Boundary Layer Characteristics Under Surface Solitary Wave on Continental Shelf and Coastal Zone

Sediment deposition and sea-bed erosion are two main phenomena associated with tsunami waves propagating into the near-shore zone. If accurately assessed, they can serve as a mean to estimate the times and recurrence intervals of past tsumai events. These two sediment-specific processes are mainly driven by the near-bed flow characteristics, i.e., turbulence and bottom stresses. It is these stresses that are responsible for significant wave drag in the coastal environment which impacts larger-scale energetics through the associated dissipation. To this end, an in-depth understanding of the near-bottom hydrodynamics and turbulence transition characteristics in the bottom boundary layer (BBL) under such waves is imperative, with the study of the BBL instability being a prerequisite.

In this study, numerically-simulated evidence of two possible scenarios to turbulence transition, as first reported in the laboratory by Sumer et al. ( J. Fluid Mech. vol. 646, 2010,p. 207) is presented. The particular experiments mimicked the BBL flow under solitary wave, adopted as a canonical model of transient long waves, using a U-shaped water tunnel in which the flow is driven by a soliton-like pressure gradient. The primary scenario is associated with the breakdown of the exponentially growing 2-D Tollmien-Schlichting waves. The alternative scenario consists of a characteristically different path to transition resulting from the formation of localized turbulent spots that lead to an earlier bypass transition to turbulence.

As for the first transition scenario, most evident in the decelerating phase of the wave, a detailed map that summarizes the different instability regimes, as a function of the base flow Reynolds number, is established. Furthermore, the different characteristics of the resulting 2D coherent structure are explored as a function of the different regimes. In terms of the alternative bypass transition, occurring during the accelerating phase of the wave, the formation of the turbulent spots is interpreted, theoretically, using non-modal stability analysis and, numerically, by means of direct numerical simulation. Implications of our findings for the energetics of wave-rich regions in the nearshore zone will be explored as well as the role of the above turbulence in driving sediment resuspension.