Unlocking the Secret of Waves
Wave dynamics are yet to be truly understood because their movement is so rapid and complex. Physicist Marc Buckley from the Helmholtz-Zentrum Geesthacht now utilises a watertight laser system to, in a sense, freeze the movement of waves to discover how wind energy is transformed into wave motion. From the results, he hopes to gain important insights into how hurricanes are formed and uncover information that can be used to optimise mathematical climate models.
Dynamic energy conversion through wind and waves. Photo: Ralf Weiße
Wherever there’s water, there’s waves. All over the world, in all seas, at all times. The oceans cover approximately seventy percent of the planet – and therefore seventy percent of the Earth’s surface is shaped by waves. Although the wave phenomenon is so commonplace, it is still yet to be understood after more than a hundred years of marine research. It is clear that wind provides the energy to shape water into waves. However, precisely how the movement of the wind transforms to wave energy, when a wave arises and how it grows is not yet really clear. Why, for example, doesn’t a hurricane lose its energy during its journey across the ocean, though it creates breakers ten metres tall or higher—that is, does a great deal of energy flow into the formation of the waves?
A Fast Laser Captures Waves
Mark of Buckley with the laser equipment. Photo: HZG/Ina Frings
In order to answer questions like these, HZG physicist Dr Marc Buckley dives more deeply into the topic than many colleagues in the past. He developed laser equipment with which he can film, in a way, how the wind energy is passed to the wave. This special device works so quickly that it can even detect and measure rapid waves that rush by at a speed of several metres per second. “It has so far simply not been technically possible to observe such rapid movements in their entirety,” says Buckley. “We could actually measure the wave velocity but not the energy fluxes.”
Buckley developed the first prototype of his device, called AirSeaPix, on a wave channel at the University of Delaware in the United States. The principle is that the very fast digital cameras can capture the movement and change of the passing waves, which are illuminated by lasers. In addition, an apparatus consisting of water pumps, hoses and mist nozzles produces fine mist droplets of a precisely defined size that float perfectly in the air and are carried away with the wind. The mist droplets flying by are also illuminated by the laser's light and captured by the cameras. This also makes the air turbulence visible above the wave.
Buckley's photographs clearly show for the first time where turbulence forms over a wave and how this turbulence forms the wave itself. This type of approach is unique to date. “From this method, we can read where and how the energy is transferred and how the air flow’s momentum impulse continues in the wave,” says Buckley.
A Method Borrowed from the Industrial Sector
The cameras capture the particles that have been made visible. Photo: HZG/Ina Frings
Marc Buckley moved some time ago to the HZG’s Institute of Coastal Research to further develop the laboratory system into a device that is robust enough to be used in the sea. He was able to carry out experiments in the Pacific when he was invited on an American research vessel. The results show that the technology works.
In principle, the technology is based on an industry-established optical method, particle image velocimetry (PIV). In this method, small particles are added to a liquid to facilitate observation of how the liquid flows by observing the particle movement– thus, for example, the flow properties of aircraft wings or car models can be tested.
“The substantial difference to our application is that those objects aren’t moving – with us, however, the entire system consisting of wind and waves is moving very quickly, a considerable challenge for the camera and image processing technology,” says Buckley. But it works.
Practice Test in the Baltic Sea
Measuring pile and equipment are first tested on the HZG premises. Photo: HZG/Ina Frings
Buckley is currently setting up his AirSeaPix on an eleven-metre-tall measurement pile, which will be installed in the Baltic Sea this year. The equipment is to continuously record waves and the flow of the mist droplets there. Fortunately, there aren’t any hurricanes in the Baltic Sea, but Buckley hopes the results will bring many new insights. “By measuring the energy fluxes between air and water, we can better understand the principles of wave formation. We can then hopefully extrapolate the results to phenomena like hurricanes.”
It is, however, not just about hurricanes, but is also about basic phenomena that are of great interest during times of climate change. Climate depends to a great extent on how energy is exchanged on a large scale between atmospheric layers, between the atmosphere and the land, and between the atmosphere and the sea. The sun warms up the land masses and sea to very different degrees, whereby air masses arise with different air pressures. The different air pressures in turn lead to air fluxes and wind, and these in turn generate waves on the sea.
“Because the oceans cover such a large surface, wave dynamics are particularly important in understanding the energy fluxes at the boundary to the atmosphere,” says Buckley. “Only when these energy fluxes are correctly described can mathematical climate models accurately take them into consideration.” So far, the "wave" phenomenon has been integrated into the models in a mathematical manner that is heavily abstracted. In the future, Buckley wants to provide more realistic values for a wave model, which should help to further improve the climate calculations.
There is another aspect that AirSeaPix will possibly be able to better estimate in the future: absorption of oxygen, and especially the greenhouse gas carbon dioxide from the atmosphere into the sea. Because Buckley’s equipment also makes fine sea spray visible, it is easy to see how heavily air is mixed into the sea. “Carbon dioxide winds up in seawater when it is enveloped by waves and spray – our camera technology can detect that very well.
Measuring pile and plant on the HZG site Photo: HZG/Ina Frings
First, however, the measuring pile needs to be erected in the Baltic Sea. It will jut out approximately four metres from the water, equipped with batteries, lasers, cameras and fog machine. Even before the pile is set up, Buckley already has the next stage planned for his AirSeaPix. “In the future, the cameras should also film the turbulences under water so that we can understand the dynamics of the waves and the energy fluxes even better.”
Text: Tim Schröder/Science Journalist