  Wind-wave interactions impose wind forcing on wave surface and wave effects
on turbulent wind structures, which essentially influences the wind-wave
loading on structures. Existing research treats the wind and wave loading
separately and ignores their interactions. The present study aims to
characterize the turbulent airflow over wave surfaces and analyze the coupled
wind-wave loading on offshore wind turbines. A high-fidelity two-phase model is
developed to simulate highly turbulent wind-wave fields based on the
open-source program OpenFOAM. A numerical case study is conducted to simulate
extreme wind-wave conditions, where coupled wind-wave fields are applied. The
intensity of wave induced turbulences and the height of wave influenced regions
are affected by the wind velocity and wave heights. Higher wind velocities
induce greater turbulence, which can be increased by over 100%. Then the
combined wind-wave loading on offshore wind turbines is simulated under
operational and extreme conditions. Under operational conditions, the wind-wave
coupling effect on the combined loading is minimal. However, under extreme
conditions, the coupled wind-wave fields lead to an increase in the average
aerodynamic loading and a significant amplification of the fluctuation in the
aerodynamic loading. Specifically, the maximum bending moment Fy at both the
tower bottom and the monopile bottom experiences an increase of around 6%. The
standard deviation of the shear force at the tower bottom increases by up to
45%. Also, the standard deviation of the bending moment at the tower bottom
increases by approximately 27%. This study reveals the importance of
considering the wind-wave coupling effect under extreme conditions, which
provides valuable insights into the planning and design of offshore wind
turbines.
