Mitigation of Roughness-Induced Boundary-Layer Transition on Airfoils Using Shielding Strips

Abstract

Boundary-layer transition in the wake of a discrete roughness element results in a turbulent wedge that can adversely affect 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 spanwise-oriented shielding strips upstream or downstream of potential roughness elements accumulation sites. Shielding strips have been shown to be effective at eliminating transition in zero-pressure-gradient, flat-plate boundary layers. This work documents the shielding performance near the leading edge of an NACA 63(3)-418 airfoil where the pressure gradient is strongly favorable. Infrared thermography is used to evaluate the laminar or turbulent state of the boundary layer behind multiple roughness elements with and without shielding strips. The performance of various upstream and downstream shielding configurations is evaluated, as well as the performance improvement of combining these two strategies simultaneously. Direct numerical simulations provide validation and additional insight into the mechanisms responsible for the shielding phenomenon.