Spatio-temporal characteristics of secondary instabilities in swept wing boundary layers

Abstract

Stationary waves approximately aligned with the flow direction develop in the laminar boundary layer of swept wings in low freestream turbulence conditions. These, so-called Crossflow vortices, undergo spatial amplification and amplitude saturation and deeply modify the boundary layer causing the destabilization of secondary high frequency instabilities. The rigorous amplification of these secondary modes has been identified as the cause of turbulent breakdown by recent investigations encompassing experimental, numerical and theoretical studies. In this paper experiments are conducted towards the identification and measurement of secondary crossflow instabilities. Detailed hotwire experiments are performed on a swept wing model at the location of primary instability saturation. In order to deterministically capture the spatio-temporal evolution of the secondary structures, measurements were phase conditioned using concurrent forcing of relevant frequencies by means of a DBD plasma actuator mounted upstream of the measured domain. The actuator effect was to enhance the amplitude of the forced modes. Additionally, the deterministic locking of the instability phase to the phase of the actuation allowed phase averaged reconstruction of the spatio-temporal evolution of these structures.