Flow- and wave-fields associated with micro-scale wave breaking as measured simultaneously by particle tracking and wave slope visualization

Longuet-Higgins predicted the development of a capillary roller at the crest of a short gravity wave, being the consequence of the existence of parasitic capillaries, (Capillary rollers and bores, J. Fluid Mech, Jan. 1992). According to the theory any free surface, at which the tangential stress vanishes, a surface vorticity is produced. The mean vorticity at the edge of the Stokes layer is of the order of 2(ak) f, ak being the slope and f the radian frequency of the wave. Hence capillary waves, with a high value of a are accompanied by particular large vorticity feeding the vortex roler at the crest of a short gravity wave. This phenomenon has been investigated at the circular facility of the Institute for Environmental Physics. At the same location, simultaneous time series of wave slope images and the Lagrangian flow field were obtained using an integrated optical set-up, including particle-tracking and wave-slope instruments. The particle tracking measurement yields drift- and bulk-velocities, Reynold stresses and the vorticity distribution beneath the wave, while the imaging wave slope gauges gives the 2D-slope and the height of the waves. First results from the measurement campaign of early 1995 will be presented.