Measurements of Crossflow Vortices and Tollmien-Schlichting Waves on a Slotted, Natural-Laminar-Flow Airfoil

Wing sweep can increase critical Mach number but also increases the risk of laminar-to-turbulent transition due to the crossflow instability. At higher pitch angles, adverse pressure gradients on the suction side of the wing destabilize the Tollmien-Schlichting (TS) instability. Numerical prediction tools typically consider each of these modes independently. To build confidence in the reliability of numerical prediction tools and understand the factors that could lead to simultaneous TS and crossflow modes, more information is needed. The objective of this work was to explore disturbance conditions in which TS or stationary crossflow modes could exist. Different sets of discrete roughness element (DRE) arrays, 2-D roughness strips, and tuned sound were applied to target each mode. Hotwire anemometry surveys were used to measure the boundary layer’s response. In agreement with trends in the literature, crossflow vortices were observed when DRE arrays of sufficient roughness-Reynolds numbers were applied. TS waves were observed when 2-D strips were applied. Tandem-applied DREs and 2-D roughness initiated simultaneous crossflow and TS modes. Pockets of higher-amplitude TS waves were observed to coincide with vortices. The contribution of each roughness to unsteady spectra was quantified.