Interactions Between Internal Solitary Waves and Floating Canopies

TL;DR

This study addresses how internal solitary waves (ISWs) propagate in a two-layer stratified fluid when encountering floating porous canopies. It combines laboratory experiments with 2D numerical simulations, modeling the canopy as a porous zone whose hydraulic resistance follows the Kozeny–Carman framework and using the extended Korteweg–de Vries solution to drive the wave maker. The key findings show that transitional canopies minimally affect ISWs, whereas dense canopies drive strong nonlinear interactions via a vortex-induced jet, leading-edge shoaling, phase-speed reductions, and energy dissipation, with an energy budget indicating $E_t=E_k+E_a$ and dissipation scaling roughly as $O(a^2)$ for small amplitudes. These results enhance understanding of ISW–floating-structure interactions and have practical implications for coastal mixing, nutrient transport, and macroalgal farming designs near floating canopies.

Abstract

Interactions between internal solitary waves and floating canopies of varying length and porosity are examined via laboratory experiments and complementary simulations for a miscible, two-layer system. In both approaches, internal solitary waves of varying amplitudes are generated by a jet-array mechanism that is driven by the nonlinear eKdV solution. Pycnocline displacements, phase speeds, and velocity fields are obtained using synchronized planar laser-induced fluorescence and particle imaging velocimetry systems in the experiment. In the simulations, the canopy is represented as a porous zone with prescribed porosity and hydraulic conductivity determined by the Kozeny-Carman model, which is validated by comparing simulated and measured horizontal velocity profiles. The higher-porosity (transitional) canopy produces a nearly monotonic, albeit minor, amplitude reduction and negligible wave energy dissipation after the interaction. However, the shear layer developed at the bottom edge of the lower-porosity (dense) canopy grows to a comparable strength as the shear sustained by the internal solitary wave profile at the pycnocline. The vortex pair generated by this shear accelerates the upper-layer fluid beneath the canopy, leading to complex nonlinear amplitude modulation and significant wave transformation. With an extended canopy length, the internal solitary waves settle to a quasi-steady state with a significant phase speed reduction. Upon the wave exiting the canopy, flow separation at the downstream edge of the canopy again pairs with the shear at the pycnocline, inducing an intensified jet. This complex interaction leads to energy transfer between kinetic and potential energy under the dense canopy.

Figures (23)

• Figure 1: (a) Schematic for the experimental set-up of interactions between ISWs and floating canopies, (b) a photo image of the jet-array wave maker.
• Figure 2: Cross-sectional view and nomenclature of the canopy structure.
• Figure 3: Isometric views of the canopy structures. (a) Transitional canopy with porosity $n=0.964$. (b) Dense canopy with $n=0.648$.
• Figure 4: An illustration of the two-step filling method to ensure sufficient particle density in the upper layer.
• Figure 5: (a) Schematic of the synchronized PLIF and PIV measurement system. (b1) Observation window coverage for short canopy, (b2) for long canopy conditions.