Interactions of large-scale free-stream turbulence with turbulent boundary layers

The scale interactions occurring within a turbulent boundary layer are investigated in the presence of free-stream turbulence. The free-stream turbulence is generated by an active grid. The free stream is monitored by a single-component hot-wire probe while a second probe is roved across the height of the boundary layer at the same streamwise location. Large-scale structures occurring in the free-stream are shown to penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. It is speculated that depending on the extent of the penetration, i.e. based on the level of freestream turbulence, the near-wall turbulence production peaks at different wall-normal locations than the expected location of y+≈15 for a canonical turbulent boundary layer. It is shown that the large-scales dominating the log region have a modulating effect on the small-scales in the near wall region; this effect becomes more significant with increasing turbulence in the free-stream, i.e. similarly increasing Reλ0. This modulating interaction and its Reynolds number trend have similarities with canonical turbulent boundary layers at high Reynolds numbers where the interaction between the large-scales and the envelope of the small-scales exhibit a pure amplitude modulation (Hutchins & Marusic 2007a; Mathis et al. 2009). This similarity has encouraging implications toward generalising scale interactions in turbulent boundary layers.