TY - JOUR

T1 - Increased Mixing and Turbulence in the Wake of Offshore Wind Farm Foundations

AU - Schultze, L. K.P.

AU - Merckelbach, L. M.

AU - Horstmann, J.

AU - Raasch, S.

AU - Carpenter, J. R.

N1 - Funding information: This project was conducted within the Polar Regions and Coasts in the Changing Earth System II program (PACES II) funded by the Helmholtz Foundation, which we gratefully acknowledge. The authors would like to thank the support of R. Kopetzky, M. Heineke, B. Peters, T. Kock, M. Cysewski, M. Stresser, the Captain, and the crew of the research vessel (RV) and the RV during the towed chain measurements. The RV ship time was provided through the Grant AWI_HE445_00. We gratefully acknowledge the German Federal Ministry of Economic Affairs and Energy (BMWi) and the Project Management Jülich (PTJ), which provided access to the temperature measurements of the platform FINO3. We would like to thank the PALM group at the Leibniz Univertität Hannover, especially Björn Maronga and Matthias Sühring, for their support with the PALM model. Ludwig Prandtl Heincke Heincke This project was conducted within the Polar Regions and Coasts in the Changing Earth System II program (PACES II) funded by the Helmholtz Foundation, which we gratefully acknowledge. The authors would like to thank the support of R. Kopetzky, M. Heineke, B. Peters, T. Kock, M. Cysewski, M. Stresser, the Captain, and the crew of the research vessel (RV) Ludwig Prandtl and the RV Heinckeduring the towed chain measurements. The RV Heincke ship time was provided through the Grant AWI_HE445_00. We gratefully acknowledge the German Federal Ministry of Economic Affairs and Energy (BMWi) and the Project Management J?lich (PTJ), which provided access to the temperature measurements of the platform FINO3. We would like to thank the PALM group at the Leibniz Univertit?t Hannover, especially Bj?rn Maronga and Matthias S?hring, for their support with the PALM model.

PY - 2020/7/31

Y1 - 2020/7/31

N2 - 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.

AB - 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.

KW - mixing

KW - mixing efficiency

KW - monopiles

KW - offshore wind farms

KW - stratification

KW - tidal shelf seas

UR - http://www.scopus.com/inward/record.url?scp=85089904831&partnerID=8YFLogxK

U2 - 10.1029/2019JC015858

DO - 10.1029/2019JC015858

M3 - Article

AN - SCOPUS:85089904831

VL - 125

JO - Journal of Geophysical Research: Oceans

JF - Journal of Geophysical Research: Oceans

SN - 2169-9275

IS - 8

M1 - e2019JC015858

ER -