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180 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION 5.1.7 Combined slope/height measurements of short wind waves : ISHG Participating scientists Roland Rocholz, Martin Schmidt Abstract A new technique for the simultaneous measurement of wave height and wave slope of water surface waves has been developed. The Imaging Slope/Height Gauge (ISHG) is an imaging system that is designed to measure waves at different length scales. Using a high speed camera, the high spatiotemporal resolution provides an insight into hydrodynamic processes such as microscale wave breaking and small scale wave-wave interaction. f 2 high speed camera (lens set to infinity) lens (L 2 ) for telecentric imaging (f 2 = 50 cm) water body lens (L 1 ) for telecentric illumination (f 1 = 25 cm) diffusing screen rays are only for illustration (entrance pupil of the camera lies at the focal point of lens L 2 ) f 2 (mean water height is at focal distance to L 2 ) water height (6 to 16 cm) f 1 (diffusing screen is at focal distance to L 1 ) programmable LED-array (320x1W, λ = 880nm) Figure 5.7: Sketch of the wave imaging system for the simultaneous measurement of waves height and slope. Background Since water surface waves strongly influence transfer velocities due to the induced turbulence, such wave measurements are used for monitoring the conditions in gas exchange experiments. Unfortunately, the slope calibration of typical imaging wave measurement techniques like the color imaging slope gauge used at the Heidelberg Aeolotron so far is depending on the wave height of larger waves. To overcome this source of systematic errors, a new technique is under development with the following aims: • The slope of the waves should be measured without biases due to the wave height • The image size should not change with wave height • The wave slope and height should be measured simultaneously • Simple set-up with a single camera Funding DFG, Ja395/13: ”Mechanismen des Gasaustausches zwischen Atmosphäre und Ozean: Laborversuche und Modellierung” Methods and results Like the color imaging slope gauge the slope measurement of the new Imaging Slope/Height Gauge (ISHG) is based on light refraction (shape from refraction) and makes use of an area-extended light source with defined spatial variation of radiance, produced by a programmable LED array. This Imaging Slope Gauge gives a 2-D slope map of the water surface. The Imaging Height Gauge gives the 2-D height map. This is also used to correct the slope map for height dependencies. The method is based on light absorption and works with light in the near infrared range. The optical setup provides a telecentric imaging. The camera is virtually located at infinity so that height variations of the water surface do not affect the image scaling. Outlook/Future work The ISHG system at our small wind wave flume is still under construction. The calibration and first wave measurements are expected to take place at the end of year 2005. It is planned to assemble the same system at the wind-wave tank of the University of Hamburg and at the wind-wave facility AEOLOTRON, University of Heidelberg. Combining the ISHG with infrared imaging will be the next step of extending the system. Main publication Jähne et al. [2005c]
5.1. SMALL-SCALE AIR-SEA INTERACTION 181 5.1.8 Development of a depth resolving boundary layer visualization for gas exchange at free water surfaces Participating scientist Tobias Schwarz Abstract To directly visualize gas concentration transport in the water-sided boundary layer, a combined LIF-measurement system consisting of a laser scanning unit and a confocal microscope has been developed and constructed. Furthermore, methods to reconstruct a 2-D depth profile had been investigated. Figure 5.8: An example of the fluorescence intensity changes through local gas concentrations, illuminated with the laser scanning system. Background As detailed knowledge of gas exchange between atmosphere and ocean is of great importance for predicting and modeling future climate situations, this diploma thesis aimed at the development of a measurement system for improved gas exchange measurements. Funding Graduiertenkolleg 1114 “Optische Messtechniken <strong>für</strong> die Charakterisierung von Transportprozessen an Grenzflächen” (TU Darmstadt and U Heidelberg) Methods and results A measurement setup is introduced which makes it possible to directly measure two-dimensional, vertical concentration profiles of gases in the water sided boundary layer using Laser-Induced Fluorescence (LIF). While it is impossible to gain knowledge of the physical processes involved in gas exchange using measurements of transfer rates and mass balances, the introduced method makes it possible to directly visualize the physical processes of matter transport in the layer. The measurement method is based on two basic principles: First, a fluorescence indicator is used, whose fluorescence intensity is proportional to the local pH-value, thus allowing a spatial resolved measurement of the concentrations of dissolved alkaline or acidic gases. Second, to create a depth resolution, a second, absorbing dye is added, whose absorption maximum lies inside the fluorescence spectrum, so that spectra from different depths show changes in their spectral shape due to the different light path lengths through the absorber. Thus the measured spectrum is the superposition of all depth spectra, which provide the basis of a linear inverse problem. Models for the reconstruction of the depth information will be introduced in the course of this thesis, and the solvability will be analyzed. As the stability of the solution of the inverse problem is almost exclusively determined by the invertibility of the basis function matrix, a confocal microscope was constructed, which allowed the direct measurement of depth spectra. Thereby it was made possible to numerically analyze and evaluate the conditioning of the matrix invertibility. Outlook/Future work Work to be continued within Graduiertenkolleg 1114 Main publication Schwarz [2005]
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