Modal Investigation of Vortex Ring Shedding from an Oscillating Disc

Vortex shedding from an oscillating disc with a chamfered tip is investigated at a Reynolds number of 8715 and a Keulegan–Carpenter number of 0.6. The experiments are performed using a high-speed Particle Image Velocimetry (PIV) system with 160 Hz frame acquisition frequency. Six full cycles are recorded with a total of 1999 recordings, i.e. 333 velocity measurements resolving a single cycle. The 175-mm-diameter disc oscillation frequency was 0.48 Hz with a 20 mm total displacement amplitude in water. Dynamic image masking is performed to remove the disc shape from the PIV raw images, using rigid object tracking and image stabilization techniques. This implies a coordinate transformation and allows the investigation of flow field results with respect to the disc, as if it was stationary. This allows the use of statistical analyses and phase-locked averaging. The results indicate that the vortex formation and shedding from-cycle-to-cycle is very stable and predictable. The chamfered disc tip geometry has great influence in the vortex dynamics: When the motion is towards the chamfered side, a big, attached trailing vortex is present on the flat side; and when the motion is reversed towards the flat side the big trailing vortex is shed off outwards from the center, forming a shear layer with a 45° orientation. These findings have been supported by modal analyses using Proper Orthogonal Decomposition (POD) and Oscillating Pattern Decomposition (OPD). The static and dynamic modes of POD and OPD successfully describe the underlying flow physics of vortex ring shedding from an oscillating disc and reveal intricate details about the flow field which cannot be captured by statistical analysis or phase locked averaging.