Experiments on the laminar to turbulent transition under unsteady inflow conditions

Abstract

Natural laminar flow (NLF) airfoils are key to the performance of sailplanes and wind turbines. They provide a significant reduction of friction drag by delaying the transition from laminar to turbulent boundary layer. However, the most common method for transition prediction in NLF airfoil design, the e^n method (Mack 1977), has limited capabilities for taking inflow turbulence into account. The current work employs wind tunnel experiments to study how the transition on an NLF airfoil is affected by free-stream turbulence. The effect of small- and large-scale turbulence is studied separately, as well as in combination. In the wind tunnel, turbulence grids generate small-scale turbulence, and a gust generator induces inflow angle oscillations corresponding to large-scale turbulence. The study includes a detailed characterization of the turbulence generated by grids placed in the settling chamber of the wind tunnel. The Reynolds number Re = 3400000 and the airfoil pressure distribution is matched to cruise or dash flight of general aviation aircraft. The results are compared with direct numerical simulation, linear stability theory (LST) and flight measurements. The results show that small-scale turbulence does have an influence on the transition location in the investigated range of turbulence level, 0.01% < Tu < 0.11%. The modified e^n method (Mack 1977) captures the general trend, but the sensitivity to Tu is airfoil dependent. The effects of 2D, single-mode inflow angle oscillations are investigated in the range of reduced frequency 0.06 < kappa < 1.7. In this range, the transition process changes from quasi-steady to clearly unsteady, but a fully convective transition mode is not formed. This is an intermediate range of unsteady flow in which trajectory-following LST is able to capture the main features of the unsteady transition process. No significant interaction between the effects of small- and large-scale turbulence are observed in the investigated range of Tu and kappa, indicating that the effects can be superposed.