Spatially Developing Moderate APG Turbulent Boundary Layer Subjected to Freestream Turbulence*

Turbulent boundary layers (TBLs) experiencing adverse pressure gradients (APGs) are more industrially relevant compared to zero pressure gradients (ZPGs). However, limited knowledge exists regarding the impact of elevated freestream turbulence (FST) on TBLs with moderate APGs. To address this, we conducted wind tunnel experiments to investigate the effects of FST on a developing TBL over a NACA 0015 aerofoil. Varying the angle of attack adjusted the pressure gradient, and hotwire anemometry measured boundary layer properties at different chordwise positions. The FST level was increased using static grids, resulting in levels ranging from 0.15 to 6%. The chord-based Reynolds number was kept constant at around 250,000 for all configurations. We find that the increased FST levels led to a systematic decrease in shape factor within the APG TBL, similar to recent experiments on ZPG TBLs subjected to FST. Enhanced mixing within the APG TBL, akin to ZPG TBLs, contributed to significant changes in mean flow. The wall-normal profile of streamwise turbulent kinetic energy shows substantial modifications with increasing FST intensity. Moreover, elevated FST levels revealed considerable spectral content in the outer region of the boundary layer. Notably, the outer peak in APG TBLs appeared enhanced with higher FST levels, suggesting a potential modification of vertical momentum exchange along the aerofoil with increasing the pressure gradient. We focus on understanding the spectral content of APG TBLs subjected to FST and the underlying physical mechanisms responsible for significant modifications in mean turbulent statistics.