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124 CHAPTER 3. TERRESTRIAL SYSTEMS 3.1.3 Fingered Flow Through Initially Dry Porous Hele-Shaw cell Participating scientist Fereidoun Rezanezhad, Hans-Jörg Vogel, Kurt Roth Abstract Fingered flow is an instability that occurs during infiltration into dry, coarse-textured, and uniform porous media. We study the dynamics of such fingers in porous Hele-Shaw cells with dimensions 1.6 × 0.6 × 0.003 [m]. High-resolution measurements of the water saturation are obtained from light transmission images that have been calibrated by X-ray transmission. Background Unstable flow of water during infiltration into unsaturated porous media belongs to the class of preferential flow. It has been studied for many years and we have a quite complete description of the general phenomena. Glass and Nicholl (1996) reviewed the classical understanding and DiCarlo (2004) summarizes the newer developments which focus on the nature of the saturation overshoot in the finger tip. Fingered flow is important for some practical issues, e.g., for rapid contaminant transport with irrigation in many arid regions. However, it also illuminates our very understanding of the physics of multiphase flow in porous media. Indeed, the current theory – Richards equation – cannot explain the existence of such fingers. Funding DFG RO 1080/9-1&2 Methods and Results The key to understanding fingered flow are rapid high-resolution measurements of the water saturation. To facilitate this, we work with Hele-Shaw cells – two parallel glass plates separated by a few millimeters – that are filled with sand. Light transmission is the used as a proxy for water saturation. A simple digital camera then yields the required rapid measurements. Since the relation between transmission and saturation is nonlinear, it is calibrated with X-ray transmission which is accurate but too slow and also too expensive for routine measurements. In our experiments, fingers are initiated at the transition from a fine-textured layer, where the flow is very uniform, to a coarse-textured Figure 3.3: Experimental setup for X-ray and light transmission measurements. The front view shows the highly localized flow paths that originate from the flow instability in the uniform part of the medium. layer. They eventually disappear into the simulated groundwater at the lower end of the cell. At different stages, a dye tracer is added to the flow in order to study the flow field behind the finger tip. Finally, pressure sensors are installed to monitor the potential energy of the water. First, a number of experimental findings of other groups were reproduced. These are in particular (i) the maximum saturation that occurs in the finger tip, (ii) the stability of the resulting flow channel on short time scales and its diffusive decay on very long time scales, and (iii) the existence of a mobile core and an immobile fringe. Some newer findings include (i) the demonstration that flow channels are destroyed when they encounter heterogeneous layers, (ii) the detection of pressure drops in the overlaying fine-textured layer upon initialization of a new finger, and (iii) the detection of correlated intermittency between different fingers. Outlook/Future Work The future work will concentrate on the theoretical understanding of the experiments. This necessitates a still higher spatial resolution, however, which in turn requires a more detailed representation of the light transmission through the porous medium. Measurements of the point spread function in the nearinfrared as well as in the visible region are under way (project 3.1.4). On another route, a microscope was installed that allows the resolution of the pore space and the detailed observation of the water phase during the passage of a finger tip.
3.1. SOIL PHYSICS 125 3.1.4 Near Infrared Imaging Spectroscopy of Water States in Porous Silicate Media Participating scientist Nadiya Smolyar, Kurt Roth, Bernd Jähne Abstract We have examined capillary and adsorbed water in porous silicates (quartz sand, clay, silica gel) by using of Near Infrared Image Spectroscopy, NIRIS. The interaction of water with the porous media, the spatial distribution of water content, and the physics of different water states, as well as their transformation will be studied. Figure 3.4: Spectral transmission images of capillary water in sand at different times t1(a) < t2(b), showing the distribution of water content and state. Background NIRIS examinations are important for understanding the molecular mechanisms of water interacting with porous silicate media. Even today, the role water plays in the quality of soils and in further environmental effects is not unterstood in many aspects. Methods and results Capillary and adsorbed water is examined spectrally in both dry and saturated silica specimens. The frequency spectra are measured in near infrared (ν = 3300 − 11000cm −1 ; λ = 700 − 3000nm) with a UV-NIR spectrometer Scan 500 (Varian,USA). The frequency values, the characteristic intensities, and appearance of new spectral bands of water in SiO2 media yield numerous data as elasticity constants, bond geometries, and adsorption mechanisms. The analysis of OH vibration modi permits the determination of water content along with changes of water states in the silicate medium. We have found that the water content can be determined best by using the intensive bands 2νOH and νOH + ν2, and the water state according to the bands 3νOH and 2νOH+ν2. A new innovative NIRIS method has been developed for investigations on the spectral optical qualities of water in porous specimens with spatial resolution. To this end, an imaging multispectral 32-channel CCD spectrometer has been set up. The corresponding CCD camera takes sequences of spectral images at certain wavelengths using band pass filters. The images are by spectral digital image processing analyzed. Outlook/Future work The NIRIS concept permits the determination of molecular interaction forces of water on solid boundaries and micro pores together with the physics of water states and measuring the water distribution on porous silicates. The results will aid in gaining deeper understanding of the thermodynamical, structural, and dynamic characteristics of water in silicate media. We are planning the following investigations of water in porous silicate media: -Relation of multiple light scattering for determination of true values of water absorption; - Investigation of different water states, their spatial distribution and concentration; -Investigation of the stationary and dynamic water regimes; - Spectral analysis of the influence of physical factors on the interaction of water and medium; - Investigation of multi-phase systems with various kinds of intermolecular interaction. Main publication Smolyar N., M. Korniyenko and K. Roth (2005) Near Infrared imaging spectroscopy: A new tool for studying water states and movement in porous media, ’Geophysical Research Abstracts’, EGU05-A-07910. Smolyar N. (2003) Bildgebende Spektroskopie an Pflanzenblättern. Dissertation, <strong>Universität</strong> Heidelberg, Interdisziplinäres Zentrum <strong>für</strong> Wissenschaftliches Rechnen (IWR), 195 S. Garbe C., N. Smolyar, M. Korniyenko and U. Schurr (2003) Water relations in plant leaves, Springer Verlag. Lecture Notes in Computer Science, LNCS 19: 377-401.
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Contents 1 Introduction/Overview of
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CONTENTS 7 3.2.1 Glaciological pilo
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