An experimental investigation of the flow in a diffusing S-duct

TLDR

The study discusses the separated flow region and the mechanisms driving this complex phenomenon. Measurements of 3‑D velocity, total and static pressures, and surface static pressures were taken at five cross‑sectional planes in a Mach 0.6, Re 2.6 × 10⁶ diffusing S‑duct. Experimental data reveal extensive streamwise flow separation and pressure‑driven secondary flows that form counter‑rotating vortices, degrading total‑pressure uniformity and magnitude, providing benchmark data for designers and code validation.

Abstract

Compressible, subsonic flow through a diffusing S-duct has been experimentally investigated. Benchmark aerodynamic data are presented for flow through a representative S-duct configuration. The collected data would be beneficial to aircraft inlet designers and is suitable for the validation of computational codes. Measurements of the 3D velocity field and total and static pressures were obtained at five cross-sectional planes. Surface static pressures and flow visualization also helped to reveal flow field characteristics. All reported tests were conducted with an inlet centerline Mach number of 0.6 and a Reynolds number, based on the inlet centerline velocity and duct inlet diameter, of 2.6 x 10(exp 6). The results show that a larger region of streamwise flow separation occurred within the duct. Details about the separated flow region, including mechanisms which drive this complicated flow phenomenon, are discussed. Transverse velocity components indicate that the duct curvature induces strong pressure driven secondary flows, which evolve into a large pair of counter-rotating vortices. These vortices convect the low momentum fluid of the boundary layer towards the center of the duct, degrading both the uniformity and magnitude of the total pressure profile.