Characterization of Controlled Flow Separation

Inherent flow separation within a diffuser duct is delayed using fluidic actuation, and the dynamics and structure of the natural and migrated separation (Mo = 0.4) in the absence and presence of actuation, respectively, are investigated experimentally. Specifically, the flow features within measurement domains localized near separation are explored using conditional averaging, spectral analysis, and proper orthogonal decomposition. Variable actuation input by an integrated array of fluidic oscillating jets migrates the separation downstream through the imposed adverse pressure gradient. The time-averaged velocity distributions of both the natural and migrated separation are dominated by the presence of a shear layer away from the surface while the shear stress at the surface diminishes and vanishes approaching separation. The present investigations show that by extending flow attachment via flow control, the characteristic cross stream scale of the separating velocity distribution increases significantly and develops an additional inflection point close to the surface. Regardless of the distinct differences in cross stream scales of the mean velocity between the base and controlled flows, scaling by outer shear layer parameters yields a reasonable global collapse of the velocity profiles for both flow conditions, except very close to the surface. A combined outer and inner shear layer scaling is shown to further improve the collapse compared to the outer scaling alone. Conditional averaging of the flow fields highlights an overall increase in the turbulent kinetic energy levels as separation migrates under flow control compared to the base flow. Spectra of the velocity fluctuations about the migrated separation exhibit an increase in the energy of the large-scale motions that is coupled to a transfer to small scales at lower frequencies than in the base flow. Proper orthogonal decomposition analysis is used to explore the underlying flow structure through separation in the absence and presence of actuation. The modal decomposition indicates that despite the pronounced increase in low frequency energy, the local vorticity modes in the natural and migrated separation are remarkably similar, and simply undergo tilting and stretching as the separation is forced to migrate downstream.