Focusing of internal waves generated by an oscillating torus

International audienceInternal waves generated by the tidal motion over the topography of the abyssal sea play an important role in ocean dynamics, and are relevant to climate prediction in view of the potential mixing and consequent change of the ocean heat distribution (Ferrari 2014). In continuation of former studies on the generation of internal waves by oscillating objects (Voisin 2003; Voisin, Ermanyuk & Flór 2011; Voisin 2020) we consider the waves generated by a horizontally oscillating torus. Circular ring topography produces convergent waves that increase in amplitude towards the focal point, and are therefore especially conducive to nonlinear effects and mixing (Bühler & Muller 2007; Ermanyuk, Shmakova & Flór 2017; Shmakova & Flór 2019; Shmakova et al. 2021).The structure of the waves is set by the Stokes number St and ranges from unimodal waves at moderate St (moderate viscosity) with thick waves beams of the width of the torus section, to bimodal waves at high St (low viscosity) with thin wave beams, having the shape of shear layers tangential to the section. Other parameters are the aspect ratio ε of the torus, and the Keulegan–Carpenter number Ke (see e.g. Voisin et al. 2011).For a very large range of Stokes numbers, set by tori of diameters ranging from 24 to 180 cm, we compare experimental results obtained from PIV measurements to a 3D theoretical model of focusing waves that includes viscous effects. We discuss nonlinear effects such as the generation of sub- and higher harmonics, triadic resonance, wave breaking as a function of Stokes number, and the generation of a mean flow. In particular, the Stokes drift is calculated from the linear theory, and separately calculated from the experimental data, and exactly opposes the measured mean flow