Resolving the wakes of offshore wind infrastructure in two layer flows

To enable a global net zero future the next decade will see exponential growth of offshore wind renewable energy. This scale of development necessitates expansion into deeper, seasonally stratified, waters, for the first time. This transition from shallow well-mixed regions to deeper waters marks a fundamental change in the marine environment, due to the importance of vertical density gradients on fluid dynamics. Seasonal stratification is a vital control on marine ecosystems; primary production, biogeochemical cycling, and water column structure are all intricately linked through vertical mixing processes across the pycnocline. Understanding turbulence introduced in the water column by tidal flows past offshore infrastructure, and the subsequent effect on vertical mixing, is therefore vital for predicting and managing renewable energy impacts on the marine environment.

 

This work is focussed on understanding the fundamental fluid dynamics of offshore wind infrastructure wakes in stratified flows, using direct numerical simulations. The tidal flows past the structures are approximated by a uniform quiescent background flow with a two-layer density profile. The flows past two types of infrastructure are investigated: A uniform vertical cylinder approximating a monopile, and a truncated cylinder approximating a floating spar-buoy or semi-submersible structure. The truncated cylinder has its length equal to the pycnocline depth, such that it penetrates through the upper half of the pynocline.

 

Through these simulations we identify the process through which turbulence generated in the wake of the structures leads to vertical mixing across the pycnocline. While both cylinders weaken the pynocline by a similar amount, the processes that lead to vertical mixing differ significantly. The truncated cylinder directly leads to mixing in the lee of the structure due to the vertical displacement of fluid beneath it. In contrast, the full cylinder requires a transition from horizontally sheared flow to 3D turbulence before vertical mixing occurs. In addition to direct mixing of the pycnocline through turbulence, internal gravity waves are also observed in the wake of both cylinders, identified through a spectral decomposition of instantaneous slice data. Future work will aim to understand the interaction between waves and turbulence in the cylinder wakes, the influence of shear in the background flow on mixing in the wakes, and will develop sub-grid-scale closures appropriate for modelling full-scale tidal flows past offshore wind infrastructure.