Synthetic jet actuation strategies for momentumless trailing edge wake Maher Ben Chiekh a,n , Mohsen Ferchichi b , Marc Michard c , Mohamed Sadok Guellouz a , Jean-Christophe Be´ra d a LESTE, ENIM, University of Monastir, 5019 Monastir, Tunisia b Royal Military College of Canada, Kingston, ON, Canada c LMFA, Ecole Centrale de Lyon, 69134 Ecully Cedex, France d Universite´ Lyon 1, U1032 INSERM, Lyon, France article info Article history: Received 3 June 2012 Received in revised form 22 November 2012 Accepted 7 December 2012 Available online 24 January 2013 Keywords: Trailing edge actuation Synthetic jet Momentumless wake Vortex synchronization Momentum injection Vortex-shedding suppression abstract Periodic addition of momentum employing a Trailing Edge Synthetic Jet Actuation (TESJA) to con- trol the wake of a blunt trailing edge plate was investigated experimentally at Reynolds number Re h ¼U N h/n ¼7200, based on the trailing edge thickness, h. A pair of synthetic jets were symmetrically placed at the model base. Two-component PIV measurements were reported for natural wake and actuated wakes under different actuation strategies namely asymmetric actuation, symmetric synchro- nous dual-actuation, and out-of-phase dual-actuation. A pair of synthetic jets were symmetrically placed at the model base. Two positions of the TESJA on the model base (peripheral or central position) were also studied and discussed. The forcing was applied at a frequency of 0.75 the natural vortex shedding frequency, while a high momentum addition coefficient of up to 24% was applied to balance the momentum deficit and to accomplish a near momentumless state. The measurements revealed that the application of TESJA reduced the extent of the separation bubble and redistributed energy among the Reynolds stresses. A perfect lock-in of vortex shedding in the wake was achieved. The actuation efficiency depended on both the applied control strategy and the actuators’ positions. Forcing was found to be most effective in achieving a near momentumless state when applied at base model extremities (peripheral configuration) to permit the interaction of synthetic jet with the neighbouring shear layers and a large entrainment. The above strategies and positions led generally to three actuated wake states: (1) vortex pair shedding vectoring for asymmetric forcing, (2) symmetric vortex shedding under in-phase synchronous forcing, and (3) alternate vortex shedding with increased vorticity and turbulence, combined with a large momentum addition when out-of-phase forcing was applied. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Fundamental understanding and the ability to control wake flows is of great importance in a wide range of applications in particular in wind engineering and in optimisation of aerodynamic devices. Despite their practical importance notably in current ground vehicles, elongated bluff body wakes have received con- siderably less attention than those of cylinder. The latter is a simple and classical configuration that has been extensively studied in the wake control literature. Control strategies are aimed at delaying the separation of the boundary layer that develops on the continuous surface and preventing the alternate shedding from top and bottom edges, thus postponing the formation of a vortex shedding (Griffin and Hall, 1991; Sakamoto and Haniu, 1994; Be´ ra et al., 2000; Glezer and Amitay, 2002). As an example, a detailed study of the effect of synthetic jet actuation on a cylinder can be found in Be´ ra et al. (2000). Additionally, Glezer and Amitay (2002) empha- sized the importance of the actuator azimuthal position which leads to different actuated flow configurations and discussed the respective contributions of the blowing and suction phases of the synthetic jet actuation. For a sharp-edged bluff body, however, the separation point is fixed and the boundary layer control becomes ineffective. Moreover, control strategies upstream of separation are unlikely to be effective and may produce reverse effects as indicated in Akansu et al. (2004). Direct wake-control strategies are the most promising. Several studies provided insight into the physical mechanisms involved in blunt trailing edge wakes. These wakes were governed by the Karman vortex shedding and their flow control passes through the control of this instability. To achieve this, two possibilities exist as described by Bearman (1965) and Henning and King (2005): the first involves the suppression of the vortex shedding so that the rolling up of detrimental vortices starts Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jweia Journal of Wind Engineering and Industrial Aerodynamics 0167-6105/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jweia.2012.12.002 n Corresponding author. Tel./fax: þ216 73907176. E-mail address: maher.benchiekh@topnet.tn (M. Ben Chiekh). J. Wind Eng. Ind. Aerodyn. 113 (2013) 59–70