Transition on a Wing with Spanwise Varying Crossflow and Linear Stability Analysis

A new test case for challenging current transition prediction methods for three-dimensional boundary layers is designed with special focus on the effect of spanwise varying mean flow. The wind-tunnel model of this test case has a sickle-shaped planform. It is equipped with pressure taps and a special skin layup to increase the contrast of infrared transition measurements. The wing is designed to generate three-dimensional boundary layers with increasing crossflow toward the wingtip. The sickle-shaped planform with distinct kinks creates spanwise gradients, and the assumptions of linear local stability theory are therefore challenged. It is found that, at locations of high spanwise gradients, the transition location moves upstream, due to strong amplification of stationary crossflow vortices. Hot-film measurements are used to identify and characterize traveling modes, and to quantify disturbance amplifications. The wing exhibits areas of rapidly growing Tollmien–Schlichting modes, which behave in good agreement with linear stability theory, whereas crossflow-dominated regions display both stationary and traveling crossflow modes.