Investigation of the multi-scale interactions between an offshore wind turbine wake and the ocean-sediment dynamics in an idealized framework.

Tim Nagel, Julien Chauchat, Achim Wirth

Friday 3 july 2015

14:30 - 14:45h at Europe 1 (level 0)

Themes: (T) Hydro-environment, (ST) Renewable energy resources

Parallel session: 16G. Environment - Renewable

While the world assists to an increasing development of offshore renewable wind energy turbines, most the interactions between offshore wind farms and their local environment need further research. In the present contribution, 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 and the ocean and sediment layers together with the retro-action 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 Kelvin-Helmholtz instabilities or vortex streets for some parameter values. Shallow ocean layers (typically below 10m) 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. Considering the morphodynamics three cases are observed, depending on whether the ocean dynamics is laminar (i), has a localized (ii) or global (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 global turbulence case (iii), instantaneous seabed changes are of the order of a few centimeters 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 water depth is a key parameter for the coupled atmosphere-ocean-sediment system around wind turbines. Considering the ocean velocity in the atmospheric friction at the ocean surface leads to a decrease of 4 % of the power lost by the friction at the atmosphere-ocean interface. Ocean dynamics could thus have a significant feedback on the wind power available for the turbines and its variability.