Numerical and experimental investigations of oblique boundary layer transition

A transition scenario initiated by two oblique waves is studied in an incom pressible boundary layer. Hot-wire measurements and flow visualizations fro m the first boundary layer experiment on this scenario are reported. The ex perimental results are compared with spatial direct numerical simulations a nd good qualitative agreement is found. Also, quantitative agreement is fou nd when the experimental device for disturbance generation is closely model led in the simulations and pressure gradient effects taken into account. Th e oblique waves are found to interact nonlinearly to force streamwise vorti ces. The vortices in turn produce growing streamwise streaks by non-modal l inear growth mechanisms, This has previously been observed in channel flows and calculations of both compressible and incompressible boundary layers. The flow structures observed at the late stage of oblique transition have m any similarities to the corresponding ones of K- and H-type transition, for which two-dimensional Tollmien-Schlichting waves are the starting point. H owever, two-dimensional Tollmien-Schlichting waves are usually not initiate d or observed in oblique transition and consequently the similarities are d ue to the oblique waves and streamwise streaks appearing in all three scena rios.