Investigation of particle-bed pore effect on turbulent structure in oscillatory boundary layer by RIM-PIV measurement

In surf/swash sediment transport in coastal processes, the influence of pore flow near the surface of the sediment bed cannot be ignored. Understanding how the pore flow affects the turbulent structure of the oscillating boundary layer is crucial for comprehending the mechanisms of bottom sediment transport. However, measurement is difficult and the turbulent structure in the porous sublayer is not well understood. As water is usually used as the fluid in hydraulic experiments, turbulent structures, including pore flow, can be measured by RIM-PIV (refractive index matching - particle image velocimetry) if the sediment bed is formed using a material with the same refractive index as water. Hydrogel particles have the same refractive index as water, but are soft and lightweight, making them unsuitable for forming stable sediment beds. Hence, in this study, sediment beds were formed using MEXFLON Ⓡ (NOK Corporation) particles and the turbulent structure of the oscillatory boundary layer, including the porous sublayer, was measured by RIM-PIV. Measurements of the turbulent structure, including the effect of infiltration and exfiltration caused by pore flow, indicated the presence of anisotropic fluctuating velocities and intermittent turbulent structure. The magnitude of the stress fraction, which indicates momentum exchange due to fluctuating velocities in the porous sublayer, was slight compared to the roughness and interface layers, but contributed strongly to the development of turbulent structures in the oscillatory boundary layer, especially in the acceleration phase of the forward flow. It was suggested that the presence of the porous sublayer causes strong nonlinearities in the turbulent structure of the boundary layer.