Localized Shear-layer Disturbances Affected by Laminar Flow Separation

Evolution of laminar flow localized perturbations generated in shear layers by low-frequency local vibration of the wall is investigated in a wind tunnel. The experimental data are obtained at low subsonic velocities by hot-wire measurements. A laminar boundary layer on flat plate placed parallel to the oncoming flow represents the basic examined configuration. The local surface vibration generates longitudinal velocity disturbances in the form of spatially localized quasi-stationary deformations of the near-wall flow region. According to recent experimental data, in such conditions the high-frequency instability is stimulated at the forward and rear fronts of the longitudinal structures. Then, the basic flat-plate flow is modified by a 2D backward-facing step mounted on the plate which creates a local region of boundary-layer separation. The characteristics of velocity perturbations induced by surface vibration are compared in the attached and separated shear layers. As is found, laminar flow separation promotes the amplification of high-frequency oscillations at the fronts of streaky structures, thus, stimulating the transition to turbulence.