Control of Flow Separation Over a Curved Surface using Fluidic Actuator Arrays with Variable Spanwise Periodicity

摘要

Control of a separation cell that forms in the cross flow over a nominally 2-D curved surface that models the suction surface of a VR-12 airfoil is investigated experimentally in a small-scale transonic wind tunnel. Actuation is effected by exploiting the unsteady interactions between spanwise arrays of fluidic oscillating jets of varying periodicity and vorticity concentrations within the separation cell. The effect of these interactions on the topology of the separated flow and its ultimate reattachment are investigated using high-resolution stereo particle image velocimetry with specific emphasis on the evolution of spanwise distributions and characteristic scales of engendered streamwise vorticity concentrations. The time-averaged flow exhibits nominally spanwise-periodic coupled counter-rotating streamwise vorticity concentrations whose spanwise wavelength and characteristic cross stream scale are commensurate with the periodicity of the actuation jets. It is remarkable that the unsteady shear flow associated with the actuation jets leads to the formation of multiple strands of small-scale streamwise vorticity concentrations of alternating signs within each of the "cores" of the time-averaged single-sense vortices. The streamwise vortical structures are accompanied by spanwise-alternating upwash and downwash regions between and along the axes of neighboring actuation jets, respectively, that lead to the formation of high and low concentrations of turbulent kinetic energy (TKE) as the advected vortical structures stretch into the cross flow. Surface oil visualization shows that the actuation divides the central domain of the separation cell into multiple smaller cells that are bounded by the streamwise vortices and terminate in the attached flow.