Boundary-Layer Transition for Low-Speed Flows via Random Excitation: The objective of this research was to provide a physics-based understanding of the nonlinear stages and breakdown mechanisms of the transition process for a ‘natural’ transition scenario in low-speed attached boundary layers. This research was motivated by the experiments of Schubauer and Skramstad (1946) [1] and Kendall (1990) who investigated ‘natural’ transition for a plat-plate boundary layer. Toward this end, highly resolved Direct Numerical Simulations (DNS) were employed to investigate the effects of randomized external disturbances with a broad range of frequencies and wavenumbers on the nonlinear stages of boundary-layer transition prior to the breakdown to turbulence. An unexpected finding from the DNS data is that in the nonlinear stages of the transition process tertiary vortices (TV) appear which are attached to the legs of the lambda structures (see figures a-b and bottom plots). These tertiary structures seem to be linked to the disturbances with a frequency range that is orders of magnitude higher than the one associated with Tollmien-Schlichting waves. It was conjectured that the generation of the TV is linked to the inviscid instability mechanism of the intense inflectional velocity profiles of the shear layers. Conributors: Shirzad Hosseinverdi, Hermann F. Fasel