Fluidic Control of Separation Over a Hemispherical Turret

Abstract

Active control of flow separation over a hemispherical turret, that is, a surface-mounted hemispherical shell, is demonstrated in wind-tunnel experiments at Reynolds numbers up to 706,000. Control is applied using high-frequency(St D ) > 10) actuation effected by a meridional array of individually addressable synthetic jet actuators. The control effectiveness is assessed and characterized using high-resolution particle image velocimetry, hot-wire anemometry, and surface pressure distributions. Measurements of the baseline flow indicate that separation occurs in a horseshoe pattern across the hemisphere with the apex at the plane of symmetry. High-frequency actuation results in a substantial reduction in the extent of the recirculating flow domain downstream of the turret by delaying separation, decreasing the reattachment length, and concomitantly suppressing the energy of the fluctuating motions. As a consequence, the core of the recirculating vortex is displaced toward the juncture between the hemisphere and the surface and its cross-sectional area is substantially diminished. Furthermore, the estimated turbulent kinetic energy in the controlled flow is decreased significantly, especially at the large scales. Deliberate tripping of the flow just upstream from the actuators' array improves the spanwise control authority. It is also demonstrated that the motions within the recirculating domain can be regularized by amplitude modulation of the actuation waveform, thereby inducing large-scale coherent motions that have externally referenced phase and passage frequency, which would make them suitable for adaptive optical corrections.