The stream wise oscillation of a spring-mounted square-section cylinder set at angles of incidence, α, in the range from 0° to 45° is investigated in the reduced-velocity range 3 < U/ND < 13 and Reynolds-number range from 3.2 × 103 to 1.4. × 104. The mass-damping parameter used for the investigations is 1.6 and this gives vibration amplitude up to 0.12D. For small angles of incidence (i.e. α < 10°), vibration occurs mainly near U/ND = ½S, where S is the Strouhal number for the stationary cylinder. In the neighbourhood of α = 13.5°, which is where one of the separated shear layers is expected to reattach, vibration occurs near U/ND = 1/S. As α approaches 45° the amplitude observed near U/ND = 1/S diminishes and small-amplitude vibration appears near U/ND = ½S.
At α = 0°, vortices help to sustain oscillations by shedding when the cylinder is moving upstream. The mean drag of the oscillating cylinder drops and may reach less than half the stationary-cylinder value. When the amplitude of vibration is small, vortices of opposite sense of rotation are shed alternately and the familiar von Karman vortex street is formed. For moderately high values of amplitude, two vortex patterns fundamentally different from that of the stationary cylinder are observed. Intermittently, pairs of vortices are then shed symmetrically from both sides of the cylinder. When this occurs a pair of contra-rotating vortices forms every cycle of the vibration. When vortices of opposite sign are shed alternately, one vortex from each side of the cylinder forms every two vibration cycles. In this latter case, it appears that each vortex is elongated and split into two parts. Split vortices of opposite sign pair up and then form a vortex street.