The influence of sweep on the linear stability of a series of swept laminar separation bubbles

Abstract

The effect of sweep on the linear stability of a series of pressure-induced laminar separation bubbles is investigated for a range of sweep angles Ψ = 0 ° , 15°, 30°, 45° by means of linear stability theory and solutions of the linear parabolised stability equations. An application of the independence principle for infinite swept configurations ensures identical chordwise cross sections of the separation bubbles in the DNS-base flows, which enables direct comparisons. Systematic investigations of the local stability and the properties of the linearly most amplified disturbances for each sweep angle show that Tollmien–Schlichting waves, not cross-flow instabilities, dominate even for higher sweep angles. Similar to the situation in attached boundary layers oblique Tollmien–Schlichting waves, which propagate approximately in free stream direction, experience the strongest linear growth in the swept cases. Compared to the strong growth in the separated shear layer, however, their maximum amplification increases only moderately with the sweep angle. The general influence of cross-flow instabilities is weak in the given configuration, despite a relevant cross-flow inside the separation bubble. Finally, the investigation yielded that the comprehensive influence of the independence principle on the base flow is not extendible to the linear stability equations, so that with respect to disturbance amplification in general each sweep angle constitutes a unique case.