Inner-outer interaction in a rapidly sheared boundary layer

Abstract

In the present work we study the inner-outer interaction in wall turbulence using a novel experimental arrangement of first generating a shearless boundary layer over a moving ground plane in the presence of grid turbulence, which is then passed over a stationary floor downstream resulting in a rapidly sheared boundary layer. The velocity spectra in such a boundary layer are shown to mimic the spectral features typical of a canonical turbulent boundary layer over a range of Reynolds numbers. This suggests that the rapidly sheared boundary layer consists of coherent structures that are qualitatively similar to the large-scale motions and superstructures observed in a canonical turbulent boundary layer. Static pressure fluctuations measured using a specially-made “needle” probe reveal the variation of the pressure field inside the rapidly sheared boundary layer. The pressure fluctuations in the free stream are seen to be highly correlated with wall pressure, especially when the boundary layer is sufficiently thin, supporting the view that the pressure fluctuations can play an important role in coupling turbulent eddies in the inner and outer regions. Further, we show that the present experimental arrangement is well-suited to studying the relative importance of the “top-down” and “bottom-up” mechanisms in wall turbulence in a systematic manner. The results obtained so far suggest that the top-down mechanism is dominant near the leading edge of the stationary surface with the bottom-up mechanism becoming progressively important as the boundary layer grows downstream.