to the leading edge in terms of magnitude and as function of pitching angles when compared to industry standards. The boundary layer measurements have indicated that panel code simulation data has errors connected to transition location rather than boundary layer development, which reflects in polar data and transition location.

Overall, this study emphasized that ZZT is more versatile than the industry standard setup, highlighting the importance of considering different application variations for accurate assessments of LER impact, while indicating possible panel code improvements related to transition location
as observed in performance polars and boundary layer thickness.