A triple decomposition of the fluctuating motion below laboratory wind water waves

Abstract

Understanding of the dynamics of the top meters of the ocean requires an improvement of present knowledge about interactions between wind waves, mean sheared current, and turbulence. To achieve this goal, a key point lies in relevant definitions and evaluations of orbital and turbulent motions. The aim of this paper is to build and validate a separation technique allowing one to distinguish all three crucial contributions of the fluctuating motion, namely the potential and rotational parts of the orbital motion, as well as turbulent fluctuations. The whole method is first developed and tested for periodic rotational waves. The first step of this technique consists of a determination of the instantaneous stream function associated with the potential motion induced by the waves in presence of a linear sheared current. The second step consists of a linear filtration of the remaining motion from which the orbital rotational motion is extracted. The strong hypotheses involved in the technique are then carefully checked and shown to be relevant in the case of laboratory wind waves. The method is finally applied to experimental data obtained by laser Doppler velocimetry measurements in a wind‐water laboratory facility. Influence of the mean sheared current on the prediction of orbital velocities is pointed out, and the orbital rotational contribution is found to have a significant magnitude.