Increased Mixing and Turbulence in the Wake of Offshore Wind Farm Foundations
Schultze, L. K. P.; Merckelbach, L. M.; Horstmann, J.; Raasch, S.; Carpenter, J. R., 2020: Increased Mixing and Turbulence in the Wake of Offshore Wind Farm Foundations. In: Journal of Geophysical Research: Oceans, Band 125, 8, DOI: 10.23689/fidgeo-4253.
The addition of offshore wind farms (OWFs) to stratified regions of shelf seas poses an anthropogenic source of turbulence, in which the foundation structures remove power from the oceanic flow that is fed into turbulent mixing in the wake downstream. The loss of stratification within the wake of a single OWF structure is observed for the first time by means of field observations, which enable a qualitative characterization of the disturbed flow downstream. These results are complemented with high‐resolution large eddy simulations of four different stratification strengths that allow for a quantification of turbulence and mixing quantities in the wake of a foundation structure. The turbulent wake of a structure is narrow and highly energetic within the first 100 m, with the dissipation of turbulent kinetic energy well above background levels downstream of the structure. A single monopile is responsible for 7–10% additional mixing to that of the bottom mixed layer, whereby ∼10% of the turbulent kinetic energy generated by the structure is used in mixing. Although the effect of a single turbine on stratification is relatively low, large‐scale OWFs could significantly affect the vertical structure of a weakly stratified water column. Further, rough estimates show that the rate of formation of stratification in the study area is of the same order of magnitude as the additional mixing promoted by the structures, thus OWFs could modify the stratification regime and water column dynamics on a seasonal scale, depending on local conditions and farm geometries.Plain Language Summary: Advances in the renewable energy sector have enabled the construction and operation of wind farms in bodies of water deep enough to present vertical temperature differences across the water column or thermal stratification. In coastal regions dominated by tidal motion, the presence of offshore wind farm (OWF) structures brings about additional turbulence and mixing of stratification. The present study combines field measurements and numerical simulations to characterize the wake of single OWF structures and quantify the amount of turbulence and mixing generated by them. Our results suggest that the effect of OWF structures is small compared to other naturally occurring mixing mechanisms, however can be comparable to the rate of stratification buildup. Stratification in certain regions of shelf seas could be impacted by OWFs if these are built over a large area.Key Points: Enhanced mixing and disturbed stratification in the wake of monopiles is traceable in field and turbulence‐resolving numerical experiments Elevated turbulent dissipation and mixing are found in a narrow region downstream of monopiles, with a bulk mixing efficiency of 8–14% The enhanced mixing generated by the offshore wind farm structures could contribute to significant changes in stratification in shelf seas
- Geographie, Hydrologie 
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