Effect of Step Surface Imperfections on Boundary Layer Transition

Abstract

Surface imperfections due to manufacturing tolerance, such as steps, gaps, and waviness, can cause early boundary layer transition, thus reduce the practicability of the natural laminar flow (NLF) technology for aircraft drag reduction. In this study, a new numerical methodology is proposed and implemented to predict the effect of backward-facing step (BFS) and forward-facing step (BFS) on boundary layer transition over NLF aircraft body. The method is based on a hypothesis that the effect of surface imperfections on boundary layer flow can be mimicked as that of surface roughness due to the relevant small size of the imperfection compared to the local boundary layer thickness. The predicted transition location on BFS/FFS surface agrees very well with the experimental validation data. It implies the applicability and capability of this approach for transition location prediction over more general surface imperfection arrangements. The study thus provides an efficient, practical prediction tool to determine the manufacturing allowance on surface imperfection dimension on NLF body for aerodynamic engineers.

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