Investigation of the Flowfield Downstream of a Discontinuous Backward-Facing Step on a Swept Flat Plate

Abstract

An experimental study of a swept flat plate has been performed in the University of Wash ington 3’ by 3’ Low-Speed Wind Tunnel to improve the understanding of iced swept-wing flow fields. 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-wing ar tificial ice shapes while reducing the geometric complexity of 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 and solid features. Configurations of the flat plate were tested at four trailing-edge flap settings, and three Reynolds numbers based on step height of 2.50×104, 3.78×104, and 5.03×104, which correspond to Mach numbers of 0.088, 0.132, and 0.176. The experimental techniques used included fluorescent-oil surface flow visualization, surface pressure measurements, and five-hole pressure probe measurements. From measurements made using a five-hole pressure probe, it was shown that the trailing edge flap had significant effects on the local flow angularity about the leading-edge. These angu ii larity changes varied across the span and had visible effects on the resulting oil flow visualization. Type II streamwise flow features were identified as having one of two sets of traits using two oil flow visualization techniques, For configurations with negative flap deflection, the flowfield be hind the step was dominated by clear streamwise streaks, each appearing to emanate from a gap in the step. A detailed surface oil flow visualization technique presented evidence that this type of streamwise feature may be the result of pairs of counter-rotating streamwise vortices that form in the wake behind each solid feature in the step. This composition was distinct from the second type, which was seen at high flap angles in the oil flow as a superposition of a leading-edge separation vortex and streamwise streaks of vortical flow. These streaks were larger than the first case, and did not correspond to any single gap or solid feature. This behavior is similar to the flowfields as sociated with certain low-fidelity representations of scallop artificial ice shapes. The second type was hypothesized to be a result of a streamwise instability in the separated shear layer; however, additional work is required. A coating of 24 grit on the solid step produced small effects in the pressure distribution and tended to increase the mean shear layer reattachment length behind the step. Finally, the effect of Reynolds and Mach number was seen to be small, both in the pressure measurements and in the oil flow visualization. This result was in agreement with past research on iced-wing aerodynamics and swept backward-facing steps.