The Effects of Freestream Turbulence on Wing-Gust Interactions

There is a growing interest in the experimental study of discrete vertical gusts motivated by the proliferation of small Uncrewed Air Systems. However, gusts are not the only unsteady flow disturbances that these vehicles are expected to encounter. Freestream turbulence is also a serious consideration. Previous work has indicated that freestream turbulence has significant effects on the flow around airfoils, so this work investigates the interaction of these two unsteady flow disturbances. Airfoils are pitched to mimic the flow angle change caused by a gust at different levels of incoming freestream turbulence. The lift generated during the gust encounters with elevated incoming freestream turbulence show that the effects on the performance of the airfoil are dependent on the turbulence intensity. Low turbulence intensity has beneficial effects, delaying stall and increasing the maximum lift, whereas higher freestream turbulence intensity delays stall without a corresponding increase in the maximum lift. A preliminary investigation of 15-trial ensemble-averaged flow fields suggests that freestream turbulence dominates the flow, but the flow features associated with the dynamic lift seen in the "clean" case are still somewhat present. However, lift curves and flow fields from individual trials indicates that the turbulence increases the unsteadiness in the interaction, but the general trends are dominated by the gust interaction. Further work is still needed to better determine the flow physics of these interactions and the interplay between repeatable and stochastic flow phenomena.