The ability of Large-Eddy Simulation (LES) to predict transitional separation bubbles is investigated, with particular emphasis being placed on the response to free-stream-turbulence. The principal objective is to quantify the penalties, relative to Direct Numerical Simulations (DNS), that arise from the coarser resolution and the use of subgrid-scale models. Two flow configurations are considered: a flat-plate boundary layer, subjected to different free-stream-turbulence levels, ranging from 0 to 2% (at the point of separation), and the flow over a compressor blade at 0 and 3.25% turbulence levels. For both cases, results are compared with DNS data. A number of challenges associated with the use of LES in transitional flows are addressed, including the representation of the decay of free-stream turbulence and the mesh resolution needed for a correct description of the growth of instability waves in the early stage of transition.


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