T4: Energy Fluxes at the Air-Sea Interface

Our goal is to resolve the small-scale processes that dominate the energy exchange as well as to identify the individual mechanisms as a function of the wind wave conditions.

Hello everyone, my name is Malte Loft and I work on the ”T4 Surface Wave-Driven Energy Fluxes at the Air-Sea Interface” subproject as a PhD student at the Hamburg University of Technology (TUHH).

I studied dual mechanical engineering at the Hamburg University of Applied Sciences and specialised in fluid mechanics at the University of Rostock as part of a Master’s degree. In September 2021, I started my PhD to investigate the energy fluxes at the air-sea interface using high-resolution CFD simulations (WP2).

Our goal is to resolve the small-scale processes that dominate the energy exchange as well as to identify the individual mechanisms as a function of the wind wave conditions, e.g. the wave age or wave slope of the current sea state. Due to mostly very high Reynolds numbers, it is hardly possible to perform Direct Numerical Simulations (DNS). Therefore, a hybrid turbulence model (Detatched Eddy Simulation, DES) is used for our simulations. First, a numerical wind-wave tank is developed to reproduce relatively simple laboratory conditions and to validate the numerical model with experimental results (WP1). In the animation shown, a non-linear surface wave can be seen propagating from left to right, involving strong wind forcing. Air separation events and highly turbulent structures are clearly visible. Due to our fully coupled model, we are able to extract the pressure fields and surface stresses at any point in space and can also include the influence of surface tension effects in our investigations. Furthermore, we produce large amounts of data during our simulations in order to determine phase-averaged quantities using triple decomposition. In other words, fields of pressure or velocity that correlate with the respective sea state, detached from turbulent fluctuations. With all this data, we hope to gain deep insights into the physical processes that determine the mechanical energy flow at the air-sea interface.

In the future, we will extend the application of our model to more complex scenarios, e.g. to highly non-linear sea states of the Baltic Sea, including further phenomena such as wave breaking. Another goal is to formulate the findings into improved parameterisations, in particular to improve the boundary conditions of current ocean models (WP3).

Here you can see a short video.

Last year I was asked if I would like to participate in a wind-wave project at the Alfred C. Glassell, Jr. SUSTAIN Laboratory in Miami, USA, for three weeks. After listening to the song "Miami" by Will Smith several times, I felt well prepared and started organizing the trip, especially the funding by the TRR. The wind-wave tank is top-notch, and I was very excited when everything was approved. After my arrival, I met the scientists from Columbia University, U.S. Naval Research Laboratory, University of New Hampshire, and of course University of Miami. From now on, we spent almost every day in the dark lab with no daylight – thanks to the Particle Image Velocimetry measurements. Outside it was summer and mosquito season, so we did not complain much. We survived working on weekends with strong Cuban coffee (do you really want the real one and no sleep for a week?). But the experimental work did not only take place in the lab; discovering the great dive sites of Miami was also part of my tight schedule. Shortly before my return flight, we cooled the tank, whereupon it began to leak as all the silicone seals contracted. This reminded me of the rainy weather in Hamburg, and I knew it was time to come home. I am very grateful for this experience and would recommend everyone not miss the opportunity to do a research stay.