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Numerical simulation of active separation control by a synthetic jet

Published online by Cambridge University Press:  15 February 2007

JULIEN DANDOIS ,
ERIC GARNIER  and
PIERRE SAGAUT
JULIEN DANDOIS
Affiliation:
ONERA, Applied Aerodynamic Department, BP 72, 29 av. de la division Leclerc, 92322 Châtillon Cedex, France
ERIC GARNIER
Affiliation:
ONERA, Applied Aerodynamic Department, BP 72, 29 av. de la division Leclerc, 92322 Châtillon Cedex, France
PIERRE SAGAUT
Affiliation:
Laboratoire de Modélisation en Mécanique, Université Pierre et Marie Curie – Paris 6, Boite 162, 4 Place Jussieu 75252, Paris cedex 5, France ONERA, CFD and Aeroacoustics Department, 92322 Châtilon Cedex, France
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Abstract

Direct numerical simulation (DNS) and large-eddy simulation (LES) are carried out to investigate the frequency effect of zero-net-mass-flux forcing (synthetic jet) on a generic separated flow. The selected test case is a rounded ramp at a Reynolds number based on the step height of 28 275. The incoming boundary layer is fully turbulent with Rθ=1410. The whole flow in the synthetic jet cavity is computed to ensure an accurate description of the actuator effect on the flow field. In a first step, DNS is used to validate LES of this particular flow. In a second step, the effect of a synthetic jet at two reduced frequencies of 0.5 and 4 (based on the separation length of the uncontrolled case and the free-stream velocity) is investigated using LES. It is demonstrated that, with a proper choice of the oscillating frequency, separation can be drastically reduced for a velocity ratio between the jet and the flow lower than one. The low frequency is close to the natural vortex shedding frequency. Two different modes of the synthetic jet have been identified. A vorticity-dominated mode is observed in the low-frequency forcing case for which the separation length is reduced by 54%, while an acoustic-dominated mode is identified in the high-frequency forcing case for which the separation length is increased by 43%. The decrease of the separation length in the low-frequency forcing case is correlated with an increase of the turbulent kinetic energy level and consequently with an increase of the entrainment in the separated zone. A linear inviscid stability analysis shows that the increase of the separation length in the high-frequency forcing case is due to a modification of the mean velocity profile suggested by Stanek and coworkers. The result is a lower amplification of the perturbations and consequently, a lower entrainment into the mixing layer. To our knowledge, it is the first time that Stanek's hypothesis has been assessed, thanks to numerical simulations of fully turbulent flow.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 574 , 10 March 2007 , pp. 25 - 58
Copyright
Copyright © Cambridge University Press 2007

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