Study of inlet perturbations on excitation of a laminar separation bubble through LES

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.A laminar boundary layer over a solid surface occasionally separates under the influence of adverse pressure gradient or sudden change of curvature, undergoes transition due to amplification of the disturbances, becomes turbulent and finally reattaches to form a separation bubble. In this paper, effects of inlet perturbations manifesting free-stream turbulence (fst) are examined to highlight the physics of unsteady flow and heat- transfer induced by a change of surface curvature using Large- eddy simulations (LES). Two kinds of inlet perturbations are considered: one case with a deterministic frequency (denoted as case1) and other being a band of frequencies (denoted as case2). The Reynolds number based on inlet velocity and leading edge diameter is 3450. The LES resolves the instability of the separated shear layer and its breakdown forming large- and energetic small-scale structures, which retain their appearance far downstream. When compared with the literature, it suggests that the excitation of boundary layer and thus, history of transition is sensitive to the imposed perturbations, although time-averaged flow characteristics are similar.