Research & Projects

Large Eddy Simulation of the turbulent wake behind a monopile offshore wind farm foundation structure. The colours show the magnitude of the horizontal (left to right) velocity with blue indicating high velocities and red indicating low velocities. The structure is marked by the green circle, and the current direction (due to tides) is shown on the left.

With the accelerated development of offshore wind farms comes the need to understand and quantify the impacts that these farms have on the ocean.

The Small-scale Physics and Turbulence group is working on both measuring, and building theoretical and numerical models, to quantify the possible impacts these farms have on the physical conditions of coastal seas, and the North Sea in particular. More specifically, the focus is on the turbulence created as the ocean currents push water back and forth through and around the wind farm foundation structures. The ocean currents will produce an increased turbulence in the wake of the foundations that may cause changes in the vertical distribution of temperature, nutrients, oxygen, and suspended particles. This could then in turn influence the biology and ecology of coastal seas.

In addition, we are studying any possible alterations in ocean currents that may be caused by the increased frictional drag of the foundation structures.

Through the ERC Consolidator Grant FOXSTORM, we are researching the feedbacks between ocean turbulent mixing and the strength of extreme storms. The project uses autonomous underwater glider vehicles equipped with a number of state-of-the-art sensors that allow it to measure the strength of turbulent mixing, and to relate this mixing to the physical forcing by the storm. One of the goals of the project is to develop an observationally-based understanding of the mechanisms of rapid mixing and sea surface cooling. This will be done together with turbulence-resolving modelling of the upper ocean mixing by large eddy simulations. The results will be used to improve the representation of ocean mixing in extreme storms in forecast models through revised turbulence parameterisations.

Through the Collaborative Research Centre "Energy Transfers in Atmosphere and Ocean" (TRR181), funded by the Deutsche Forschungsgemeinschaft (DFG) since 2016, we are investigating the energetics of the ocean surface layer, and its exchanges with the atmosphere and deeper ocean. New insights have been gained into the following research topics.

Wind-wave interaction

Although theoretical models for the interaction of wind and ocean waves have existed for many decades, there has been little direct experimental and observational evidence of the coupling between wind and waves. Through this work we have demonstrated direct evidence to support specific mechanisms that couple the wind to ocean waves. Through novel new advanced airflow visualisation techniques, we are able to view the wave-affected turbulent airflow to within fractions of a millimetre from the ocean surface. This has led to new techniques to reconstruct the pressure field and quantify the momentum and energy transports between atmosphere and ocean through the wavy ocean surface, as well as an identification of mechanisms that enact these transfers.nd intense turbulent energy dissipation.

Energy dissipation in the ocean surface

Within the upper layer of the ocean we investigate processes that affect the dissipation of energy. This is done primarily using autonomous ocean glider vehicles that have been outfitted with specialised turbulence measuring sensors. Using this method we have identified strong energy transfers in frontal features (so-called "submesoscale fronts"), as well as intense mixing in the ocean response to storms. Both of these energy dissipation processes occur through the triggering of instabilities in the ocean's mean state, which lead to the generation of enhanced turbulence. Finally, we have also observed and quantified enhanced turbulence in the very near-surface of the ocean in conditions of strong heating. The formation of diurnal heating layers in the upper metres of the ocean can result in the development of strong currents and intense turbulent energy dissipation.

An autonomous underwater glider equipped with turbulence microstructure package (black tube on top) to measure ocean turbulence.-Image: Jeff Carpenter/Hereon-

The Small-scale Physics and Turbulence group has specialised in the operation and development of the autonomous glider platform. This robotic platform allows us to measure upper ocean variables over long periods, in extreme conditions such as extreme storms and sea states, as well as independently measure very near-surface ocean processes that are otherwise contaminated by research vessels. We are continually integrating new combinations of sensors, developing algorithms for describing glider behaviour and sampling strategies, as well as researching new technological advances for glider platforms. Many of these current activities are supported through the Helmholtz-funded MUSE project. These research activities are described below.

Describing glider flight

A model of the flight behaviour of gliders has been developed in order to better plan missions, decouple the glider hydrodynamics from ocean physical processes, and understand the limits of various sensing technologies. A glider flight model has therefore been developed and tested using various sensors to quantify important aspects of the flight, such as the angle of attack. This model has also been extended to model the effects of unsteady forcing, such as ocean surface waves, and to understand the limitations of such processes on our ability to measure ocean turbulence using gliders. The model has also been used to help quantify ocean convection processes in a region of deep convection.

Designing hydrogen fuelled gliders

To transition the glider and other low power marine vehicles towards sustainable hydrogen fuel, we are working together with the materials scientists at Hereon to design hydrogen fuel systems for gliders. This involves the harvesting of oxygen from ocean waters by specially designed "artificial gills" that have been patented by Hereon. A working prototype of this design has been built, and is being optimised for glider use.