Flow symmetry/asymmetry over an inclined cylinder by forebody bleed

Abstract The receptivity of the natural bi-modal flow asymmetry along a finite circular cylinder at high incidence in a uniform cross-flow to perturbations is investigated in wind tunnel experiments. Azimuthal perturbations along its forebody are effected by spatially and temporally varying distributed bleed actuation driven by pressure differences across an array of surface ports. While the base flow can evolve into a random one-sided asymmetric mode that is characterized by a pair of counter-rotating streamwise vortices of unequal circulations and consequently unequal aerodynamic loads, it is shown that azimuthally segmented bleed actuation can lead to either prescribed two-sided flow asymmetry or prescribed flow symmetry. Local interactions of the bleed flow along the forebody can either disrupt the formation of the one of the CW or CCW vortices in a prescribed manner by enhancing its circulation relative to its counterpart and displacing it away from the surface or can form a symmetric pair of counter-rotating vortices of nominally equal circulations. Consequently, control of the flow symmetry can lead to either side forces of prescribed sign (up to C s ≈ −3.5 or 3.5) or nearly balanced side forces. Temporal azimuthal regulation of the bleed about the point of flow symmetry leads to rapid corresponding variations in the aerodynamic side loads by azimuthal excursions of bleed orientations of only ±15°. These findings indicate that the characteristic random onset of flow asymmetry and side loads over cylindrical bodies at high incidence can be overcome by forebody azimuthal aerodynamic bleed to yield either balanced or prescribed aerodynamic loads. Furthermore, asymmetry can also be effected at lower angles of incidence when the flow is naturally symmetric.