Two-dimensional monophasic flow past a circular cylinder intersecting or close to a free surface at Reynolds 180 is numerically investigated using the Smoothed Particle Hydrodynamics (SPH) method. The wake behaviour for a range of conditions with Froude numbers between 0.3 and 2.0, a gap ratio of 0.55 and for a half-submerged cylinder are studied. The SPH technique allows for a non-diffusive computation of the free surface evolution, even while breaking and fragmentation may occur. Vorticity shed by the cylinder, vortex generation due to free surface breaking, mixing processes, and drag and lift coefficients behaviour are discussed. It has been found that, for certain Froude numbers and for the analysed submergence ratio, a vorticity layer remains spatially localized between the cylinder and the free surface and a large recirculating wake area develops, which eventually gets detached after several shedding cycles, being advected downstream, a previously unreported form of wake instability, to the authors’ knowledge. This behaviour is blocked when the cylinder is intersecting the free surface.

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