Velocity-field measurements of an axisymmetric separated flow subjected to amplitude-modulated excitation

Active flow control was applied at the point of separation of an axisymmetric, backward-facing-step flow. The control was implemented by employing a Helmholtz resonator that was externally driven by an amplitude-modulated, acoustic disturbance from a speaker located upstream of the wind tunnel. The velocity field of the separating/reattaching flow region downstream of the step was characterized using hotwire velocity measurements with and without flow control.; Conventional statistics of the data reveal that the separating/reattaching flow is affected by the imposed forcing. Triple decomposition along with phase averaging was used to distinguish periodic disturbances from random turbulence in the fluctuating velocity component. The outcome of this analysis showed that the forcing triggered large-scale, organized structures that formed at regular intervals near the separation point. The structures convect downstream and grow to a size comparable to the step height at a location approximately half way to reattachment. A significant outcome of the present study is that it demonstrates that amplitude-modulated forcing of the separated flow alters the flow in the same manner as the more conventional method of periodic excitation.