Near-wake of a yaw-oscillating circular cylinder at subcritical flow

Abstract

In this experimental study, the near wake of a circular cylinder undergoing yaw oscillations is investigated. The yaw angle during oscillations varies from θ=0o (exposing cylinder directly to crossflow) to 30o at two different Reynolds numbers of 5×103 and 1.5×104. Planar Particle Image Velocimetry measurements are used to study the flow topology in the near-wake region of the yaw-oscillating cylinder for a range of oscillation frequencies, which when expressed non-dimensionally in terms of reduced frequencies (K) vary between K=0.25 to 4. For a cylinder undergoing yaw oscillation, the wake closure length and wake width vary with the phase angle of the oscillation cycle for a given oscillation frequency. At moderate reduced frequencies, the phase-averaged recirculation region is suppressed completely when the cylinder is closer to a certain yaw angle, which was found to be, a yaw angle of θ=15o or above in the return cycle (depending on the value of K). The yaw-oscillating cylinders generally show longer wake closure lengths in their return cycle compared to the equivalent yaw angles in the first half of their oscillation cycle. It is shown that although the wake closure length shows changes depending on the phase in the oscillation cycle for a given oscillation frequency, as the frequency of oscillation is elevated, the vortices generally shed nearer to the surface of the cylinder. Thereby, for moderate reduced frequencies of yaw oscillation, the extent of the yaw-averaged recirculation region downstream of the cylinder reduces considerably as the frequency increases, suggesting an increase in the base suction pressure, which may be speculated to result in a drag increase locally in the measurement plane (at the midspan of the cylinder) in comparison to the static counterpart. For the highest reduced frequency investigated (K=4), the phase-averaged recirculation region was observed to be suppressed completely during all phases of oscillation.