Journal of applied science in the thermodynamics and fluid mechanics Vol. 4, No. 1/2010, ISSN 1802-9388 HEAT TRANSFER CONTROL OF SEPARATED AND REATTACHING FLOW BY LOCAL FORCING – EFFECT OF RICHARDSON NUMBER *Zouhaier MEHREZ, *Mourad BOUTERRA, *Afif EL CAFSI *Ali BELGHITH **Patrick LE QUERE *Faculté des sciences de Tunis Campus universitaire, 1060, Tunis, Tunisie Phone:+ 216 22880134 , + 216 97408388 Email: zouhaier.mehrez@yahoo.fr ** LIMSI-CNRS Bat. 508 - B.P. 133 - 91 403, Orsay Cedex, FRANCE A numerical study, using Large Eddy Simulation (LES) methodology, is performed to control, by local forcing, heat transfer of separated and reattaching flow over a backward facing step. The local periodic forcing is realized by sinusoidal oscillating jet at the step edge. The Reynolds number is fixed at 33000 and the Richardson number is varied in the range 0.1≤Ri≤1. The found results show that the flow structure is modified and the local heat transfer is enhanced by the applied forcing. The observed changes depend on the Richardson number and vary with the frequency and amplitude of the local forcing. For the all Richardson numbers, the largest augmentation of heat transfer is obtained at the optimum forcing frequency St = 0.25. At this frequency the local heat transfer enhancement is improved by increasing the forcing amplitude. Keywords: Backward-facing step; Large Eddy Simulation; Heat transfer; Local forcing 1 INTRODUCTION The fluid flow over a backward-facing step occurs in many engineering applications like gas turbine engines, heat exchangers, combustors, chemical reactors, electronic equipment, energy system equipment, environmental control systems and many others applications. A significant amount of mixing of high and low energy fluid occurs in the reattached flow region in these devices, thus affecting their heat transfer performance. Backward-facing step geometry is one of the most important benchmark problems used in computational fluid dynamics (CFD) to validate either a new algorithm or a computer code developed because of its rich flow physics in spite of its simple geometry. This same flow includes four typical zones of different types: a separated shear layer when the incoming fluid reaches the step edge, a recirculating flow region, formed by two counter-rotating vortexes, limited by the reattachment zone followed by a relaxation region. So, to control flow separation, many investigations by numerous authors have been conducted in fluids engineering. The suppression or desired control of separation phenomena has been addressed in the mechanics community for many decades. There has been much research interest on the periodically forced turbulent separated flow. Singurdson [1], Chun and Sung [2] are interested to the active control of the reattachment process, in which the enhancement of momentum transport across the separated shear layer plays a major role. Several authors are interested to the excitation of the instability and vortex formation inherent to the separated shear layer (Bhattacharjee et al. [3]; Kiya et al. [4]; Chun and Sung [5]; Yoshioka et al. [6]; Uruba et al. [7]; Mehrez et al. [8]). Among them, a couple of common results have been observed, for example, larger perturbation achieves larger flow modification so that the flow reattachment length is drastically minimized. Another point worth to note is that an optimum forcing frequency exists, that is, the largest flow modification or the minimum flow reattachment length is obtained at a certain forcing frequency. The main objective of the present study is to simulate, by the Large Eddy Simulation, the heat transfer enhancement in separated and reattaching flow over a backward-facing step by periodic local forcing. 2 GOVERNING EQUATIONS AND NUMERICAL METHODOLOGY In this paper, the L.E.S approach is used to solve the Navier–Stokes equations for its principle benefit: simulating only the large scales of turbulence and taking into account the small scales by a mixed subgrid- scale model proposed by Ta Phuoc [9] and Sagaut [10]. 1