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Effect of two-dimensional surface irregularities on swept wing transition An experimental investigation was carried out to examine the effect of two-dimensional Forward Facing Steps surface irregularities, on the laminar-to-turbulent boundary-layer transition on a 45 • swept-wing. For the clean reference case, the numerical boundary-layer flow is calculated from pressure measurements, and a thorough linear stability analysis is performed for all variations of Reynolds number and angle of attack. Infrared thermography is employed to determine the transition-front location which is associated to an N -Factor, calculated from the linear stability analysis. The change in the amplification factor ∆N , caused by the addition of the surface irregularity, is analyzed. The reduction in the critical N -factor is observed to correlate with the estimated cross-flow instability vortex core height to step height ratio and the relative step height. The work presented in this paper is part of an ongoing research project to characterize the effect that surface irregularities have on boundary layer transition. The ∆N -method offers an overview of the phenomena related to FFS, capable of guiding future investigations into the underlying flow mechanisms.

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