Direct numerical simulation of surface tension dominated and non-dominated breaking waves

Surface tension effects onto the two-dimensional wave breaking flow produced by a hydrofoil moving beneath the free surface are investigated. The study is carried out numerically by a finite difference approach which solves the Navier-Stokes equations. The air-water interface is embedded in the computational domain and it is captured via a Level-Set technique. A heterogeneous unsteady domain decomposition approach is used, allowing to focus the computational effort onto the free surface vicinities, while the flow about the body is approximated by a potential flow model. Surface tension effects are investigated by progressively reducing the length scale, while keeping Froude and Reynolds number constant. Different flow regimes are recovered, ranging from intense plunging jet, eventually resulting in large amount of entrapped air, up to a micro-breaker, in which case air entrapment is suppressed and the jet is replaced by a bulge growing on the wave crest. At this scale, the surface tension is responsible for the large curvature at the toe of the bulge, and when the bulge slides upon the forward face of the wave, an intense shear layer develops from the toe. Instabilities of this shear layer are observed, and, when increasing the Reynolds number, the shear layer breaks up into coherent vortex structures that interact with the free surface, eventually leading to the formation of large surface fluctuations which propagate downstream.