Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Ali Al Sam (1) F P Robert Szasz (1) Johan Revstedt (1)
(1) Lund University, Lund, Sweden
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Mr Ali Al Sam is a PhD candidate in the departments of Energy science at Lund University, Sweden. His area of research includes the study of wind-wave interactions and the effects of this interaction on offshore wind energy. His approach is based on computational fluid dynamic and in particularly Large Eddy Simulations and moving mesh. His project is a part of OffWind Project (Prediction tools for offshore wind energy generation).
Extrapolate surface velocity to hub height depending on sea state
The accurate prediction of flow characteristics in the offshore environment requires knowledge of the wind-wave interaction phenomena. Atmosphere and ocean are coupled dynamically by the momentum exchange at the air-sea interface. The wave effects are commonly thought to be limited to a few meters above the water surface and usually considered as an aerodynamic roughness.
In order to understand the influence of sea state on vertical momentum flux and turbulent flow, Large Eddy Simulations are used to investigate turbulence flows over various waving boundaries. Airy and Stokes waves are considered with a wide range of wave phase speed and wave steepness.
Main body of abstract
In order to derive the logarithmic wind profile from Monin-Obukhov (MO) similarity theory, the aerodynamic roughness is consistently treated in offshore wind energy applications either as a constant or as a function of friction velocity without regard to its dependency on the sea state(e.g. wave- age, steepness and shape). More elaborate work treats the aerodynamic roughness as a fetch dependent. Because of the difficulties of conducting accurate measurements close to the sea surface and scarcity of simultaneous measurements of ocean waves and winds, the sea state effects on momentum fluxes, and by that on the atmospheric boundary layer, are unclear.
Field observations and numerical simulations have shown that the impact of the waves, in particularly the non-locally generated waves, on the atmospheric surface layer might be stronger than previously assumed. The waves can affect the near surface momentum transfer  and alters the wind velocity profile . Under the conditions of low wind and fast waves, waves cause an upward momentum flux from the ocean toward the atmosphere . No- locally generated waves can cause a significant misalignment between winds and wind stress which invalidates the use of the MO similarity theory . More recently Di Yang et al.  examined the effect of moving waves on the offshore wind farm numerically by using LES and found that the waves have appreciable effect on the wind turbine power performance and a higher wind power extraction rate is founded under the action of fully developed wind-sea condition comparing to that under the fetch-limited condition.
Detailed analysis of airflow and turbulent structures over moving waves give insight on their dependence on the sea state. The results show that the wave effects on the ABL are significant. In the case of fast wave the wave-induced stress acts against the turbulence stress, resulting in reduction in the total wind stress and hence larger wind velocity and lower wind vertical shear. By increasing the wave to wind speed ratio or/and the wave amplitude, the effects of the wave increase and higher deviation of the velocity and turbulence intensity profiles from the profiles of the logarithmic are noticed.
The validity of MO similarity theory is dependent on sea state.