Cross-stream pollutant dispersion in the wake of a cylindrical marine structure under oscillatory currents

Pollutant dispersion in the wakes of fixed cylindrical marine structures is strongly influenced by interactions between vortex shedding and oscillatory currents, yet the role of their relative timescales in cross-stream spreading remains unclear. We use a two-dimensional large-eddy simulation (LES) model, validated by obtaining mean drag coefficients (CD) and Strouhal numbers (St) well within the ranges reported in classic benchmark studies, to examine wake dynamics and passive-scalar transport around a fixed circular cylinder representing an idealized cylindrical marine structure. The inflow combines a steady current with a sinusoidal oscillatory component. Because the steady velocity exceeds the oscillatory amplitude, the total flow remains unidirectional while varying periodically, with instantaneous Reynolds numbers of 500–1500. When the oscillatory component opposes the steady current, reduced advection weakens Lamb-vector-diagnosed transverse confinement and increases the initial cross-stream offset of newly shed vortices. Farther downstream, slower transport prolongs vortex residence and interaction, promotes vortex collision, merging, and cross-stream (lateral) deflection. Elevated scalar concentrations remain closely associated with high-vorticity regions, directly linking plume spreading to vortex evolution. The estimated cross-stream dispersion coefficient varies non-monotonically across the tested oscillation frequencies and peaks when the estimated vortex-shedding and inflow-oscillation timescales are comparable. Under this condition, it exceeds the steady-current value by more than fivefold. These results suggest that the observed cross-stream pollutant dispersion is closely associated with the wake-vortex organization corresponding to the prescribed nominal frequency ratios, offering process-based insights into localized plume dynamics that form a mechanistic hypothesis for scalar transport around idealized cylindrical structures under unsteady forcing.