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174 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION 5.1.1 Investigation of transport processes across the sea-surface microlayer by active thermography Participating scientist Uwe Schimpf and Christopher Popp Abstract The thermal boundary layer is considered as a black box in linear system theory. The input is a periodically varying heat flux forced onto the water surface by a carbon dioxide laser, the output is the amplitude and phase shift of the water surface temperature measured by an infrared camera. 100 Watts CO 2 laser (10.6 µm) IR camera (3-5 µm) 240 cm 100 cm Paddle temperature probe humidity probe temperature probe 62 cm X-Y scan unit flow meter wind direction wind speed 1.4 m/s wind speed 3.2 m/s wind speed 8.2 m/s Figure 5.1: Setup of the controlled flux technique in the AEOLOTRON wind-wave facility. A carbon dioxide laser forces a periodic heat flux onto the water surface. The temperature response of the water surface is measured by an infrared imager. From the image sequences the amplitude damping and the phase shift of the temperature signal is calculated. Background In order to improve the parameterizations and the models of air-gas exchange, the transport mechanisms in the boundary layer and their space and time scales have to be understood in detail. The active controlled flux technique [Jähne et al. , 1989] gives a detailed insight into the turbulent processes controlling the transport across the sea surface microlayer. Funding Aktives Thermografie System, HBFG- 125-667 (Hardware), DFG Ja395/13-1 (Joint project with Detlef Stammer, Insitut für Meereskunde, Universität Hamburg) Methods and results Heat is used as a proxy tracer for gases. The heat flux is controlled by forcing infrared radiation onto the water surface and the temperature variation is directly measured with an infrared imager. For slow temporal variations of the infrared radiation the transfer velocity is given by the ratio of the heat flux density and the temperature variation at the water surface. If the heat flux is switched off, the temperature increase will decay with a characteristic time constant of the transport across the thermal sublayer. The measured phase shifts clearly indicate that the transfer process is strongly intermittent. Outlook/Future work The developed system will be deploy in the DFG project ”Impact of Wind, Rain, and Surface Slicks on Air-Sea CO2 Transfer Velocity - Tank Experiments” in cooperation with the Institute of Oceanography, University of Hamburg. The goal is to improve the understanding of the parameterization of air-sea gas exchange with emphasis on CO2. Main publication Schimpf et al. [2004]
5.1. SMALL-SCALE AIR-SEA INTERACTION 175 5.1.2 Water flow measurements in environmental and biological systems Participating scientist Christoph S. Garbe Abstract Novel measurement techniques are developed for accurately determining water flows in biological and environmental systems. These techniques are closely linked to the development of refined image processing techniques, making temporally and spatially highly resolved field measurements feasible. a) b) c) Figure 5.2: In a) a thermal image of the air-water interface is shown with the extracted motion field. In b) the motion of a microfluidic application is shown, and in c) the flow velocity of Xylem in a plant leaf can be seen. Background For a number of exchange processes both in environmental and life sciences, the transport of water plays a fundamental role. Systems of interest are turbulent transport at water surfaces for air sea heat and gas exchange, water flow in plant leaves, and microfluids. Density distributions of tracers are visualized with modern cameras and their motion as well as density changes estimated from digital image processing techniques. Examples of such tracers are heat which can be applied with lasers and visualized with highly resolved thermographic imagers or caged dyes for microfluidic applications. Results of these flow measurements can be used for modelling physical transport processes, deepening our understanding of the systems analyzed. Funding Research Centre Jülich (Forschungszentrum Jülich), Jülich DFG SPP 1147, “Bildgebende Messverfahren für die Strömungsanalyse” DFG SPP 1114, “Mathematische Methoden der Zeitreihenanalyse und digitalen Bildverarbeitung” Methods and results A fluid is visualized using density distributions as tracers. These can be heat or dyes. At the air-water interface, usually a net heat flux is present which allows visualizing turbulences directly without additional tracers. The motion of density distributions is then modelled as a linear partial differential equation (PDE). These PDEs can be solved from the image sequences with techniques developed in digital image processing. This allows to estimate the motion as well as density changes in the image sequences. Through estimating motion as well as other parameters of the transport model, a deeper understanding of the transport processes involved can be reached. Through this technique, the net heat flux at the air-water interface was estimated directly for the first time, both temporally and spatially highly resolved. Furthermore, the flow of water in plant leaves was also measured in a relatively simple set-up. Outlook/Future work The developed techniques will be used for studying transport processes for shear driven as well as convective driven exchange of heat and mass at the air-water interface. Furthermore, the transport of Xylem in plant leaves will be measured under varying environmental forcings. This will make spatially resolved measurements of water flows in plant leaves possible for the first time, closing a missing link in the understanding of water relations in plants. The low Reynolds number flows in microfluidic mixing machines will also be measured, allowing to improve the design and mixing capabilities for such devices. Main publication Garbe et al. [2004a]; Zhang & Garbe [2004]; Garbe et al. [2004b]
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CONTENTS 7 3.2.1 Glaciological pilo
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