Transition process in separated–reattached flows plays a key role in many practical engineering applications. Hence, accurately predicting transition is crucial since the transition location has a significant impact on the aerodynamic performance and a fundamental understanding of the instability mechanisms involved in transition process is required in order to make significant advances in engineering design and transition control, for example, to delay the turbulent phase where laminar flow characteristics are desirable (low friction drag) or to accelerate it where high mixing of turbulent flow are of interest (in a combustor). The current understanding of instabilities involved in the transition process in separated–reattached flows is far from complete and it is usually very difficult to theoretically and experimentally study the transition process since theoretical studies suffer from the limitation imposed by nonlinearity of the transition process at later stages and experimental studies are limited by temporal and spatial resolution; hence, a thorough description of the transition process is lacking. Nevertheless, significant progress has been made with the simulation tools, such as large eddy simulation (LES), which has shown improved predictive capabilities and can predict transition process accurately. This paper will first briefly present LES formalism followed by its applications to study the transition process in separated–reattached flows, reviewing our current understanding of several important phenomena associated with the transition process and focusing on the instabilities in particular.
instability, large eddy simulation, separated–reattached fl ows, transition process
 Langtry, R.B. & Menter, F.R., Transition modelling for general CFD applications inaeronautics. AIAA 2005-522, Reno, Nevada, 2005.
 Smagorinsky, J., General circulation experiments with the primitive equations: I – thebasic experiment. Monthly Weather Review, 91, pp. 99–164, 1963. doi: http://dx.doi.org/10.1175/1520-0493(1963)091<0099:GCEWTP>2.3.CO;2
 Schmid, P.J. & Henningson, D.S., Stability and Transition in Shear Flows, Springer:New York, 2001. doi: http://dx.doi.org/10.1007/978-1-4613-0185-1
 Lesieur, M. & Metais, O., New trends in large eddy simulations of turbulence. AnnualReview of Fluid Mechanics, 28, pp. 45–82, 1996. doi: http://dx.doi.org/10.1146/annurev.fl .28.010196.000401
 Sagaut, P., Large Eddy Simulation for Incompressible Flows: An Introduction, 2nd edn,Springer: Berlin, 2003.
 Kajishima, T. & Nomachi, T., One-equation sub-grid scale model using dynamic procedurefor the energy production. Transaction of ASME, 73, pp. 368–373, 2006.
 Germano, P., Piomelli, U., Moin, P. & Cabot, W.H., A dynamic sub-grid scale eddyviscosity model. Physics of Fluids, 3(7), pp. 1760–1765, 1991. doi: http://dx.doi.org/10.1063/1.857955
 Lee, S., Lele, S. & Moin, P., Simulation of spatially evolving turbulence and theapplicability of Taylor’s hypothesis in compressible fl ow. Physics of Fluids A, 4,pp. 1521–1530, 1992. doi: http://dx.doi.org/10.1063/1.858425
 Druault, P., Lardeau, S., Bonnet, J.-P., Coiffet, F., Deville, J., Lamballais, E., et al.Generation of three-dimensional turbulent inlet conditions for large-eddy simulation.AIAA Journal, 42(3), pp. 447–456, 2004. doi: http://dx.doi.org/10.2514/1.3946
 Veloudis, I., Yang, Z., McGuirk, J.J., Page, G.J. & Spencer, A., Novel implementationand assessment of a digital fi lter based approach for the generation of large-eddysimulation inlet conditions. Journal of Flow, Turbulence and Combustion, 79, pp. 1–24,2007. doi: http://dx.doi.org/10.1007/s10494-006-9058-y
 Benhamadouche, S., Jarrin, N., Addad, Y. & Laurence, D., Synthetic turbulent infl owconditions based on a vortex method for large eddy simulation. Progress in ComputationalFluid Dynamics, 6(1–3), pp. 50–57, 2006. doi: http://dx.doi.org/10.1504/PCFD.2006.009482
 Tabor, G.R. & Baba-Ahmadi, M.H., Inlet conditions for large eddy simulation: areview. Computers & Fluids, 39, pp. 553–567, 2010. doi: http://dx.doi.org/10.1016/j.compfl uid.2009.10.007
 Ho, C.M. & Huerre, P., Perturbed free shear layers. Annual Review of Fluid Mechanics,16, pp. 365–424, 1984. doi: http://dx.doi.org/10.1146/annurev.fl .16.010184.002053
 Yang, Z. & Voke, P.R., Large-eddy simulation of boundary layer separation and transitionat a change of surface curvature. Journal of Fluid Mechanics, 439, pp. 305–333,2001. doi: http://dx.doi.org/10.1017/S0022112001004633
 Chandrasekhar, S., Hydrodynamic and Hydromagnetic Stability, Clarendon Press:Oxford, 1961.
 Abdalla, I.E. & Yang, Z., Numerical study of the instability mechanism in transitionalseparating-reattaching fl ow. International Journal of Heat and Fluid Flow, 25,pp. 593–605, 2004. doi: http://dx.doi.org/10.1016/j.ijheatfl uidfl ow.2004.01.004
 Roberts, S.K. & Yaras, M.I., Large-eddy simulation of transition in a separation bubble.ASME Journal of Fluids Engineering, 128, pp. 232–238, 2006. doi: http://dx.doi.org/10.1115/1.2170123
 McAuliffe, B.R. & Yaras, M.I., Numerical study of instability mechanisms leading totransition in separation bubbles. ASME Journal of Turbomachinery, 130, pp. 1–8, 2008.doi: http://dx.doi.org/10.1115/1.2750680
 Lang, M., Rist, U. & Wagner, S., Investigations on controlled transition developmentin a laminar separation bubble by means of LDA and PIV. Experiments in Fluids, 36,pp. 43–52, 2004. doi: http://dx.doi.org/10.1007/s00348-003-0625-x
 Roberts, S.K. & Yaras, M.I., Effects of periodic unsteadiness, free-stream turbulenceand fl ow Reynolds number on separation-bubble transition. ASME-GT2003-38626,2003.
 Volino, R.J. & Bohl, D.G., Separated fl ow transition mechanism and prediction withhigh and low free stream turbulence under low pressure turbine conditions. ASMEGT2004-53360, 2004.
 Metcalfe, R.W., Orszag, S.A., Brachet, M.E. & Riley, J.J., Secondary instability of atemporally growing mixing layer. Journal of Fluid Mechanics, 184, pp. 207–243, 1987.doi: http://dx.doi.org/10.1017/S0022112087002866
 Huang, L.S. & Ho, C.M., Small-scale transition in a plane mixing layer. Journalof Fluid Mechanics, 210, pp. 475–500, 1990. doi: http://dx.doi.org/10.1017/S0022112090001379
 Winant, C.D. & Browand, F.K., Vortex pairing: the mechanism of turbulent mixing- layergrowth at moderate Reynolds number. Journal of Fluid Mechanics, 63, pp. 237–255,1974. doi: http://dx.doi.org/10.1017/S0022112074001121
 Malkiel, E. & Mayle, R.E., Transition in a separation bubble. ASME Journal ofTurbomachinery, 118, pp. 752–759, 1996. doi: http://dx.doi.org/10.1115/1.2840931
 McAuliffe, B.R. & Yaras, M.I., Passive manipulation of separation-bubble transitionusing surface modifi cations. ASME Journal of Fluids Engineering, 131, pp. 021201:1–16, 2009.
 Yang, Z. & Abdalla, I.E., Effects of free-stream turbulence on large-scale coherentstructures of separated boundary layer transition. International Journal for NumericalMethods in Fluids, 49, pp. 331–348, 2005. doi: http://dx.doi.org/10.1002/fl d.1014
 Yang, Z & Abdalla, I.E., Effects of free-stream turbulence on a transitional separatedreattachedfl ow over a fl at plate with a sharp leading edge. International Journal of Heatand Fluid Flow, 30, pp. 1026–1035, 2009. doi: http://dx.doi.org/10.1016/j.ijheatfl uidflow.2009.04.010
 Kiya, M. & Sasaki, K., Structure of a turbulent separation bubble. Journal of FluidMechanics, 137, pp. 83–113, 1983. doi: http://dx.doi.org/10.1017/S002211208300230X
 Cherry, N.J., Hillier, R. & Latour, M.E.M.P., Unsteady measurements in a separatingand reattaching fl ow. Journal of Fluid Mechanics, 144, pp. 13–46, 1984. doi: http://dx.doi.org/10.1017/S002211208400149X
 Kiya, M. & Sasaki, K., Structure of large-scale vortices and unsteady reverse fl ow inthe reattaching zone of a turbulent separation bubble. Journal of Fluid Mechanics, 154,pp. 463–491, 1985. doi: http://dx.doi.org/10.1017/S0022112085001628
 Abdalla, I.E. & Yang, Z., Numerical study of a separated-reattached fl ow on a bluntplate. AIAA Journal, 43, pp. 2465–2474, 2005. doi: http://dx.doi.org/10.2514/1.1317
 Hussain, A.K.M.F., Coherent structures and turbulence. Journal of Fluid Mechanics,173, pp. 303–356, 1986. doi: http://dx.doi.org/10.1017/S0022112086001192
 Cantwell, B.J., Organised motion in turbulent fl ow. Annual Review of Fluid Mechanics,13, pp. 457–515, 1981. doi: http://dx.doi.org/10.1146/annurev.fl .13.010181.002325
 Kim, H.T., Kline, S.J. & Reynolds, W.C., The production of turbulence near a smoothwall in a turbulent boundary layer. Journal of Fluid Mechanics, 50, pp. 133–160, 1971.doi: http://dx.doi.org/10.1017/S0022112071002490
 Smith, C.R. & Metzler, S.P., The characteristics of low-speed streaks in the near-wallregion of a turbulent boundary layer. Journal of Fluid Mechanics, 129, pp. 27–54, 1983.doi: http://dx.doi.org/10.1017/S0022112083000634
 Yang, Z., Large-scale structures at various stages of separated boundary layer transition.International Journal of Numerical Methods in Fluids, 40, pp. 723–733, 2002.doi: http://dx.doi.org/10.1002/fl d.373
 Abdalla, I.E., Yang, Z. & Cook, M., Computational analysis and fl ow structure of atransitional separated-reattached fl ow over a surface mounted obstacle and a forwardfacingstep. International Journal of Computational Fluid Dynamics, 23, pp. 25–57,2009. doi: http://dx.doi.org/10.1080/10618560802566246