Abstract
Travelling waves in an unstable three-dimensional boundary layer are studied experimentally with the use of hot-wire anemometry. For the sake of realistic comparisons with stability theory, the tests were performed on a swept flat plate where infinite swept-wing conditions were approximated by means of contoured end plates. The required pressure gradient was imposed by a displacement body. The Reynolds number for the first appearance of travelling waves is roughly the same as that of stationary vortices. The frequencies of the most amplified waves depend on the Reynolds number. It is shown with the aid of a twin probe that in a more strongly amplified state, the travelling waves propagate in a direction different from that of the mean flow. Further upstream, where stationary waves first become visible in the oil-flow pattern, a uniform direction could not be identified. Under certain conditions, travelling waves of two frequency ranges are amplified that propagate in different directions. The present work is part of the transition experiment started at the DFVLR. It is closely connected to the theoretical work by Dallmann and Bieler.
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