High-Resolution 3D CFD Simulations of Wind Turbine Aerofoils with 3D Scanned Leading Edge Roughness

The impact of leading edge roughness (LER) and erosion on wind turbine aerodynamic performance remains highly uncertain, due to a lack of high-resolution topographic roughness data of in-service blades. Within the LERCat project headed by DTU Wind a workflow was established that extracts roughness topographies from high-resolution 3D scans of real-world LER and applies them digitally to any other aerofoil surface that can be assessed aerodynamically using 3D CFD or wind tunnels. Two of the extracted roughness topographies - one with few spanwise distributed shallow damages (≤ 0.8 mm) towards the suction side and another with deep erosion (≤ 4.0 mm) all along the span - are applied to two commonly used wind turbine aerofoils with 21% and 25% relative thickness and evaluated with RANS CFD by highly resolving (≈100 µm) the erosion damages. The effect of roughness on boundary layer transition is evaluated by using the γ-Reθt transition model; simulations are performed at Reynolds numbers of 3 and 6 million. The aerofoils react similarly to the tested erosion, with the shallow damages triggering transition on the suction side over the entire span around operating angles-of-attack at 6 million. Pressure side transition is not affected as the stagnation point moves below the eroded region. The deep erosion on the other not only triggers transition, increasing viscous drag, but also strongly increases pressure drag when approaching stall. Furthermore 2D chordwise slices are extracted from the 3D aerofoil with LER and simulated to quantify the error from using 2D representations of 3D LER for evaluating roughness related aerodynamic performance loss.