Investigating Iced Swept-Wing Flowfields Using a Swept Flat Plate and a Discontinuous Backward-Facing Step

An experimental study of a swept flat plate has been performed in the University of Washington 3’ by 3’ Low-Speed Wind Tunnel to improve the understanding of iced swept-wing flowfields. The swept flat plate model included two interchangeable backward-facing steps that were mounted to the full span of the plate leading edge, as well as an adjustable trailing-edge flap. Each of the steps was designed to recreate one of two key flowfields characteristic of swept wings with artificial ice shapes while reducing the geometric complexity relative to the swept wing model. The first was a spanwise-running leading-edge vortex produced by certain low-fidelity artificial ice shapes. This type of flowfield is referred to as Type I, and was produced using the solid backward-facing step. Characteristic streamwise-running streaks of oil identify the second flowfield in surface flow visualization. This flow pattern was referred to as Type II, and was recreated with a modified backward-facing step that included spanwise-periodic gaps. Configurations of the flat plate were tested at four trailing-edge flap settings at a Reynolds number based on step height of 5.03 × 10⁴, which corresponds to a Mach number of 0.176. The experimental techniques used included fluorescent-oil surface flow visualization, surface pressure measurements, and five-hole pressure probe measurements. Two tentative categories of Type II flowfield features were identified. In the first category, referred to as “vortex generatortype,” each Type II flowfield feature was correlated to a single discontinuity in the step. Flow visualization showed that these are likely the result of counter-rotating vorticity that forms in wakes of each solid feature in the discontinuous step. In the second category, referred to as “shear layer instability-type,” Type II features did not correspond to a single discontinuity. This behavior may be attributed to the discontinuities exciting an instability in the separated shear layer, or streamwise vorticity coalescing off of the flat plate surface.