Large Eddy Simulation Study of Synthetic Jet Frequency and Amplitude Effects on a Rounded Step Separated Flow P.-Y. Pamart 1 , J. Dandois 2 and E. Garnier 3 ONERA, Meudon, France, 92190 and P. Sagaut 4 Université Pierre et Marie Curie, Paris, France, 75252 Paris Cedex 5 Numerical simulations of active separation control by means of synthetic jet are carried out to study the forcing frequency and amplitude effects on the flow. The chosen test case is a rounded ramp at a Reynolds number based on the step height of Reh = 30192. The incoming flow is fully turbulent with Re θ = 1350 at the separation point. The whole flow in the synthetic jet cavity is computed to ensure an accurate description of the actuator effect on the flow-field. Through 21 LES simulations, it is shown that the optimal frequency is relative to the considered objective. In the present case, no frequency simultaneously improves every evaluated criterion. The effect of amplitude seems to be independent of the frequency one. I. Introduction Separated flows occur in a variety of engineering applications and has generally a negative impact on performance. Recently, numerous attempts have been made to control turbulent separated flows in order to improve the aerodynamic performance. Among all the available actuators for flow control, synthetic jets appear to be promising since they have been proven to effectively control separation (Seifert et al. [24]), enhance mixing (Chen et al. [3]) and vector thrust (Smith & Glezer [26]). The advantage of synthetic jets over steady blowing or suction is that they need less momentum by one or two orders of magnitude to produce equivalent effects (Seifert et al. [24]). They also do not require complex plumbing system because the momentum expulsion is only due to the periodic motion of a diaphragm or a piston on the lower wall of a cavity. Among all synthetic jet actuation parameters (frequency, amplitude, position, injection angle, ...), the forcing frequency is one of the most sensitive. It depends on the flow and if not well chosen, actuation can have no effect or worse, a deleterious impact on the separated flow. The jet amplitude needs also to be chosen appropriately in order to satisfy synthetic jet formation criterion and contribute significantly to the forcing efficiency. Frequency and amplitude are the two parameters on which we focus on in this study. Active separation control over ramps or backward-facing steps have been studied experimentally and numerically by many authors. Bhattacharjee et al. [2], Chun & Sung [4], Yoshioka et al. [30,31], Wengle et al. [29] and Dejoan & Leschziner [10], have reported a reduction of the recirculation length by about 30 % for an optimal Strouhal number Sth ≈ 0.2. This frequency does not correspond to the low frequency “flapping” of the shear layer ( Sth = 0.02, see Chun & Sung [4]) but rather to the mixing layer shedding mode. This is what we call the low frequency forcing strategy F  =fL sep / U ∞ =O  1 . Chun & Sung [5] have found a higher optimal forcing Strouhal number Sth = 0.477 and attribute this discrepancy to the low Reynolds number of their experiment. Liu et al. [16] have also found a slightly higher optimal Strouhal number Sth ≈ 0.275. Neumann & Wengle [19] have performed a large-eddy simulation on separated control over a rounded step. They have also noticed an optimal Strouhal number Sth ≈ 0.2. 1 Ph.D. Student, Applied Aerodynamic Department, 8 rue des Vertugadins, pierre-yves.pamart@onera.fr. 2 Research Engineer, Applied Aerodynamic Department, 8 rue des Vertugadins, julien.dandois@onera.fr. 3 Research Engineer, Applied Aerodynamic Department, 8 rue des Vertugadins, eric.garnier@onera.fr. 4 Professor, Institut Jean Le Rond d'Alembert, Boîte 162, 4 Place Jussieu; also Consulting Scientist, CFD and Aeroacoustics Department, ONERA, 92322 Châtillon Cedex, France. American Institute of Aeronautics and Astronautics 1 5th Flow Control Conference 28 June - 1 July 2010, Chicago, Illinois AIAA 2010-5086 Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.