Abstract

In this work we studied the wave-induced vortex generation and shedding from a semi-submerged vertical square cylinder interacting with an upcoming undular bore wave train under a shallow water configuration. This unsteady and rapid process was investigated by means of numerical and experi- mental approaches. A numerical simulation, solving the full turbulent viscous Navier-Stokes equations was carried out in order to study and characterize both the undular bore wave properties and the vortex dynamics triggered during this unsteady interaction. Starting with fluid at rest the undular bore was generated by the impulsive translational motion of a piston wavemaker at laboratory scale in both a numerical and an experimental wave tank. When the undular bore impinges on the cylinder, filamen- tary vortex structures were formed at the four cylinder’s edges synchronised with the propagating wave motion, leading to a vortex shedding phenomena at a frequency which matched the wave instanta- neous frequency. These vortices extended along the entire cylinder span under the water column, from the free-surface to the seabed. At the trailing edge of the cylinder, a pairing process of two shed vortices was observed, similar to an Lamb-Oseen vortex pair. These vortices were present during the whole undular bore wave train dynamic forcing. Overall agreement was found with the experimental version of the bore-cylinder interaction, carried out in a physical wave tank. Laser sheet bore profiling and particle image velocimetry (PIV) measurements of the velocity field confirmed undular bore prop- erties, the onset of vortex formation, subsequent shedding and pairing in the experiments performed in similar conditions with the numerical approach.