In-flight experiments for delaying laminar—turbulent transition on a laminar wing glove

Abstract

This article describes in-flight measurements to delay laminar—turbulent transition by means of active Tollmien—Schlichting (TS) wave cancellation. The damping of unstable TS waves in the boundary layer leads to downstream shifting of the laminar—turbulent transition and therefore to the reduction of skin friction. In-flight experiments were carried out using a laminar wing glove for a sailplane. A sensor—actuator system attached to the wing glove consisted of an array of surface hot-wire reference sensors to detect oncoming TS-waves upstream of a membrane actuator and surface hot-wire error sensors downstream of the actuator. The method applied to delay laminar—turbulent transition is based on damping of naturally occurring instabilities through superimposition of the counter wave, which is calculated by a fast digital signal processor using a closed-loop feed-forward control algorithm. The experiments were carried out at flight velocities in the region of 20 m/s, which corresponds to a chord Reynolds number of about 2 million. The results show a damping of ∼ 50 per cent reduction of the local amplitudes of the instabilities. It is anticipated that using an actuator with a high resonance frequency and a minimal reaction time will improve damping significantly in future experiments.