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148 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION 5.1.5 3D fluid flow measurement close to free water surfaces Markus Jehle Abstract A novel measurement technique is developed for 3D reconstruction of velocity vector fields close to free water surfaces. The new method overcomes the restriction of planar dimensionality by both a sophisticated experimental setup and data analysis based on contemporary image processing techniques. 1.5 1 0.5 0 -0.5 -1 -1.5 0 0.5 1 z = 9.5mm z = 7.5mm z = 5.5mm z = 3.5mm z = 1.5mm z = 0.5mm umean/vmean [pixel/frame] urms/vrms [pixel/frame] wmean [cm/frame] wrms [cm/frame] 0.8 0.6 0.4 0.2 0.01 0.005 0 -0.005 0 -0.01 0 z[cm] 0 0.5 1 z[cm] 0 0.5 1 z[cm] 0 0.5 1 z[cm] Figure 5.5: Flow in a convection tank driven by buoyancy and evaporation. Top: Velocity vector fields starting from the deepest layer moving upwards (the w-component is colour-coded). Bottom: Vertical profiles of the mean and rms velocities (red: u, blue: v, black w). Background In order to examine the air-water gas exchange a detailed knowledge of the waterside flow-field is needed. Therefore important quantities such as shear stresses and velocity profiles have to be determined. Because the interesting flow is 3D, instationary and close to the wavedriven water-surface, conventional techniques like particle imaging velocimetry (PIV) using laser light sections are not applicable. A technique, that is similar to the one proposed here, is applied in the field of biofluidmechanics successfully, where it is important to get knowledge about the flow in artificial blood vessels. Methods and results A fluid volume is illuminated by LEDs. Small spherical particles are added to the fluid, functioning as a tracer. A camera pointing to the water surface from above records the image sequences. The distance of the spheres to the surface is coded by means of a supplemented dye, which absorbs the light of the LEDs according to Beer-Lambert’s law. By using LEDs flashing with two different wavelengths, it is possible to use particles variable in size. The 0.025 0.02 0.015 0.01 0.005 velocity vectors are obtained by using an extension of the method of optical flow, an established technique in computer vision. The vertical velocity component is computed from the temporal change of brightness. Hardware and algorithmics are tested in several ways: i) A laminar falling film serves as reference flow. The predicted parabolic profile of this stationary flow can be reproduced very well. ii) Convective turbulence acts as an example for an instationary inherently 3D flow (see figure 5.5). Outlook/Future work The presented technique constitutes the first part of a comprehensive research project whose ultimate goal is the spatio-temporal analysis of flows close to moving and wave-driven curved interfaces. Funding DFG priority program “Bildgebende Messverfahren für die Strömungsanalyse”. Main publications [Jehle & Jähne, 2006b; Jehle, 2006; Jehle et al. , in preparation]
5.1. SMALL-SCALE AIR-SEA INTERACTION 149 5.1.6 Small-scale turbulence in the sea-surface microlayer Uwe Schimpf Abstract Heat is used as a proxy tracer for gases to study the transport processes across the seasurface microlayer. Infrared imaging techniques permit fast measurements of heat transfer velocities and give an insight into the transport mechanisms across the thermal sublayer. IR camera (3-5 µm) 240 Watt carbon dioxide laser (10.6 µm) optics wind wave facility mirror x-y scan head wind speed: 2.0 ms 0.5 s 1.0 s 1.5 s 2.0 s 2.5 s 3.0 s wind speed: 7.0 ms 0.5 s 1.0 s 1.5 s 2.0 s 2.5 s 3.0 s Figure 5.6: Setup of the controlled flux technique in the wind-wave facility at IRPHE, University of Marseille. The carbon dioxide laser forces a periodically varying heat flux onto the water surface. The temperature response of the water surface (amplitude and phase shift) is measured by an infrared imager. Background The turbulent mixing processes in the watersided thermal boundary layer controlling the transport of heat across the sea surface microlayer are investigated with high temporal and spatial resolution by means of the active controlled flux technique [Jähne et al. , 1989]. Methods and results The heat flux density is controlled by forcing infrared radiation onto the water surface and the variation of surface temperature is measured with an infrared camera. For slow temporal variations of the infrared radiation the heat transfer rate is given by the ratio of the net heat flux density and the temperature change 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 improved ACFT system was deployed for 4 weeks in wind-wave facility at IRPHE, University of Marseille. A detailed comparison of the local heat transfer and the synchronously measured surface slope of short wind waves (section 5.1.7) might allow to identify micro scale breaking of capillary waves. Funding Aktives Thermografie System, HBFG- 125-667 (Hardware), DFG Ja395/13-1 (Joint project with Institut für Meereskunde, Universität Hamburg. Main publications [Schimpf et al. , 2006b], [Schimpf et al. , 2006a]
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