The unsteady three-dimensional separated flow on a wall induced by a square
protrusion (approximately twice the local boundary layer thickness in width and
height), and its control by means of passive suction through holes, is
investigated using wind tunnel experiments at Mach 2.87. The baseline flow
without any control was characterized and compared against the cases with
bleed. A bow-shaped separation line on the wall with a mid-span separation
length of 5.57δ from protrusion face was traced from oil-flow
visualization. The averaged pressure distribution surveyed using static
pressure ports placed on the wall has mapped plateau, high-pressure, and a
low-pressure region in the separated flow, distinctive to three-dimensional
interactions. Ten control configurations were tested with suction holes placed
along mid-span in the different pressure zones. Significant spanwise `Mean
Reduction in Separation Length' of up to 0.93δ was observed from
oil-flow visualization. A comparison of observations from various control
configurations suggested that bleeding the flow from the high-pressure region
could in general delay the separation and reduce the bubble size. Further,
time-resolved schlieren visualizations have confirmed reduction in both
`mid-span separation length' and `shock-intermittent-region' with the
introduction of suction in high-pressure region. Fourier and Proper Orthogonal
Decomposition analysis done on the schlieren data has confirmed the presence of
low-frequency separation-shock oscillations at Strouhal Numbers of order
10−2, both with and without control. Furthermore, the amplitudes of
separation-shock oscillations in the spectrum were reduced with the
introduction of suction simultaneously from two holes placed in high and
low-pressure regions