Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

Abstract

Using flight measurements conducted between altitudes of 700 and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar–turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien–Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel are postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.