Wave-induced boundary layers underneath a flexible structure

• First experimental observation of wave-induced boundary layers beneath a continuous, flexible structure throughout the wave cycle. • For second-order, low-steepness waves, an adapted Stokes model accurately predicts boundary layer thickness, overshoot, and velocity decay with depth. • For attenuating, moderately steep second-order waves, the model fails to reproduce the observed velocity decay, boundary layer thickness, and peak values. • In third-order wave conditions, the decay profile aligns with theory, but the boundary layer. The interaction between propagating waves and flexible floating structures remains poorly understood, particularly in terms of interface boundary layer characteristics, which play a role in influencing wave attenuation and energy dissipation. In this study, phase-locked Particle Image Velocimetry was used to capture velocity fields beneath a continuous, compliant structure subjected to regular waves of varying order and steepness. The results confirm the formation of a distinct wave-induced boundary layer at the fluid-structure interface. For second-order waves with low steepness ( H / λ = 0.02 ), an adapted form of Stokes’ second problem predicts the boundary layer thickness, velocity overshoot, and exponential decay with depth reasonably well. However, for steeper second-order or third-order waves ( H / λ = 0.033 –0.064), the experimental data reveal significant deviations from the model - specifically in the boundary layer structure and in the slope of the velocity profile for attenuating waves. These discrepancies suggest that classical linear theory is insufficient in capturing the full complexity of wave-structure interactions as wave steepness increases and nonlinear effects become more pronounced.