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A quantitative analysis illustrating salient features of a Laminar SeparationBubble (LSB), its transition forming coherent structures, and associated heattransfer has been performed on a flat plate for varying free stream turbulence(fst) between 1.2% to 10.3%. A well-resolved Large Eddy Simulation (LES)developed in-house is used for the purpose. Flow separation has been induced byimposing an adverse pressure gradient on the upper boundary of a Cartesiandomain. Isotropic perturbations are introduced at the inlet to mimic gridturbulence. With an increase of fst, an upstream shift in the mean reattachmentpoint has been observed while the onset of separation remains almost invariant,shrinking the bubble length significantly. The transition of the shear layer istriggered by the Kelvin-Helmholtz (K-H) instability for fst of less than 3.3%,while Klebanoff modes (Kmodes) dictate the flow transition at fst greater than6.5%. Further, a mixed mode, i.e., both K-H and K-modes, contribute to the flowtransition at a moderate level of fst, lying between 3.3% and 6.5%. Thus, theroll-up of the shear layer appears in the second half of the bubble sheddinglarge-scale vortices that keep their identity far downstream at low fst levels.On the contrary, the streamwise streaks via K-modes prior to the separation arefound to interact with the LSB, resulting in an earlier breakdown of the shearlayer with abundant small-scale vortices downstream at the moderate to high fstlevels. However, higher surface-normal heat flux is associated with large-scaleenergetic coherent vortices.

Aero-thermal analysis of a laminar separation bubble subjected to varying free-stream turbulence: Large Eddy Simulation