Preliminary Numerical Study on the Influence of a Wind Field on Wave-Induced Load on a Circular Cylinder

The design load for offshore structures can be established from experimental and numerical investigations. When these are conducted, only the indirect effect of wind is taken into account. I.e. the wave spectrum is defined from fetch and wind speed. Nevertheless, the wind can have a direct effect on steep waves if airflow separation and vortexes develop above the waves. This could potentially cause increased wave-induced loads or change the breaking probability for waves and thereby the load statistics. This paper presents preliminary results from numerical simulations on how a local wind field affects wave kinematics and wave-induced loads on a cylinder. The waves are generated as spatio-temporal focused wave train. The wave field, including surface elevation and kinematics, is computed with the fully nonlinear potential solver program, OceanWave3D. The wave-induced load on the cylinder is computed from the output of the kinematics and the FNV force model. The wind forcing term is modelled by means of Jeffrey's sheltering mechanism. Wave field and wave-induced loads are compared for different wind velocities and configurations of a focused wave. The presence of wind above a steep non-breaking wave increases the surface elevation until breaking is initiated for high wind velocity. The maximal wave-induced load for an initial non-breaking wave is obtained for the highest wind velocities due to the sudden initiation of breaking. The capability of the wind to increase surface elevation and load for wind above initially breaking waves is more questionable. The numerical model simply exchanges the energy transfer between breaking dissipation and wind energy differently depending on wind velocity and wave field; nevertheless, no significant increase in surface elevation or load is discovered in this case. The highest wind velocity can, on the contrary, lead to a second breaking wave, which increases the line force. Finally, the numerical simulations are validated successfully against experimental investigations without wind.