Laminar to Turbulent Transition at Unsteady Inflow Conditions: Wind Tunnel Measurements at Oscillating Inflow Angle

Wind tunnel measurements have been performed to investigate the effect of large scale turbulence on the laminar to turbulent transition on a natural laminar flow airfoil through cyclic 2D variations of the inflow angle. The investigated range of reduced frequencies is \(k=\pi \cdot f\cdot c/U_{\infty }=0.22-1.67\) (corresponding to \(f=2{-}15\) Hz) and an equivalent angle of attack amplitude of \(\pm 0.5^{\circ }\). Transition locations are determined using the M-TERA intermittency method and time-dependent frequency spectra are calculated by a continuous wavelet transform. At high frequencies of inflow oscillation the downstream moving transition front is slower than the change in pressure gradient and the Tollmien-Schlichting peak is significantly reduced, indicating a “convective” transition mode. The downstream turning point of the transition location during the inflow angle cycle moves upstream for \(k>1.0\) while the upstream turning point is essentially unchanged for \(k\le 1.67\).