Dynamics Of Internal Solitary Wave And Bottom Boundary Interaction

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Aghsaee, Payam
Sediment Re-Suspesion , Stratified Flow , Wave Breaking , Global Instability , Environmental Fluid Dynamics , Internal Solitary Waves
The breaking of internal solitary waves (ISWs) of depression shoaling upon a uniformly sloping boundary in a smoothed two-layer density field was investigated using high-resolution two-dimensional simulations. The simulations were performed for a wide range of boundary slopes S∈[0.01,0.3] and wave slopes. Over steep slopes (S≥0.1), three distinct breaking processes were observed; surging, plunging and collapsing breakers which are associated with reflection, convective instability and boundary layer separation, respectively. Over mild slopes (S≤0.05), nonlinearity varies gradually and the wave fissions into a train of waves of elevation after it passes through the turning point where solitary waves reverse polarity. The dynamics of each breaker type were investigated and the predominance of a particular mechanism was associated with a relative developmental timescale. The breaker type was characterized in wave slope S_w versus S space and the reflection coefficient (R), modeled as a function of the internal Iribarren number, was in agreement with other studies. The same 2D model was applied to investigate boundary layer separation-driven global instability, which is shown to play an important role in breaking of shoaling ISWs. The simulations were conducted with waves propagating over a flat bottom and shoaling over relatively mild (S=0.05) and steep (S=0.1) slopes. Combining the results over flat and sloping boundaries, a unified criterion for vortex shedding is proposed, which depends on the momentum thickness Reynolds number and the non-dimensionalized ISW-induced pressure gradient at the point of separation. The criterion is generalized to a form that may be readily computed from field data and compared to published laboratory and field observations. During vortex shedding, the bed shear stress, vertical velocity and near-bed Reynolds stress were elevated, implying potential for sediment re-suspension. Laboratory experiments were also performed to study three-dimensionality (3D) of global instability. Our results agree with previous laboratory experiments, using the proposed pressure gradient parameter and Reynolds number. The 3D effects prevent the vortices from ascending as high as they do in 2D simulations. The instabilities were not able to re-suspend sediments with 20 µm median diameters, which suggests applying lighter sediments, as finer sediments will be cohesive and dynamically different.
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