INVESTIGATION OF THE MULTI-SCALE INTERACTIONS BETWEEN AN OFFSHORE WIND TURBINE WAKE AND THE OCEAN-SEDIMENT DYNAMICS IN A INDEALIZED FRAMEWORK

摘要

A coupled two dimensional idealized numerical model of the ocean and sediment layers, forced by an offshore wind turbine wake is used to investigate the complex interactions between the wake, the ocean and the sediment layers, together with the retroaction on the wind energy. Results show that the turbine wake has an impact on both, the ocean and the sediment layers. The turbine wake impacts the ocean surface and generates instabilities or vortex streets for some parameter values. Shallow ocean layers (typically below 15m) are laminar. When water depth is higher, large scale instabilities are generated, leading to a turbulent dynamic in the ocean layer. The size of the generated vortices in the ocean increases with water depth and decreases with the quadratic-law bottom friction coefficient. Considering the morphodynamics three cases are observed, depending on whether the ocean dynamics is laminar (i), has a localized (ii) or domain wide (iii) turbulent behavior. In the first case, changes in seabed elevation are around a few millimeters per month. Results are similar for the localized turbulence case with small spatial variations. For the domain wide turbulence case (iii), instantaneous seabed changes are of the order of a few millimeters per month, whereas the transport averaged over several days decreases to a few tenths of millimeter per month. This behavior is easily explained by the oscillating local velocity which transports sediments back and forth. The above emphasizes that the water depth is a key parameter for the coupled atmosphere-ocean-sediment system around wind turbines. Furthermore, considering the ocean velocity in the atmospheric forcing at the ocean surface leads to a decrease of 4 % of the power lost by friction at the atmosphere-ocean interface. Ocean dynamics could thus have a non-negligible feedback on the wind power available for the turbines and its variability.