Aerospace Science and Technology 13 (2009) 36–48 www.elsevier.com/locate/aescte The effects of pulse frequency and duty cycle on the skin friction downstream of pulsed jet vortex generators in an adverse pressure gradient turbulent boundary layer J. Kostas ∗ , J.M. Foucaut, M. Stanislas Laboratoire de Mécanique de Lille, Bd. Paul Langevin, Cité Scientifique, Villeneuve d’Ascq, 59655, France Received 13 March 2007; received in revised form 6 March 2008; accepted 6 March 2008 Available online 20 March 2008 Abstract An investigation on the viability of pulsed jets as active vortex generator devices was conducted. The devices were installed and tested on an adverse pressure gradient turbulent boundary layer designed to simulate the suction side of a conventional aircraft wing. Both co-rotating and counter-rotating jet geometries were used. The duty cycle and frequency of pulsation were varied and their effects were investigated by measuring the skin friction gains at a predefined location (the location of the minimum skin friction for the un-actuated situation) on the adverse pressure gradient turbulent boundary layer. Pulsing the jets proved to be successful in increasing the wall skin friction and therefore potentially delaying separation. The improvements in wall shear stress were approximately proportional to the duty cycle. The frequency of jet pulsation was found to be important for attaining optimal gains, however no clear relationship between frequency and shear stress gain was observed. Phase averaged wall shear stress measurements far downstream of actuation indicate that quasi-steady structures are introduced by the vortex generators when actuating with a sufficiently high pulse frequency. In this situation interactions between successive structures produced by the jets were likely to be occurring.  2008 Elsevier Masson SAS. All rights reserved. Keywords: Flow control; Pulsed jets; Vortex generators; Wall shear stress; Adverse pressure gradient turbulent boundary layer 1. Introduction Pulsed jet vortex generators (VGs) have been gaining widespread interest for use in flow control experiments. Their principal benefit is a reduced mass flow compared to VGs that are operated continuously at similar conditions. Continuous jet VGs have been shown to beneficially modify the wall shear stress [10,11], however the effect of jet pulsation (i.e. duty cy- cle <100%) on the wall shear stress or structure of the turbulent boundary layer structure requires further investigation. It is now well established that active VGs have a similar ef- fect on the flow structure to passive VGs [7,9–11,16] with the added advantage of no parasitic drag when they are not required e.g. for take-off, landing and dynamic manoeuvres. Active VGs therefore lend themselves to the possibility of being used within * Corresponding author. E-mail address: dimitrios.kostas@eng.monash.edu.au (J. Kostas). a closed loop control system, as demonstrated by Jacobson and Reynolds [14] and Rathnasingham and Breuer [26]. Both active and passive types of VGs generate streamwise vortex structures which entrain high momentum fluid towards the wall, hence energising the boundary layer, increasing wall shear stress and potentially delaying separation. Increases in wall shear stress obtained whilst using a duty cy- cle less than 100% are of interest to the aircraft industry, since the flowrate through the actuators and hence energy would be conserved in comparison to using a continuous jet arrangement. The power expended in actuating the valves must also be taken into consideration in order to obtain the net benefit over the un-actuated and continuous jet configurations. Additionally, the size and weight of the pulsing apparatus must also be consid- ered in aircraft applications. Table 1 presents a summary of pulsed jet, active VG control experiments in the literature. A number of experiments have been conducted on an isolated, pulsed jet in a turbulent bound- 1270-9638/$ – see front matter  2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ast.2008.03.002