Cross-Flow Instability: Flow diagnostics and control of swept wing boundary layers

The research presented in this booklet focusses on the cross-flow instability. Applying traditional and advanced flow diagnostics, the boundary layer evolution is studied in detail. The topology and evolution of both primary and secondary instability mechanisms is revealed with unprecedented detail for experimental research paving the way for new advanced-diagnostics investigations. Important confirmations of the outcomes of past experimental, numerical and theoretical studies are achieved together with the description of a newly-reported flow phenomenon. The latter consists of a low frequency motion of the "stationary" primary vortices. While this phenomenon is considered not relevant for the transition evolution, it is deemed important for experimental investigations as it encompasses very high levels of turbulent kinetic energy.

Advanced flow control experiments based on alternating current dielectric barrier discharge plasma actuators are also performed following different instability control approaches. The primary instability is conditioned by the external forcing either in the wavenumber spectrum (by inducing selected spanwise modes) or in intensity (by weakening or enhancing the cross-flow velocity). The secondary instability modes are conditioned in the frequency spectrum and phase.
These efforts achieved the intended scopes. Although, when selected stationary modes were forced, the boundary layer fluctuations were enhanced. These fluctuations can directly cause the turbulent breakdown vanishing the beneficial effect of the performed instability control. The cross-flow forcing, making use of newer actuators reaching higher frequencies, resulted successful yielding transition promotion or delay depending on the forcing direction.