William M. Drennan

Flux measurements from a SWATH ship in SWADE

Abstract The Surface Wave Dynamics Experiment (SWADE) took place east of the U.S. Coast in the winter of 1990–1991. A major objective of the research program is to refine our understanding of the relationship between fluxes to the sea surface and the sea state as determined from directional wave spectra. Simultaneous measurements of turbulent fluxes of mass, momentum and energy between sea and air, with the directional wave spectra, were required to meet this objective. In this short article we describe the process of obtaining turbulent flux measurements from a small water-plane-area twin hull (SWATH) ship. We measured turbulent fluxes of momentum, heat and water vapor from a tall mast at the bow of the SWATH ship Frederick G. Creed by the eddy correlation method, while the ship was moving into the wind. Directional wave spectra were obtained from a wave staff array ahead of the bow of the ship. The motion of the ship was recorded and a coordinate rotation was performed for each data sample. After all instrument response and motion corrections have been accounted for, we compare our calculated turbulent fluxes with values computed from another standard method, viz. the inertial dissipation method. This approach is not susceptible to platform motion but relies on assumptions that are not always valid. However, the two methods agree on average within 12%, 20% and 31% for momentum, water vapor and heat fluxes, respectively.

Observations of the Enhancement of Kinetic Energy Dissipation Beneath Breaking Wind Waves

Most attempts to characterize the kinetic energy dissipation in the upper 20 metres of the water column revert to simple wall layer scaling — proportional to the cube of the friction velocity u ✱ and inversely proportional to depth, ∈αu ✱ 3 z −1With a concomitant logarithmic velocity profile, this is consistent with a total kinetic energy flux from the wind of 3 ✱ . However, in fully rough flow and strongly forced waves the energy input may be one to two orders of magnitude greater. Where does this energy go? Why is it not reflected in most of the upper layer measurements? This paper attempts to answer these questions and to demonstrate that there are two regimes of kinetic energy dissipation in the near surface layers under breaking waves. Near the surface, the dissipation rate is very high and scales with the wave characteristics. At greater depths the dissipation rate drops quickly and reverts to wall layer scaling. In the intermediate region the dissipation decays more rapidly than z−1. This may be viewed as a transition region between the deeper shear layer and the near surface region, with intense patches of breaking-imposed turbulence. In the absence of density stratification, dissipation in near surface region is analogous to that due to grid-generated turbulence and decays as z−4

The velocity field beneath wind-waves - observations and inferences

An extensive set of measurements taken from a fixed tower is used to study the velocity field under wind waves. Velocity measurements, made with miniature drag spheres, are compared with linear theory estimates of the orbital velocities obtained from measured surface elevation. Results are presented in the context of how well linear theory is able to predict wave-induced forces on cylindrical structural members. Linear theory is seen to predict the flow velocities to within about 7%. Furthermore, both the inertial and drag forces are generally well predicted by linear theory, although small scale turbulent motions not accounted for by linear theory can result in significantly higher inertial forces on smaller structural members.