The curious case of the vanishing vorticity (INVITED)

Abstract eng:
The flow over a submerged horizontally translating cylinder results in vorticity of opposite sign being shed from the top and bottom. However, when the cylinder is close to a stress-free surface, the vorticity shed from the top of the cylinder seems to disappear downstream, for zero and non-zero Froude number cases. For the zero Froude number flow case, vorticity shed from the top surface of the cylinder can diffuse from the body of the fluid into the interface. For high Froude numbers, the stress-free surface curves and vorticity can stream into the body of the fluid, cross-annihilating vorticity shed from the top surface of the cylinder. For each case, it is shown that circulation is conserved in the flow, by taking account of the circulation at the stress-free interface. Such an interface is therefore a potent source of vorticity, even in the absence of external forcing, and can dramatically modify vortex formation and evolution.

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