Laminar-turbulent transition experiment on the effect of surface imperfections on an NLF airfoil in transonic flow

Abstract

In the present study, the effect of various twodimensional surface defects (forward-facing steps and ramps, backward-facing steps and ramps, gaps and steps-and-gaps) on boundary-layer transition was experimentally investigated in the transonic regime. A laminar profile was specifically designed and manufactured by ONERA so as to allow for a maximum of defects to be tested simultaneously, and to include resin pockets to accurately monitor laminar-turbulent transition using infra-red thermography. Transition was also characterised using the ∆N model based on linear stability calculations. Relatively good agreement with existing ∆N models for forward and backward facing steps as well as gaps was found, indicating that these models, which were mostly developed for incompressible flows, can still be used as a conservative estimate for compressible flows. More practical defects referred as step-and-gap were also considered.