Unsteady Fluid Dynamics of a Circular Cylinder Undergoing Yaw Oscillation at Subcritical Flow

The yaw oscillation of a bluff body is a fundamental part of the motion yielding the complex unsteady flow that occurs past athletes in many examples of competitive sports such as cycling. While the impact of the motion of limbs of athletes on the drag and large-scale flow structures over cyclists, as an example, has received recent and increasing attention, little is known about the underlying flow physics. In this thesis, a canonical case of a circular cylinder is used as a low-order model of an athlete’s limb to better understand the flow behavior associated with the yaw oscillation component of motion in a controlled experimental setting. 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 of 5Χ103 and 1.5Χ104. For these tests, cylinders with aspect ratios of 13 and 20 are immersed into a water channel. The yaw oscillation frequencies (K) have the values of 0.5 (low), 1.3 to 2 (moderate), and 4 (high). Planar Particle Image Velocimetry measurements are performed in a vertical symmetry plane and horizontal planes to investigate the flow structure in the near wake. The time-varying flow behavior such as the vortex shedding is studied using Constant Temperature Anemometry measurements. 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 is changed from low to moderate values, while at high K, the flow becomes relatively uniform again. 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. At large yaw angles, high-magnitude axial flow originating from the bottom free end develops over larger spanwise sections of the cylinder, turning into the most influential parameter controlling the flow topology in addition to the cylinder's direction of motion effect.