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The vortical structures around a yawed square cylinder oscillating in quiescent water are investigated using the particle image velocimetry technique. Following a previous study on the hydrodynamics (Lou et al., 2017), the present experiments are performed at different yaw angles (α) and Keulegan-Carpenter (KC) numbers to correlate the independence principle (IP) to vortical flow structures. At KC = 6, the vortex pair shows no shedding. Similar vortex patterns at different yaw angles result in similar hydrodynamic behaviors, which validates the IP at small KC numbers. The single and double pairs of vortex shedding regimes are observed for α= 0° at KC = 11 and 19, respectively, and the vortex shedding process is determined by the movement of the cylinder as well as the interaction between vortices and shear layers. As α increases to 45°, the shear layers are stretched and show attachment to the upper and lower sides of the cylinder for most of the time within one oscillating cycle. The shedding is only observed at the end of each half cycle and the vortices are found to reattach to the cylinder body. The subsequent drag force behavior of the yawed cylinder displays significant differences from that at α= 0° and hence the IP is no longer applicable at KC = 11 and 19. When KC increases to 25, a three pairs of vortex shedding regime can be observed at both α= 0° and 45°. A similar flow feature, characterized by the shear layer attachment when the cylinder is at the neutral position and the vortex shedding at the end of each half cycle, has been found for both α= 0° and 45°. This result indicates that the IP becomes valid for the yawed square cylinder when the KC is sufficiently large until it is analogous to the steady flow.
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