Increased Mixing and Turbulence in the Wake of Offshore Wind Farm Foundations

Abstract

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.