2. February 2021 | Magazine:

The Power of Waves and Wind A short portrait of postdoctoral researcher Christian Windt from the Leichtweiß-Institute for Hydraulic Engineering and Water Resources

Wind power plants in the German North Sea set a new record last year, delivering more energy than ever before. With floating wind turbines, the offshore wind industry could open up even more areas in the future. The Leichtweiß Institute for Hydraulic Engineering and Water Resources is also involved in this research with the project NuLIMAS. Postdoctoral researcher Dr Christian Windt is coordinating the project in the Department Hydromechanics, Coastal and Ocean Engineering. Bianca Loschinsky spoke to him about the project and his academic career so far.

Dr Christian Windt, coordinator of the NuLIMAS project. Picture credit: Christian Windt/TU Braunschweig

Mr Windt, you coordinate the NuLIMAS project at the LWI. What exactly are you researching?

NuLIMAS is the abbreviation for “Numerical Modelling of Soil Liquefaction Around Marine Structures”. So in our joint project we are investigating processes of soil liquefaction.

And what does “soil liquefaction” entail?

A seabed can become a liquid-like mass as a result of vibrations or pressure. This process, in which the stability of the seabed massively decreases, can cause damage to marine structures, such as breakwaters, and must also be taken into account for floating offshore wind energy platforms. These are in the main focus of our project.

How do you want to approach the project?

Primarily, it is about developing a computer model with which we are able to simulate this phenomenon. The physical process is already quite well understood. We are initially experimenting with our partners in Poland on a small scale. These experiments do not yet include the floating structure. So we are first looking at how waves and soil interact with each other.

In the second validation step, the floating offshore wind turbine is then added in large-scale experiments in the Large Wave Flume of our Coastal Research Centre in Hanover. We then analyse the interaction between wave, seabed and structure. With our computer model, we can simulate how the ground around an anchor of a floating offshore wind turbine liquefies and then adjust and optimise the design of the turbine. Or: if you are not sure whether the ground is completely load-bearing at a certain location, you can use our tool to learn more about the situation on site. So it expands the possibilities for site selection and prevents uncertainties.

Before you came to TU Braunschweig last year, you completed your PhD in Ireland at Maynooth University. What was the topic of your dissertation?

During my PhD I worked on wave energy. But the idea for this topic had already been put in my head long before that. I was already working on wave energy in 2014, at the beginning of my Master’s degree at the Technische Universität Hamburg (TUHH).

The project in Ireland was about the numerical simulation of wave energy power plants (“High-fidelity numerical modelling of ocean wave energy systems”). The research group of my supervising professor deals with the integration of control technology in wave energy power plants in order to increase their efficiency.

The problem is that as soon as you integrate control technology into such a device, the movement of the device is increased. And the more this floating buoy moves in the sea, the more energy is produced. That sounds very good at first. But almost all computer models for these wave energy power plants are based on the assumption of linear wave theory, which means that very small movements are assumed there. Of course, this goes against the goal of control engineering. Therefore, the question arose: How can the linear design models be improved or adjusted with correction factors to obtain better results? For this purpose, I created high-resolution simulations using CFD, i.e. Computational Fluid Dynamics.

What is the advantage of such numerical simulations?

The field of numerical simulation is obviously huge. With the OpenFOAM simulation tool, for example, you can simulate the flow of an aircraft wing. One can calculate the wave-structure interaction of a wave energy power plant, but also radio waves. Especially in coastal engineering, we can reduce costs and make predictions with simulations. These are much cheaper than a large-scale experiment or a test on the open sea. Even though numerical simulations are of course not free. For example, if I want to simulate one second of wave-structure interaction, I could easily need 1,000 seconds of computing time. That’s where the supercomputers come in.

In the simulation, we can change design parameters much more quickly. For example, if we want to investigate the flow around the floating offshore wind turbine and want to know whether the platforms for the wind turbines should better be built with four or three buoyancy bodies. This can be done extremely quickly in a numerical simulation.

Another advantage: the whole thing can take place in a much more controlled way. In successful research and development, however, experiment and numerical simulation must always act together – in combination we create added value.

How can you now use your knowledge in the NuLIMAS project?

In my doctorate, I worked on wave-structure interaction: the buoy as a structure that interacts with the wave. In NuLIMAS we also have the wave-structure interaction, with the wave and the structure of the floating offshore wind turbine. On top of that, we have the wave-structure-ground interaction, so that’s another aspect.

My expertise in this field and also in numerical simulation will of course come in handy here. Hopefully I will be able to apply this successfully in the project.

How did it come about that you did your PhD in Ireland?

I spent quite a long time choosing my PhD because I knew exactly what topic I wanted to work on. However, there was no research group in Germany that dealt with the topic of numerical simulation of wave energy power plants. Basically, the only options wee Great Britain, Sweden and Ireland. Maynooth University was a very good choice and overall it was a great experience!

What brought you back to Germany?

After three and a half years in Ireland, I had the urge to come back, although I enjoyed my time there. It was clear to me that I would like to work in Germany again at some point. Fortunately, the option with Braunschweig came up. The decision was not a hard one for me.

On the academic side, I don’t really have the right background: I studied mechanical engineering in my Bachelor’s degree and power engineering in my Master’s degree, and I did my PhD in the Department of Electrical Engineering at Maynooth University. So my education has no connection to coastal engineering. But with my topic, I’m definitely in the right place here!

Do you have a special relationship with water or waves?

(laughs) At least I didn’t grow up on the coast. My first research project involving water took place in Northern Ireland. That’s where I started surfing – in keeping with the topic. In Braunschweig, of course, that’ s a bit difficult to do now.

What inspires you about your research?

Whether wave energy or offshore wind energy, these are highly exciting combinations of different research problems that need solving. In the research field of marine renewable energies, the community is still relatively small, so you always meet certain scientists at conferences and can form a solid network very quickly. Even as a fresh postdoc, you have the feeling that you are an important part of it.

Many of the researchers in this field are also motivated not only to do interesting research, but to address pressing climate change issues. I get the impression that this is a special kind of people. I find that very pleasant and inspiring.

What relevance does your topic have for the Future City?

Of course, it’s about energy supply. That is the big theme of future living. The majority of humanity lives near the coast. Therefore, it is important to include this maritime area in energy production through renewable energies. The distance from the production site to the consumption site is short. However, you can’t supply people with energy only through wind or wave energy. There has to be even more interdisciplinary and international cooperation here.