City Research Online

Abstract

Windtunnel tests have been conducted on a two-dimensional model of a three element high lift system in a take-off configuration in City University's T2 low speed windtunnel. The high lift system was mounted between endplates and consisted of a leading edge Handley Page slat and a trailing edge Fowler flap. Endplate boundary layer control ensured two-dimensional conditions up to and beyond the stall incidence of the high lift system and was provided by blowing through two near tangential slots located flush in each endplate adjacent to the main wing element. The windtunnel tests involved:

♦ monitoring the pressure distribution around each element of the multiple element aerofoil
♦ investigating the structure of the shear layers above the main wing and flap elements of the high lift system.

Boundary layer separation was first seen at the trailing edge of the main wing and developed steadily with incidence. The stall of the high lift system coincided with the rapid divergence of the static pressure at the trailing edge of the flap. However, the stall was preceded by a loss of load at the rear of the main wing attributable to the adverse viscous effects of the confluency of the main wing upper surface boundary layer and the slat wake, resulting in a substantial growth in the thickness of the shear layers above the main wing and the appearance of separated flow on this element first.

Various vane vortex generator configurations were tested on the upper surface of the main wing of the high lift system. Each configuration had a favourable influence on the suppression of separation at the rear of the main wing. However, for all configurations the generators had an adverse effect on the normal force coefficient generated by the high lift system at incidences below the stall incidence of the cleanfoil, resulting from an increased displacement effect of the shear layers above the upper surface of the main wing.

A system of co-rotating airjet vortex generators installed in the main wing and utilizing a constant blowing pressure of 60% above freestream stagnation pressure significantly increased the maximum total normal force coefficient of the high lift system. The high lift system also exhibited an increase in total normal force coefficient at all incidences below the stall incidence of the cleanfoil, resulting from a reduced displacement effect of the shear layers above the upper surface of the main wing. A reduction in the profile drag of the high lift system can be inferred from a lower value of momentum defect in the shear layers above the flap at all incidences.

The improvements achieved by the airjets were significantly greater than those produced with the various vane vortex generator configurations and cannot be attributed to just the suppression of boundary layer separation at the rear of the main wing. The airjets and associated vortices which they generate promote enhanced mixing and momentum transfer across the complex shear layers above the main wing in ways which cannot be achieved with vane generated vortices.

It is felt the low Reynolds number and Mach number of the tests do not detract from the fundamental fluid processes at work and hence the applicability of airjets to high lift systems. However, further work needs to be done to establish the balance between aerodynamic benefit and the performance cost of installing and driving the jets.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses School of Science & Technology > Engineering

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