Detecting separation pre-cursors using on-board optical tracking of flexible pillar sensors

A novel approach for characterising specific flow phenomena unique to incipient stall in real-time on small scale UAVs and aerodynamic systems operating at low to medium Reynolds numbers is introduced. Flexible pillar sensors emanating from the suction side of an aerofoil of length approx. 3% of chord length are installed on a NACA0012 and tested at City, University of London's wind-tunnel facilities. The sensors are tracked in real-time using a high-speed camera in a simulated 'on-board' position, acting as ‘digital tufts’, and the results subsequently processed. The results show the pillars reacting to specific flow phenomena that are unique to incipient stall, and scale with angle of incidence. Namely, these are low-frequency oscillations of local streamwise velocity components which are hypothesised to be resulting from quasi-periodic vortex shedding/shear layer flapping from the breathing of the laminar separation bubble over the suction side.