Momentum transfer under laboratory wind waves

In this study, we explain contradictory previous observations of the contribution of coherent wave organised motion to the downward transfer of momentum through wave Reynolds stresses (ρu˜w˜¯) below wind waves. The generation of non-zero u˜w˜¯ is potentially significant because 1) the turbulent mixing is not then the only momentum transport mechanism under wind waves as was previously assumed, 2) this provides a wave-current energy exchange pathway that could explain inconsistencies in measured air- and water-side wind-wave energy transfer and 3) it can be a critical term in the wave-current coupling formulation under wind waves. However, such a mechanism for momentum transfer has generally been ignored, since contradictory observations were reported. Here, two new sets of wind-wave laboratory experiments are reported. For the first set, contradictory u˜w˜¯ were observed, as in previous literature. Investigating the sources of such inconsistency, we examined spatial inhomogeneity due to wave reflection through a second set of experiments, by varying instrument location and additionally considering random waves. The results resolve the inconsistencies observed in the first set of experiments and previous measurements. In addition, we emphasise the contribution of secondary circulation cells in momentum transfer under wind waves.