Interaction of free-stream turbulence with a turbulent boundary layer

The interaction of free-stream turbulence with a turbulent boundary layer is investigated with planar particle image velocimetry. An active grid consisting of a series of wings mounted to stepper motor-actuated rods is used to generate free-stream turbulence in a wind tunnel. In this study, turbulence intensities between 7\% and 13\% are investigated. The boundary layer is formed over a plate mounted downstream of the grid. When the turbulent free-stream case is compared to a canonical turbulent boundary layer with an approximately laminar free-stream, the distinct angled structure of the velocity correlation maps becomes homogeneous much closer to the wall relative to the boundary layer thickness. However, when comparisons are made between the canonical turbulent boundary layer and a case with significant free-stream turbulence for matched shear levels, their structures are approximately the same. It is also found that there is a distinct lack of uniform momentum zones in the turbulent boundary layer for high levels of free-stream turbulence. This is particularly interesting given that the outer layer of the boundary layer still exhibits a single strong low-frequency peak in the velocity spectrum.