Inner-outer interaction in a Rapidly Sheared Boundary Layer (RSBL).

We present results from an experiment in which grid-generated turbulence is passed initially over a moving ground plane to create a shearless boundary layer, which is then rapidly sheared as it encounters a stationary surface downstream. Close to the leading edge of the stationary surface, the conditions of the Rapid Distortion Theory (RDT) can be expected to be approximately satisfied. We report measurements of both velocity and pressure in the RSBL: the spectra of the streamwise velocity fluctuations display a bi-modal shape, resembling those in an equilibrium turbulent boundary layer. The streamwise evolution of the spectra indicates that the low-wavenumber peak in the spectra indicate a 'top-down' mechanism, whereas the relatively high-wavenumber part of the spectrum can be attributed to a 'bottom-up' mechanism. Static pressure fluctuations measured using a needle probe show that their correlation with the wall pressure is much higher in the RSBL, compared to a canonical layer, especially close to the leading edge of the stationary surface. This implies that the pressure fluctuations near the boundary-layer edge are strongly coupled with those in the near-wall region. The further implications are discussed.