A transition scenario initiated by two oblique waves is studied in an incompressible boundary layer. Hot-wire measurements and flow visualizations from the first boundary layer experiment on this scenario are reported. The experimental results are compared with spatial direct numerical simulations and good qualitative agreement is found. Also, quantitative agreement is found when the experimental device for disturbance generation is closely modelled in the simulations and pressure gradient effects taken into account. The oblique waves are found to interact nonlinearly to force streamwise vortices. The vortices in turn produce growing streamwise streaks by non-modal linear 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 many similarities to the corresponding ones of K- and H-type transition, for which two-dimensional Tollmien–Schlichting waves are the starting point. However, two-dimensional Tollmien–Schlichting waves are usually not initiated or observed in oblique transition and consequently the similarities are due to the oblique waves and streamwise streaks appearing in all three scenarios.

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Abstract. Using experimental data from Particle Image Velocimetry (PIV) measurements, coherent structures of a transitional spatially developing boundary layer are determined. The coherent structures are determined utilizing the Proper Orthogonal Decomposition (POD), which is based on an expansion of the flow field variables into a set of eigenfunctions or modes. For having constant and reproducible flow field conditions, the flow is artificially excited by means of periodic velocity fluctuations. The used excitation device allows the generation of different transition scenarios, where this paper focuses on the case of the fundamental transition. Phase locked excitation signals allow the recording of instantaneous velocity fields of the flow field at certain instants of time. It can be shown that PIV is a suitable technique to provide experimental data for POD. The results of the POD show that already a small number of modes cover most of the kinetic energy of the flow.

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