Multi-input multi-output phononic subsurfaces for passive boundary layer transition delay

This paper documents progress towards developing a passive boundary layer transition delay strategy using a phononic subsurface (PSub). High-order implicit large-eddy simulation (ILES) is used to explore control requirements for boundary layer transition delay in the context of guiding passive flow control strategies. Positive phasing of surface displacement relative to forcing from a Tollmien-Schlichting (T-S) wave is shown to be effective at attenuating instability for the present flow conditions. Although difficult to achieve with a single input system, positive phasing can be realized with a multi-input, multi-output (MIMO) system. To this end, a MIMO PSub was developed and a reduced-order model was coupled with the flow solver. Positive phasing, and therefore a small degree of passive transition delay, was demonstrated on a flat plate boundary layer. Further optimization of subsurface properties and placement of multiple devices are likely to improve controller efficacy.