Spanwise wake structure of a yaw-oscillating circular cylinder at subcritical flow

Abstract

An experimental study is carried out to characterize the spanwise variation of the near wake of a cylinder yaw-oscillating about its mid-span between the yaw angles of θ=0o (unyawed case) and 30o at two subcritical Reynolds numbers, Re=5×103 and 1.5×104. For these tests, a cylinder with length-to-diameter ratio of 13 is immersed into a water channel, leaving a free end at the bottom end and the free water surface at the top. The yaw oscillation frequencies, when expressed non-dimensionally in terms of the reduced frequency (K), have the values of K=0.5 (low), K=1.3 to 2 (moderate), and K=4 (high). Planar Particle Image Velocimetry measurements are performed in various planes that are parallel and orthogonal to the spanwise axis of the cylinder’s unyawed version. For a cylinder undergoing yaw oscillation, the near wake is found to be highly three-dimensional. The spanwise variation in the near wake increases substantially as the reduced frequency K is changed from low to moderate values, while at high K, the flow becomes relatively uniform over large spanwise distances as a result of the delay in flow response. The spanwise flow topology is mostly independent of the Reynolds number in the subcritical range considered. The cylinder’s direction of motion, its acceleration/deceleration, and the axial flow developing as a result of large yaw are identified as factors impacting the flow behavior. For low reduced frequencies, the cylinder’s direction of motion plays the most prominent role in the topology of the near wake until large yaw angles. This effect either weakens or strengthens the reverse flow velocity and the mean recirculation region depending on the direction in which the cylinder moves. At large yaw angles, high-magnitude axial flow originating from the bottom free end develops over larger spanwise sections reducing the reverse flow velocity in the wake and eventually leading to the suppression of the mean recirculation region. For moderate values of reduced frequencies, the acceleration/deceleration of the cylinder becomes another factor influencing the flow behavior. During the phases of oscillation where the cylinder is accelerating, the impact of the cylinder’s direction of motion on the near-wake flow intensifies. In contrast, the same impact is weakened when the cylinder decelerates. At very high reduced frequencies, the mean recirculation region is suppressed over most of the cylinder’s top or bottom half span under the dominant effect of the cylinder’s direction of motion. At such high reduced frequencies, the response of the flow to the fast oscillation motion of the cylinder is delayed, revoking the impact of acceleration/deceleration of the cylinder.