Wake-vortex transitions of a submerged horizontal cylinder under combined wave–current action

The nonlinear interaction between ocean waves and currents significantly modifies wake dynamics around submerged horizontal cylinders, with important consequences for hydrodynamic loading on offshore structures. This study combines laboratory experiments, synchronized three-component load measurements, and flow visualization to investigate wake-vortex transitions and vertical force frequency characteristics for a fully submerged horizontal cylinder in the ranges Re = 1.2 × 10⁴–4.8 × 10⁴ and Kc = 0.09–2.19. The vertical force spectra are classified into three distinct modes: wave-dominated (mode I), subharmonic (mode II), and vortex-dominated (mode III), according to the position of the low-frequency component f_l in the spectra. The non-dimensional low frequency f_lT is primarily governed by Re, the velocity ratio λ = U/U_w, and the period ratio τ = TU/D, and captures the mechanism of spectral mode transitions. Vortex identification reveals that wake-vortex patterns evolve from nearly symmetric, wave-locked structures in mode I to alternating and asymmetric shedding in modes II and III. The broadband low frequency vertical force results from transitions between different shedding modes, with mode III being more unstable and sensitive to such transitions. The results provide a quantitative basis for evaluating mode transition mechanisms for the horizontal cylinder under combined wave–current action.