Wall forcing of the secondary crossflow instability

In this experimental investigation, wall forcing is used to study the secondary crossflow instability in swept-wing boundary layers. This instability is a high frequency traveling wave which is caused by distortion of the mean flow due to the primary crossflow vortex. In previous experimental studies wall forcing has been carried out, but the influence of the forcing amplitude and the behavior of the secondary instability close to transition has not been studied in detail. In this paper, experiments carried out in the low turbulence wind tunnel at City, University of London are described. The experimental model consists of a 45 degrees swept flat plate together with a displacement body which creates the favorable pressure gradient on the plate. The excitation mechanism was placed close to the onset of the natural occurring secondary instability. The forcing amplitude was varied and the development of the flow was tracked through detailed hot-wire anemometry measurements. It was found that the saturation point of the secondary instability moved upstream with increased forcing. Furthermore, a wave interaction just prior to breakdown seems to be accelerating the transition process which will be the focus of future studies.