Session: 06-03-06 Fluid-Structure, Multi-body and Wave-body Interaction - VI

Submission Number: 177942

Structural Deformation and Hydrodynamic Load Characteristics of Flexible-Buoyant Vegetation Under Oscillatory Wave Loading: Cauchy Number Analysis and Drag Validation 

The mechanical behavior of flexible structures under hydrodynamic loading is fundamental to designing effective fluid-structure interaction systems. This study investigates the coupled mechanical response of buoyant, flexible vegetation subjected to periodic wave forcing, establishing relationships between material properties, structural deformation, and hydrodynamic forces.

Experiments were conducted in a large-scale wave basin (16.8 m × 12 m) at Queen’s Marine Laboratory using two vegetation materials with contrasting mechanical properties: EVA foam (elastic modulus E_EVA = 0.507 MPa, density ρ_EVA = 310 kg/m³) and LDPE membrane (E_LDPE = 310 MPa, ρ_LDPE = 820 kg/m³). The flexural rigidity values of 2.79 × 10⁻⁵ N·m² for LDPE and 1.63 × 10⁻⁶ N·m² for EVA highlight the substantial stiffness contrast between the two materials by a 17-fold difference in flexural rigidity. This pairing was intentionally selected to systematically examine how vegetation stiffness affects wave attenuation while maintaining consistent hydrophobic properties to ensure mechanical stability during testing. Direct hydrodynamic characterization was performed using a dedicated towing system, yielding a drag coefficient of Cd = 1.31. Wave basin experiments were conducted across five wave periods (0.9–1.8 s) and wave heights ranging from 20 to 160 mm, with a constant water depth of 550 mm.

Cauchy number (Ca), analysis reveals distinct deformation regimes. EVA vegetation exhibits Ca values from 5 to 1400, while LDPE remains semi-rigid (Ca < 2). Critically, buoyancy governs structural posture: EVA's high buoyancy maintains vertical orientation, while LDPE's near-neutral buoyancy causes pre-bending independent of wave forcing.

Drag force analysis from towing system measurements (Cd = 1.31) examined across all reconfiguration states reveals that EVA maintains buoyancy-dominated behavior (drag-to-buoyancy ratios of 1–46), enabling effective vertical posture maintenance, while LDPE exhibits drag-dominated regimes (ratios of 1–150) due to insufficient buoyancy. This demonstrates that material buoyancy for ribbon-shaped stems, not flexural rigidity alone, governs the force balance and structural stability in flexible vegetation systems.

Wave reflection characteristics were analyzed using the Mansard–Funke three-probe method. The results show that structural reconfiguration systematically alters the distribution of wave energy, with reflection coefficients ranging from 11.1% to 35.0%, depending on the deformation regime. These reflection values correspond to 1.2–12.3% of the incident wave energy, with the remaining energy distributed between transmission and dissipation. Energy conservation analysis indicates that about 90% of the incident energy is either transmitted or dissipated, primarily due to the combined effects of mechanical damping, wake generation, and geometric changes caused by reconfiguration.

These results establish the mechanistic foundation for predicting fluid-structure interaction behavior in flexible vegetation systems, providing experimental validation for coupled computational models under development. The findings advance understanding of how material properties and buoyancy jointly govern structural deformation and load response under hydrodynamic forcing.

Presenting Author: Saeideh Baghanian Queen&#39;s University Belfast

Presenting Author Biography: I’m a civil engineer specializing in coastal engineering, currently pursuing a PhD at Queen’s University Belfast, where I work with the Marine Research Group in Belfast and Portaferry. My research focuses on wave attenuation by coastal and marine vegetation—a field that blends traditional engineering with nature-based solutions to promote sustainable and resilient coastal environments in the face of climate change and natural hazards.

Authors:

Saeideh Baghanian Queen's University Belfast
Pal Schmitt Queen's University Belfast
Nipuni Odara Merenchi Galappaththi Queen's University Belfast