Modal Analysis Of Actively Controlled Flow Past A Backward Facing Ramp

An investigation based on the application of spectral proper orthogonal decomposition (SPOD) of pressure measurements and Fourier analysis of velocity measurements is carried out to further elucidate the properties of separated flow behind a backward-facing ramp in uncontrolled and controlled conditions. Several flow control conditions are considered, varying the duty cycle and the frequency of an actuation system composed of 89 pulsed jets, issuing normally to the flow direction. The SPOD is applied to unsteady pressure measurements to characterize the main features of the flow and to extract the most energetically and dynamically relevant space-time coherent structures. Fourier analysis is applied to the velocity measurements and is complementary to the SPOD analysis. In uncontrolled conditions four major modes, together with their typical frequencies, have been identified and associated with (1) presence of large-scale longitudinal roll-up vortices which oscillate at the slanted ramp side edges, (2) flapping motion of the central recirculation region, (3) large-scale vortex emission from the shear layer at the slant edge, and (4) onset of high-frequency Kelvin-Helmholtz instability occurring in the shear layer near the slant edge. The control suppresses the recirculation mode and all associated mechanisms. As expected, the mode related to the lateral roll-up vortices persists. The modal analysis also shows that distinctive peaked bands emerge, which are ascribed to the presence of the control pulsed jets. Focusing on the SPOD mode associated with the actuation allows one to analyze the extension of the receptivity region of the flow to the action of control. Some peculiar aspects of SPOD field reconstruction are suggested by the specific physical characteristics of the flow.