FLOW CONTROL OVER A RAMP USING ACTIVE VORTEX GENERATORS. C. Cuvier 1,2 , C. Braud 1,3 , J.M. Foucaut 1,2 , M. Stanislas 1,2 Univ Lille Nord de France 1 F-59000 Lille, EC Lille 2 , CNRS 3 , Laboratoire de M ´ ecanique de Lille (UMR 8107) Boulevard Paul Langevin, 59655 Villeneuve d’Ascq C´ edex, France. christophe.cuvier@gmail.com caroline.braud@univ-lille1.fr jean-marc.foucaut@ec-lille.fr michel.stanislas@ec-lille.fr ABSTRACT A parametric study of separation control using continuous jets vortex generators was conducted on a two- dimensional ramp with a mild adverse pressure gradient on a 2 m flat plate and a flow separation on a flap. Two jets diam- eters were investigated : 6 and 12 mm. For both diameters, co and counter-rotating arrangements were analysed. The control efficiency was quantified by wool-tufts visualisations and by four friction probes placed on the flap. It was found that a skewness of the output voltage of a friction probe greater than -0.4 is characteristic of flow reattachment. Dif- ferent spacing between jets, different pitch angles, different distances of the jets to the separation line and different VR were tested. The best configuration obtained is a counter- rotating one, with Φ δ = 0.03, λ Φ = 27.3, L Φ = 15 and α = 135 ◦ . Key words : Turbulent boundary layers, adverse pres- sure gradient, flow separation, control, continuous jets. INTRODUCTION Turbulent Boundary Layer (TBL) separation induces by strong adverse pressure gradient (APG) or by sudden discon- tinuity of curvature can lead to a drop in efficiency of a turbo- machinery or to a loss of aircraft control. In a way of im- proving continuously the performances and the safeness of all the machineries that interact with fluids (aircraft, turbo- machineries, cars, etc.), preventing and/or controlling turbu- lent boundary layer flow separation seems to be a crucial point that has to be solved. Since the beginning of the 1990s, many studies were per- formed on flow separation control (see Lin et al., 1990; Lin et al., 1991; Lin, 1999; Selby et al., 1992; McManus et al., 1994; Godard and Stanislas, 2006a; Godard and Stanislas, 2006b; etc.). Flow separation control experiments can be clas- sified in two types. The first one corresponds to passive con- trol strategies (Lin et al., 1990; Lin et al., 1991; Lin, 1999; Godard and Stanislas, 2006a; etc.). The second one concerns active control (Selby et al., 1992; McManus et al., 1994; Go- dard and Stanislas, 2006b; etc.). The active control strategies can be divided also in two families. The first one concerns steady continuous jets vortex generators (VGs) (Selby et al., 1992; Godard and Stanislas, 2006b; etc.), and the second one concerns unsteady VGs (like pulsed-jets in McManus et al., 1994; etc.). Good reviews of control strategies can be found in GadelHak (2000) and Lin (2002). For real flow control applications, it seems that the active strategies are the most appropriate as on an aircraft, the actu- ators can be turned off when they are not necessary, to avoid any additional drag and reactive control (closed-loop) can be achieved. Round jets are popular active VGs (Godard and Stanislas, 2006b; Selby et al., 1992; McManus et al., 1994; etc.). Their control efficiency depends on many parameters such as the diameter, the orientation, the exit velocity, the ar- rangement (co or counter-rotating), etc. (see Compton and Johnston, 1991 or Godard and Stanislas, 2006b). Moreover, the flow where the actuators is embedded has significant in- fluence on the control results as the adverse pressure gradient tends to increase interactions between vortices and thus de- crease the control efficiency (Lin, 2002). This explains the existing disagreement between investigators on the optimal active control parameters. The experiment presented here was performed on a two dimensional ramp, designed for the AVERT (Aerody- namic Validation of Emission Reducing Technologies) FP6 EC project. The ramp was tuned such as a boundary layer with mild adverse pressure gradient develops on the 2 m flat plate. At the end of this flat plate, there is an imposed separation with a flap which is used to quantified the control efficiency. THE EXPERIMENT The wind tunnel facility and the ramp This parametric active control experiment has been con- ducted in the LML boundary layer wind tunnel at U ∞ = 10 m/s. A boundary layer develops on the 20 m long lower wall 1