A natural laminar flow airfoil under the influence of oscillating inflow conditions

Abstract

Flight through the lower part of the Earth’s atmosphere with active convection is characterized by high turbulence intensities. This has important consequences for the design of smaller aircraft, UAVs, and wind turbines. Dedicated free-flight measurements with a high performance sailplane revealed an increase in friction drag for inflow conditions with high turbulence intensities. The present study aims to investigate the reasons for this drag increase. Next to the exaggeration of small scale modes in the transitional boundary layer, such as Tollmien-Schlichting waves, high inflow turbulence causes unsteady inflow conditions on scales affecting the overall airfoil pressure distribution. Employing an experimental setup in the Laminar Wind Tunnel, the influence of such large scale fluctuations is investigated. For this study, these experiments are limited to periodic, two-dimensional inflow oscillations with low reduced frequency to enable high-resolution phase-averaged results. A gust generator is used upstream of a natural laminar flow airfoil to generate the specific inflow oscillations. Lift and drag coefficients are measured with pressure sensors installed in the wind tunnel model and in a wake rake. A low response time is realized to capture the unsteady airfoil behavior. In parallel hot-film sensors are used to measure boundary layer quantities, allowing for a spectral investigation. The results presented in this publication show an increase in drag coefficient at the lower corner of the laminar bucket, consistent with observations from the free-flight measurement campaigns. Hot-film sensor data and spatiotemporally resolved wake rake measurements clearly demonstrate the temporal upstream shift of the transition front on the pressure side, as well as the resulting increase in drag. A spectral evaluation of the hot-film sensor data underlines this finding. For the suction side of the investigated airfoil, no similar behavior could be observed, obviously due to the significant differences in the base pressure distribution.