Behaviour of a natural laminar flow aerofoil in flight through atmospheric turbulence

Abstract

AbstractAtmospheric turbulence is encountered frequently in flight and it creates oncoming flow disturbances of varying direction and magnitude for aircraft passing through turbulent zones. The unique measurement set-up on a motorised glider enables the investigation of the flow processes acting on a laminar wing section in flight through atmospheric turbulence. The expected quasi-steady aerofoil characteristics are deduced from an investigation of boundary-layer transition under calm flight conditions. Spanwise frequency–wavenumber spectra and comparisons with linear stability theory (LST) yield insight into the linear and weakly nonlinear stages of transition. Simultaneous measurement of the oncoming flow, characteristic flow quantities on the wing section and the motion of the aerofoil enables correlations between these quantities and provides insight into the unsteady flight physics. Emphasis is placed on the response of laminar–turbulent transition to moderate free-stream turbulence on both sides of the wing section. On the lower side of the aerofoil significant and rapid upstream fluctuations of transition are observed, which correspond closely to variations in the pressure distribution. Wavelet analysis is applied to gain insight into the composition of these streamwise excursions of the transition front in the time-pseudo-frequency domain. It is shown that they are driven by rapid transient base-flow changes and that transition is initiated by a short growth stage of Tollmien–Schlichting (TS) waves.