Turbulence and coherent structures in fully developed open-channel flows beneath periodic surface submergence

Periodic surface submergence, which frequently occurs in floating installations such as solar farms and aquaculture systems, induce a surface boundary with roughness heterogeneity that fundamentally disrupts the boundary layer characteristics and reshapes underlying turbulence. This study experimentally investigates the turbulent structures and free-surface dynamics induced by periodic submergence in a fully developed flow using high-resolution particle image velocimetry. Spanwise-oriented semi-submerged bars with 70% open spacing were deployed periodically in the streamwise direction, at submergence depths ranging from 1% to 12%, which represents realistic prototype scenarios. Time-averaged results of the instantaneous measurements revealed that the flow asymmetry increased with the submergence depth together with turbulence intensification near the surface. At higher submergence, streamwise velocity fluctuations reached up to 18% of the local maximum velocity, while both the velocity peak and zero-shear-stress position shifted downward, reaching up to 15% of the flow depth. Two-point correlation analysis showed that the streamwise and wall-normal extent, as well as the inclination of hairpin packets, also increased with submergence, surpassing those induced by traditional two-dimensional rough walls. Spectral analysis identified a consistent dominant Strouhal number of St=0.11 for all test cases. Dynamic mode decomposition revealed dominant coherent structures aligned with the harmonics of this frequency, which became more pronounced and elongated with increasing submergence. Strong free-surface oscillations also emerged under high submergence, exhibiting amplitudes up to 0.5 times the submergence depth. These findings underscore the distinct turbulence mechanisms underneath periodic surface submergence and improve the physical understanding toward hydrodynamic assessments.