Incipient breaking of steady waves in the presence of surface wakes

The effect of free-surface drift layers on the maximum height that a steady wave can attain without breaking is explored through experiments and numer ical simulations. In the experiments, the waves are generated by towing a t wo-dimensional fully submerged hydrofoil at constant depth, speed and angle of attack. The drift layer is generated by towing a plastic sheet on the w ater surface ahead of the hydrofoil. It is found that the presence of this drift layer (free-surface wake) dramatically reduces the maximum non-breaki ng wave height and that this wave height correlates well with the surface d rift velocity. In the simulations, the inviscid two-dimensional fully nonli near Euler equations are solved numerically. Initially symmetric wave profi les are superimposed on a parallel drift layer whose mean flow characterist ics match those in the experiments. It is found that for large enough initi al wave amplitudes a bulge forms at the crest on the forward face of the wa ve and the vorticity fluctuations just under the surface in this region gro w dramatically in time. This behaviour is taken as a criterion to indicate impending wave breaking. The maximum non-breaking wave elevations obtained in this way are in good agreement with the experimental findings.