Orbital rotational motion and turbulence below laboratory wind water waves

Abstract

New experimental results describing the structure of both orbital and turbulent motions below laboratory wind water waves are presented. The data obtained by means of a submersible laser probe are processed through the triple decomposition method developed by the authors. This method allows one to distinguish three contributions in the fluctuating motion, namely the potential and rotational parts of the orbital motion, as well as the turbulent fluctuation. The results show that the orbital rotational motion has spectral properties very similar to those of its potential counterpart and represents a contribution of significant magnitude. The behavior of all three components of the turbulent motion is discussed. Their near‐surface level is comparable with that found near a wall, but their vertical decay is quite different The dissipation rate estimate confirms that under present conditions the turbulence is essentially unaffected by the orbital motion. In contrast, a study of the cross correlations between orbital rotational and potential motions shows that the rotational contribution plays a key role in energy transfers between the wave motion and the mean shear flow. Finally, the origin of the orbital rotational motion is addressed. Several theoretical mechanisms capable of contributing to the generation of a wave‐related component of the vorticity are examined. Comparison between theory and experiments supports the idea that in laboratory experiments an important part of the orbital rotational motion results from wave‐current interactions linked to the vertical variations of the mean shear.