An experimental investigation of a gapless high-lift system using circulation control K. C. Pfingsten, R. D. Cecora and R. Radespiel Institute of Fluid Mechanics (ISM), Technische Universit¨at Braunschweig Bienroder Weg 3, 38106 Braunschweig, Germany K.C.Pfingsten@gmail.com Abstract When air is blown from a slot directly upstream of a flap, the flow over the flap can bear large adverse pressure gradients without separation. This effect is used to design high-lift airfoils with low momentum coefficients of blowing. For experimental assessment of these airfoils a rectangular wing with an aspect ratio of 4.3 was built. The flow around the model in a low speed wind tunnel is analysed using pressure measurements and long distance microscopic particle image velocimetry. To measure the velocity in the vicinity of the slot and next to the surface of the flap the jet is seeded with particles. For Reynolds numbers of about Re = 1 · 10 6 the dimensionless momentum coefficient of the jet and the angle of attack of the airfoil are varied. Numerical simulations of the three-dimensional flow around the circulation control airfoil in the wind tunnel are compared to the experimental data. Good agreement is observed in terms of pressure distributions and velocity profiles. 1 Introduction In recent years noise pollution from aircraft, especially around airports, has become a huge problem. Hence there is an increasing interest in reducing the noise emitted during take off and landing. The conventional high-lift systems, consisting of slats and slotted flaps, are a major contributor of airframe noise. Therefore a gapless high-lift system without slats has a potential of reducing the overall noise emitted by an aircraft. With active flow control a gapless high-lift device is capable of generating the high lift coefficients needed for climb and landing. For circulation control a small fraction of the cold engine flow is used for blowing. The bleed air is pipelined from the engine to a slot directly upstream of the flap and thus the flow over the flap can bear large adverse pressure gradients without separation. Thus a gapless high-lift device with circulation control can generate the required lift. The low drag coefficients during climb-out, achievable with this powered high-lift system, could also allow for new low-noise trajectories, which would further reduce noise impact on the ground. The absence of slats might allow for laminar flow conditions in cruise flight, thereby reducing the drag in this flight segment. Even taking into account the additional system weight associated with the bleed air distribution for a gapless high-lift system, there is a chance of reducing the total weight of the aircraft and possibly the cost, because slats and fowler systems are no longer needed. The first experiments using blowing to improve lift were conducted in the thirties of the last century by Bamber [1] as well as Hagedorn and Ruden [2]. Circulation control was first proposed for the flow over a circular cylinder by Davidson [3] and then applied to elliptical airfoils by Kind and Maul [4]. Elliptical airfoils utilising circulation control were also investigated by Stevenson et al [3] and Novak et al [6]. At Technische Universit¨at Braunschweig systematic measurements and theoretical considerations for wings with blown flaps by Thomas [7] and K¨orner [8] yielded lift increase versus necessary momentum coefficients. The elliptical airfoil with circulation control as well as the internally blown flap were extensively investigated by Englar [9] who could demonstrate good lift-over-drag performance. The first aircraft to demonstrate the high- lift capability of circulation control was a technology demonstrator built by Loth [10]. An experimental investigation using particle image velocimetry to assess the flow around a circulation control airfoil with an elliptical trailing edge as well as the flow around an airfoil with a flap was conducted by Jones et al [11]. These configurations were assessed numerically by Baker and Paterson [12] using two-dimensional RANS simulations. Large-eddy simulations for an elliptical profile with circulation control were performed by Slomski et al [13]. Due to the promising results of preceding numerical simulations by the authors [14] further experimental investigations were conducted to analyse an airfoil with circulation control [15]. As in the preceding exper- 1