A Method for Identifying the Freestream Turbulence/Turbulent Boundary Layer Interface

The interface between a turbulent boundary layer (TBL) and freestream turbulence (FST) is investigated by performing particle image velocimetry. Three different FST intensities are investigated together with a reference low-turbulence case. Utilizing vorticity distributions, a novel approach is introduced to delineate the boundary between the TBL and FST flows by identifying individual velocity thresholds for each instantaneous field. An increase in FST intensity results in a broader range of velocity thresholds. Conditionally averaged spanwise vorticity rapidly decays to zero on the FST side of the interface, while taking on a negative value within the boundary layer. The streamwise velocity exhibits a jump across the interface, whose magnitude increases with increasing FST intensity. The interface moves closer to the wall on average for increased FST intensities, leading to decreased intermittency values in the inner region of the TBL.