Flow past a cylinder in an unbound medium gives rise to self-sustained, limit cycle oscillations involving formation of a regular, periodically alternating Karman vortex street. However, little attention has been paid to the effect of an adjacent free surface on the development of the Karman vortices and possible generation of new classes of the nearwake structure[1], see Figure 1. The technique of Particle Image Velocimetry (PIV) is a useful tool to this flow, considering that it is unsteady and multi-scale in nature. 德國LaVision PIV/PLIF粒子成像測速場儀