The transition of a separated shear layer over a at plate in presence of free-stream turbulence (FST) and regular three-dimensional (3D) roughness elements, is numerically investigated. A at plate with a contoured inviscid upper wall imposes the necessary streamwise pressure gradient to simulate the suction surface of a ‘ultra high-lift’ low-pressure turbine (LPT) blade. Results obtained from the current investigation compare favourably with extensive experimental and numerical data of previous investigations. The flow physics surrounding the transition process is examined. The benefits associated with the suppression of the separation bubble by introducing FST and surface roughness is established for the blade profile. The instantaneous flow features are an essential tool to identify the various streak formations associated with introduction of upstream disturbance. The turbulent kinetic energy budget is obtained for the various test cases and the individual and cumulative effect of FST and roughness is shown. © 2017 by Aditi Sengupta, Nagabhushana Rao Vadlamani, Paul Tucker. Published by the American Institute of Aeronautics and Astronautics, Inc.