Abstract
Collinear wave-current shear interactions are often assumed to be the same for currents following or opposing the direction of regular wave propagation; with momentum and mass exchanges restricted to the thin oscillating boundary layer (zero-flux condition) and enhanced but equal wave-averaged bed shear stresses. To examine these assumptions, a prototype-scale experiment investigated the nature of turbulent exchanges in flows with currents aligned to, and opposing, wave propagation over a mobile sandy bed. Estimated mean and maximum stresses from measurements above the bed exceeded predictions by models of bed shear stress subscribing to the assumptions above, suggesting the combined boundary layer is larger than predicted by theory. The core flow experiences upward turbulent fluxes in aligned flows, coupled with sediment entrainment by vortex shedding at flow reversal, whilst downward fluxes of eddies generated by the core flow, and strong adverse shear can enhance near-bed mass transport, in opposing currents. Current-aligned coherent structures contribute significantly to the stress and energy dissipation, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. These preliminary findings suggest a notable difference in wave-following and wave-opposing wave-current interactions, and highlight the need to account for intermittent momentum-exchanges in predicting stress, boundary layer thickness and sediment transport.
Highlights
Fluid motion in the coastal zone is often characterised by the simultaneous presence of both oscillatory and unidirectional currents
The hydrodynamic forcing, non-directional spectral properties of the incident wave field, namely the peak wave period, TP, and the mean (Hm ); significant (Hm0 ); and maximum (Hmax ) wave heights inferred from the immersed water depths, and the average flow velocities measured, at each acoustic Doppler velocimeters (ADV)
Our results show that an equal magnitude current opposing waves (Run O1) appears to have a more significant impact on time-averaged shear stresses than in the aligned case (Run A1) and high, mean and maximum shear stresses are outside the theoretical oscillatory boundary thickness, which the GM86, SC05 and MD12 models suggest to be confined to a few millimeters in the immediate vicinity of the bed
Summary
Fluid motion in the coastal zone is often characterised by the simultaneous presence of both oscillatory (e.g., surface wind and swell waves) and unidirectional currents (e.g., tidal, fluvial, wind-driven, rip currents, etc.). The interaction between currents and waves is extremely complicated [1], and has direct implications on sediment transport and bedform dynamics, as well as engineering problems such as the design of offshore structures and beach replenishment. Non-linear interactions between the steady shear of the current and the oscillating flow often result in enhanced stresses beyond the simple summation of the current-only and wave-only components [1,3]. As a result, combined wave-current flows often result in highly dynamic bedforms and active sediment transport across the continental shelf and shallow coastal waters [5,6]
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