Second-harmonic pressure loading and near-field flow around a submerged horizontal cylinder under regular waves

This study experimentally investigates second-harmonic loading and near-field flow around a submerged horizontal cylinder. Free-surface elevation, hydrodynamic forces, circumferential pressures, and near-cylinder velocity fields were measured synchronously under regular waves spanning Kc ​ = 0.252–2.116. Cycle-resolved pressure indicators and harmonic decomposition were used to quantify circumferential pressure redistribution and harmonic responses. The pressure field becomes increasingly asymmetric and localized as the wave condition intensifies. The upstream waves contain a measurable second-harmonic component, while the force responses differ markedly between directions: circumferential cancellation weakens the horizontal component, whereas the local pressure harmonics are retained more effectively in the vertical component. Both the relative and absolute second-harmonic pressure amplitudes are preferentially enhanced over the upper-right part of the cylinder. PIV measurements reveal a non-zero cycle-averaged residual flow and an intensified second-harmonic velocity component in the same region. These results indicate that the cylinder redirects and concentrates the upstream orbital motion, thereby organizing the local second-harmonic pressure response. Overall, the hydrodynamic response remains dominated by the upstream-wave frequency, accompanied by a weaker but spatially organized second-harmonic component.