The Swirl in an S-Duct of Typical Air Intake Proportions

Summary

The paper presents the results of measurement of the static pressure, total pressure and swirl in the flow through an S-shaped duct of typical air intake proportions mounted in a wind tunnel and tested at different incidences and different through-flow ratios. In order to reduce the magnitude of swirl at high incidence, two methods have been studied - one, to change the distribution of pressures by means of a spoiler and two, to re-energize the separated flow with an inflow of free stream air through auxiliary inlets. Measurements were also made, at 0° incidence, with a perforated spoiler intended to simulate roughly the taking in of low energy flow from the fuselage boundary layer. The results show a small degree of swirl at low incidence which takes the classical pattern of two contra-rotating flows, and a large degree of swirl at high incidence, in the form of a single rotating flow. Of the anti-swirl devices, the spoiler is the more powerful and can be sized either to reverse the swirl direction or to eliminate the swirl completely. A parameter of swirl coefficient, SC₆₀, has been suggested. Values of SC₆₀ at 30° incidence are 0.188 for the duct as designed, 0.068 for the arrangement with auxiliary inflow and −0.039 with a solid spoiler of width 0.15 times throat width. The various arrangements tested, together with results of an earlier study, shed useful light on the general nature of swirl in an S-duct, its method of generation and its final form and magnitude. A further experiment will be made on a duct having both a horizontal and a vertical offset.