The Air-Sea Interaction Group
The exchange of mass, momentum and heat between the atmosphere and the ocean is governed by thin boundary layers at each side of the interface. These boundary layers are a mere 10–350 μm thick in the water, and 1-2 mm in the air. The hydrodynamics in these layers is significantly different from boundary layers at rigid walls since the orbital motion of the waves is of the same order as the velocities in the boundary layer. In our research, we use non-invasive imaging techniques to understand the processes that govern the thickness of these layers. While wind is the main driver of the exchange processes by producing shear at the water surface, waves and ultimately turbulence in the water, other factors, such as the contamination of the water surface with surface active material (surfactants), the length the wind is blowing over the water surface (fetch) or the entrainment of bubbles and the production of spray by breaking waves also play a role.
The Aeolotron wind-wave tank is the largest operational annular wind-wave tank in the world with a diameter of 10 m. Its construction was finished in 1999, and it is named after Aeolus, the Greek god of the wind. Due to its annular shape, the distance the wind can travel over the water (fetch) is virtually unlimited, so that waves in the 1m deep water section can grow much larger than in the more common linear wind-wave tanks, which have a limited length of typically below 40 m. The Aeolotron is chemically clean, so that very sensitive chemical conditions can be met which is important for the imaging techniques of gas concentrations developed in our group. Also, the transfer velocities of many gases can be measured in parallel without interference by biological or chemical processes. The Aeolotron's construction also allows the use of artificial or natural sea water. Wind is produced by 4 axial fans with 2.2 kW of power each, with the highest wind speed being about 23 m/s. The Aeolotron is thermally insulated, allowing for the measurement of heat exchange. The 1.4 m high air space can be flushed with fresh air with a time constant of approx. 2 min, so that gases that have accumulated in the air space can be flushed out of the system very fast. A large window in the bottom of the tank and several other windows in the side walls and roof allow optical access for imaging systems.
The imaging techniques developed and used in our group cover a wide range of studied processes.
- The Imaging Slope Gauge (ISG) resolves the slope of the water surface in a footprint of approx. 20 cm x 22 cm by an intensity coded flashed light source below the water and a high speed camera above, so that processes such as wave creation and wave breaking can be studied.
- Boundary Layer Imaging (BLI) makes the thickness of the water-side boundary layer visible by the combination of a pH-sensitive fluorescent dye and an alkaline gas. Together with the ISG, BLI is a powerful tool to study the relationship between wave shape, size and area affected by wave breaking and the boundary layer thickness.
- Particle Streak Velocimetry (PSV) allows for measurements of the velocity of the air or water right up to the air-water interface. Turbulent, wave-coherent, pressure induced and laminar contributions to the transfer of momentum can be separated.
- The Active Thermography (AT) is used to measure the transfer velocity of heat by heating the water surface periodically with a laser and measuring the temperature response with a highly sensitive infrared camera. The bulk transfer velocity of heat can be converted to that of a gas. Combining AT, BLI and the ISG gives insights into the mechanisms responsible for transporting gas and heat across the air-water interface.
- Laser Induced Fluorescence (LIF) of the gas SO2 in the ultraviolet range allows to measure concentration profiles in the air right down to the water surface with a resolution high enough to resolve the air-side mass boundary layer. In combination with PSV, turbulent and laminar transport mechanisms of gases in the air can be studied.
This video shows rendered BLI and ISG data which were measured in the same footprint. The darker the water surface is, the thinner the mass boundary layer is. Streaky structures, which are parallel to the wind, develop. Occasionally (e.g. frames 130-150; in the upper right corner) a wave breaks without entrainment of bubbles, but removes accumulated gas from the boundary layer and leaves a trail of high turbulence.
|Aeolotron Air-Sea Interaction Laboratory
Institute of Environmental Physics
Im Neuenheimer Feld 229