e
WAVE
AND
CURRENT INTERACTION
Horizontal
mean currents can transfer energy to or from internal
waves as the waves propagate across the shelf and can also cause caustics that reflect the waves back across the shelf. The bar tropic flow is assumed to be weakly sheared, so that the effective change in the local rate of rotation caused by the mean flow relative vortices does not appreciably alter the dispersion relation for the intrinsic frequency of the wave.
Whole-field kinematics measurements are presented of steep, steady waves travelling on strongly sheared currents, concentrating on the region between trough and crest level. For slab currents, the crest kinematics and wave parameters had found to be correctly obtained by using Doppler shifting and standard wave theories. When the current was sheared, the crest kinematics had found to be well predicted by adding the results of an irrotational
model to the stretched current profile.
The mean mass transport induced by
surface gravity waves is investigated theoretically for a deep, rotating ocean with a constant eddy viscosity. The waves are periodic in time and have amplitudes that grow or decay slowly in space. The analysis is based on a Lagrangian description of motion, and the results are valid to second order in the wave steepness. An equation for the wave-induced mean Lagrangian mass transport in the oceanic surface layer is derived. It is demonstrated that there are two sources for the mean mass transport: (a) the form drag associated with the fluctuating wind
stress normal to the wave slope and (b) the horizontal divergence of the mean wave momentum flux.
The energy, momentum, and mass-flux exchanges between surface waves and underlying Eulerian mean flows are considered, and terms in addition to the classical wave radiation stress are identified. The concept of surface wave radiation stress has proven useful in many scenarios, including wave-induced set-down outside the surf zone and set-up inside as waves shoal and break; generation of long-shore currents is the result.
It has long been recognized that wind-driven surface waves can significantly influence ocean currents. Modelling studies on this issue range from how the process of wave-current interaction influences the bed friction coefficient to how waves can affect the current field by enhancing the wind stress.
Based on the spectral eddy viscosity model of bottom boundary layers, the spectral representation of bottom friction and dissipation for irregular waves is reduced to an equivalent monochromatic wave representation. The representative wave amplitude and frequency are chosen so that the bottom velocity and bottom shear stress variances of the equivalent wave model are identical to those of the spectral model.
The Importance of Wave/Current Interactions
Knowledge of representative wave kinematics is crucial in the design of offshore structures, as loadings depend heavily on the local velocities and accelerations in the water. Most waves are generated by the wind, and there is some limited evidence for a strongly sheared, wind-driven current near the surface, during storms. The study is concerned with laboratory measurements of regular waves on strongly sheared currents, with parameters representative of those, which might be found in the real sea.
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