Large eddy simulation of periodically perturbed separated flow over a backward-facing step A. Dejoan, M.A. Leschziner * Department of Aeronautics, Imperial College, Prince Consort Road, London, SW7 2AZ, UK Received 9 September 2003; accepted 24 March 2004 Available online 7 June 2004 Abstract Large eddy simulation is used to investigate the effects of a periodic perturbation introduced into a separated shear layer that borders a recirculation bubble behind a backward-facing step in a high-aspect-ratio channel. The perturbation is provoked by the injection of a slot jet, at zero net mass-flow rate, uniformly along the spanwise edge at which separation occurs. Attention is focused on one particular jet-forcing frequency, at the Strouhal number 0.2, for which experimental data show the perturbation to cause a maximum change to the properties of the unperturbed flow – in particular, the largest reduction in the time-mean recirculation length. Results are reported for time-mean and phase-averaged velocity and Reynolds stresses, and these are compared with experimental data. The time evolution of phase-averaged properties, including stream-function, pressure and turbulence energy, are investigated in an effort to identify the mechanisms responsible for the observed substantial changes to the time-mean properties. This is aided by a study of some spectral properties. The simulations are shown to reproduce the experimental observations, and these provide clear indications that the high level of sensitivity to the perturbation at the Strouhal number considered is due to a strong interaction between shear-layer instabilities, which are amplified by the perturbation, and shedding-type instabilities, which are induced by the interaction of large-scale structures developing downstream of the step with the wall, causing the shear-layer to flap. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Large eddy simulation; Turbulent recirculating flow; Backward-facing step; Separation control; Unsteady flow 1. Introduction The control of turbulent flows by external forcing, either steady or unsteady, is of much practical as well as fundamental interest. Methods of flow control in a practical context include skin-friction reduction by means of grooves and compliant surfaces, separation control on aircraft wings by means of solid vortex gen- erators and jets, the use of lobbed and serrated mixers in jet-engine exhausts to promote mixing and reduce noise, and the suppression of shedding from cylindrical bodies and blunt trailing edges of turbomachine blades by local blowing or the addition of geometric features such as dimples, bulges, serrations and spanwise fins. The fundamental mechanisms underlying the re- sponse of a flow to a controlling agency are as varied as the nature of the forcing, the type and scales of the phenomena to be controlled and the practical circum- stances. At the small-scale end of the range, surface-drag reduction by sub-boundary-layer features (of size y þ ¼ Oð10ÞÞ or transpiration appears to be effected by a modification of the structure of the near-wall streaks and their distance to the wall. At the much more ener- getic end of the range, strong jets, either steady or pul- sating, are injected into a baseline flow in order to introduce streamwise vorticity and to promote mixing across the entire boundary layer, often with the objec- tive of preventing separation under the influence of an adverse pressure gradient or of reducing the size of the separation zone. The present paper is concerned with the latter – specifically the control of reattachment and thus * Corresponding author. Tel.: +44-20-759-45061; fax: +44-20-758- 48120. E-mail addresses: a.dejoan@imperial.ac.uk (A. Dejoan), mike.lesc- hziner@imperial.ac.uk (M.A. Leschziner). URL: http://www.ae.ic.ac.uk/research/tfms/index.html. 0142-727X/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2004.03.004 International Journal of Heat and Fluid Flow 25 (2004) 581–592 www.elsevier.com/locate/ijhff