Hydrodynamic forces of a prescribed oscillating cylinder in steady currents

Predicting hydrodynamic forces on an obliquely oscillating riser in current poses challenges, especially as existing practices rely on a linear summation of forces induced by the current and riser motion, which overlooks crucial nonlinear interactions. The present study, using three-dimensional numerical simulations at a fixed Reynolds number of 500 and a small oscillation amplitude of 0.4 cylinder diameters, evaluates these nonlinear effects over a range of oscillation wavelengths (λ* from 1.0 to 25.0) and orientation angles (θ from 0° to 90°). The results reveal significant nonlinear interactions at intermediate λ*=1.5–10, forming multiple resonant wake states that either amplify or reduce the hydrodynamic forces. For instance, time-averaged force coefficients in inline and transverse directions increase by more than 2.5 times compared to the no-oscillation case, while the root-mean-squared force coefficient in the motion direction can drop to a minimum (0.1–0.3) at λ*=4–10. Physical explanations on the influences of inclination angle on the wake dynamics and hydrodynamic force coefficients are provided. These findings indicate the inadequacy of linear summation method and propose a new framework incorporating data at 0°, 45°, and 90°, offering improved predictive capability for oblique angles.