Numerical and Experimental Investigations of Oblique Boundary Layer Transition

Kurzfassung

A transition scenario initiated by two oblique waves is studied in an incompressible boundary layer. Hot-wire measurements and flow visualizations of this scenario are reported for the first time. The experimental results are compared with spatial direct numerical simulations and a good qualitative agreement is found. Also quantitative agreement is found when the experimental device for disturbance generation is closely modeled and pressure gradient effects taken into account. The oblique waves are found to interact nonlinearly to produce streamwise streaks growing downstream, related to non-modal linear growth mechanisms. The same has previously been observed in channel flows and calculations of both compressible and incompressible boundary layers. The flow structures of oblique transition have many similarities to 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