Delaying laminar-turbulent transition is considered a crucial issue with respect to the reduction of airplanes' environmental impact, allowing the reduction of skin friction drag and thus fuel consumption. As part of this theme, boundary-layer destabilizing effects of surface imperfections (assembly gaps, steps, rivets, waviness, and holes) have been investigated by ONERA scientists for many years. Experimental, numerical, and theoretical works provided fruitful results partly described in this paper, which especially focuses on gaps effects. Geometrical triggering criteria were searched and found to reproduce transition at a realistic chord percentage on reduced scale models. More recently, predicting the impact of gaps on laminar flow transition was also studied so as to provide a numerical tool for defining tolerances compatible with natural laminar flow airfoils. A specific model based on a ΔN approach has been developed to quantify the transition displacement caused by transverse gaps with a rectangular section. Detailed experimental studies that recently contributed to its validation are also presented in this paper.

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