Nonlinear Boundary-Layer Stability Analysis of a Swept, Natural-Laminar-Flow Airfoil

Under the auspices of the NASA University Leadership Initiative, an effort is underway to design a swept, slotted, natural-laminar-flow (SNLF) airfoil for transonic flight. In order to accomplish this, computational validation must be performed with experimental data. As part of a risk-reduction experiment, an analogous SNLF airfoil referred to as the X207.LS has been designed, built, and tested in the Klebanoff-Saric Wind Tunnel. The objective of this experiment is to excite relevant instability mechanisms that will be present in various flight configurations. The considered angle of attack is-5.5 degrees and sweep of 35 degrees to induce a favorable pressure gradient, exciting strong crossflow. For discrete roughness elements of a wavelength 12.7 mm and diameter 6.35 mm, computations with the nonlinear Parabolized Stability Equations (PSE) simulate the experimental measurements of a hot wire traversed across the wing at various streamwise stations. A heuristic approach is used to select the initial amplitudes of the fundamental stationary crossflow wavelength and the superharmonic amplitude, which agrees well with the observed measurements. Comparing results to the linear PSE, it is seen that the flowfield’s dynamics are, for a large extent of the chord, mainly determined by the linear superposition of the two wave-like disturbances, i.e., nonlinearities are rather weak in the measured flowfield at these conditions.