Mitigation of Airfoil Boundary-Layer Transition due to Leading-Edge Roughness Using Shielding Strips

Abstract

Boundary-layer transition in the wake of a supercritical discrete roughness element (DRE) results in a turbulent wedge that adversely affects aerodynamic performance. Even when surfaces are designed for laminar flow, roughness that accumulates near the leading edge can cause premature transition and degrade performance. One approach for mitigating such transition is to place small two-dimensional shielding strips upstream or downstream of potential DRE accumulation sites. Shielding strips have been shown to be effective in zero-pressure-gradient, flat-plate boundary layers. This work documents the shielding performance on the leading edge region of a NACA 63(3)-418 airfoil where the pressure gradient is strongly favorable. IR thermography assesses the laminar-or-turbulent state of the boundary layer behind multiple DREs located just aft of the leading edge, with and without shielding strips. Optimal upstream and downstream shielding configurations and the performance improvement of combining these two strategies simultaneously are determined. Complementary Direct Numerical Simulations provide validation and additional insights into the mechanisms.