The safe design of ships and offshore structures requires accurate predictions of the environmental loading. This includes the description of the interaction between waves, winds and currents. Indeed, there is strong evidence that wave-current interaction leads to the occurrence of extreme waves that cause the largest loading. Furthermore, strong winds, that produce extreme waves, simultaneously generate sheared currents that bring about a significant vorticity distribution, thus leading
to rotational flows. As a result, potential wave theories (such as Stokes’ theory) become invalid. The aim of the present study is to improve the understanding of the
influence of arbitrarily-sheared currents on regular waves and focused wave groups. The interaction of regular waves and currents is simulated by an irrotational
multi-layer (Cummins & Swan, 1993) and a rotational polynomial streamfunction model (Swan & James, 2001). The interaction of focused wave groups and currents is simulated using a numerical, fully-nonlinear, rotational Green-Naghdi (GN) model (Webster & Shields, 1991). Results show that surface current velocity and vorticity significantly influence the wave steepness, the surface elevation and the wave-induced velocity. For focused wave groups the steepness increases by up to 23% compared to the waves-only solution. It is further demonstrated that surface vorticity counteracts the effect of depth-uniform currents. Evidence also proves
that vorticity at the surface is important, whilst closer to the bed it is negligible. Moreover, additional rotational wave components due to vorticity were identified, which can contribute to the wave-induced velocity by up to 11%. The GN model has also been employed to investigate gradually-varying flows in which waves propagate onto currents. Under these conditions the wave amplitude can be significantly modified, which leads to much larger changes in the steepness (for example, for nonlinear focused wave groups this can be an increase of 46% compared to the waves-only solution), the crest elevation and the wave-induced velocity than observed for traditionally-studied equilibrium flow conditions. The present study provides conclusive evidence that surface vorticity must be considered in the design of offshore structures.