Modal Decomposition Analysis of Controlled Internal Flow Separation*

The effects of fluidic actuation on the dynamics and evolution of flow separation within a diffuser are investigated using modal decomposition (EMD, POD) of the velocity field. Actuation is effected using a spanwise array of fluidic oscillators placed upstream of the natural separation. The significant effects of the actuation strength (as measured by the flow rate coefficient C_q) on the global streamwise pressure gradient, flow constriction, flow rate and losses within the diffuser are characterized (M = 0.4). The flow dynamics in the immediate vicinity of the unsteady separation are investigated in detail in the absence and presence of actuation using high speed particle image velocimetry (up to 5 kHz) with specific emphasis on the effects of the actuation on the temporal and spatial migration of the local separation front. Local measurements of turbulent kinetic energy indicate that while in the presence of actuation (C_q = 0.8%) the scale and dynamics of the flow surrounding the separation front change drastically, the scaled flow structure remains nearly invariant. In particular, the underlying spectral contents of the flow centered locally about the incipient separation are compared in the absence and presence of actuation using EMD and POD.