Comparison between pressure-driven and shear-driven oscillatory flows over ripples*

The seabed in littoral zones is characterized by an oscillatory flow motion generated by sea surface waves through gravity. The so-called wave bottom boundary layer affects the whole seabed ecosystem, from remodeling the bed morphology to the transport of nutrients and substances dispersed in water. To date, a complete understanding of this flow is still lacking. Commonly, in numerical or experimental laboratory studies, the oscillations of the bottom boundary layer are induced through either shear-driven devices (such as an oscillating tray bed in a tank with still water) or pressure-driven mechanisms (using, for example, oscillating water tunnels) to approximate field conditions. There has been relatively limited attention to compare the two types of forcings. The objective of this work is a systematic comparison of shear-driven and pressure-driven oscillatory flow over a bed made of ripples, such as those that are typically encountered in littoral areas. The study will be conducted using direct numerical simulations, with the immersed boundary method to model the bed geometry. Two oscillatory-flow forcing mechanisms (pressure-driven and shear-driven) will be compared for the same conditions. Implications for the dynamics of the seabed will be analyzed and discussed.