Abstract
Numerical computations are carried out to predict the transition generated by excrescence on a platelike geometry in subsonic flow. Both forward-facing and rearward-facing steps of small roughness height are considered in the investigation. These are representative of joints and other surface imperfections on wing sections that disrupt laminar flow, thereby increasing skin friction and drag. Solutions are obtained via a high-fidelity numerical scheme and an implicit time-marching approach on an overset mesh system that is used to represent the steps. Very-small-amplitude numerical forcing is employed to generate perturbations, which are amplified by the geometric disturbances, similar to the physical situation. The flowfield just downstream of the steps is characterized by the growth and breakdown of two-dimensional fluid structures. Because all significant scales of the flow are fully resolved in this region, the solutions there correspond to direct numerical simulations. Further downstream where the flow is more fully turbulent, the calculations are regarded as large-eddy simulations. Details of the numerical procedure are summarized, and features of the flowfields are described, which help to elucidate the transition process. Comparisons are made with the available experimental data in terms of time-mean skin-friction measurements. The locations of transition and skin-friction levels predicted numerically are in close correspondence with the experiments. A grid-resolution study was carried out to confirm the accuracy of the computations. In the fully turbulent region downstream of the transition, calculations agree with the expected behavior, but have not yet evolved to the high-Reynolds-number asymptotic form.
Published Version
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