Boundary-Layer Stability Measurements of a Variably Swept and Pitched, Slotted, Natural-Laminar-Flow Airfoil

Wing sweep increases critical Mach number but also increases the risk of laminar-to-turbulent transition due to the crossflow (CF) instability. This is particularly the case on natural-laminar-flow (NLF) airfoils that are designed with favorable pressure gradients meant to stabilize the Tollmien-Schlichting (TS) instability. As sweep increases, there may be conditions when either instability mode can grow. To build confidence in the reliability of numerical prediction tools which typically only consider instabilities independently, more information is needed about coexisting modes and the parameter space boundaries where modes can exist. The objective of this work is to explore what instability modes can exist on an NLF model positioned at various sweep and pitch angles such that either or both TS and CF can be unstable. Cooperative stability computations helped determine experiment conditions. 2-D roughness and discrete roughness element (DRE) arrays were applied to initiate instability growth. Flow visualizations and velocity measurements were compared to no-roughness baselines to assess whether an instability had emerged. In agreement with previous experiments, DRE arrays of roughness-Reynolds number were conducive to CF formation. One notable finding is that, when both modes were amplified, the combination of upstream 2-D roughness plus a downstream DRE array was more conducive to CF formation than only a DRE array.