Laboratory experiments to evaluate the joint effect between heterogeneity and head fluctuation on mixing, effective porosity and tailing
- Autores: Eduardo Francisco Castro Alcalá
- Directores de la Tesis: Jesús Carrera Ramírez (dir. tes.), Daniel Fernández García (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Tanguy Le Borgne (presid.), Xavier Sánchez Vila (secret.), Francisco Javier Elorza Tenreiro (voc.), Maarten Willem Saaltink (voc.), María Pool Ramírez (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Civil por la Universidad Politécnica de Catalunya
- Materias:
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- Resumen
Tracer visualization in the laboratory at high spatial and temporal resolution can help advance the study of mixing processes. However, grain borders, fluctuations in lighting and non-uniform brightness contribute to produce noisy images of concentrations that cannot be used to directly estimate mixing at the local scale. We present a new methodology to visualize local values of mixing from noisy images of optical tracers based on a nonparametric regression algorithm. The methodology is used to provide a full visualization of the mixing dynamics that occur in a tracer experiment performed in a reconstructed heterogeneous aquifer consisting of two materials with contrasting hydraulic properties. Results show that the method is capable of providing optimal images of mixing of high quality even at complex material edges between bodies of different sand.
In transport problems, dispersivity control the mixing. Understand the behaviour of this parameter allow characterizing the mixing. Characterize this parameter is complex due to the heterogeneity and the spatial and temporal fluctuations., in that many assumptions (about the definition of dispersion, how to represent it locally, and how to handle medium heterogeneities or velocity fluctuations) are required to make it tractable. Surprisingly the results go in opposite directions. All agree that fluctuations of velocity transverse to the mean flow direction enhance transverse dispersion, some authors conclude that the enhancement is very large, others conclude that it is not so important. Researchers generally agree the e¿ects of velocity ¿uctuations on longitudinal dispersion is much smaller than on transverse dispersion.But they have found it to decrease. Others increase, others remain unchanged Ironically, despite of all the above research, little experimental work has been performed. For this reason we presents an experiment speci¿cally aimed at evaluating the effect of temporal fluctuations of velocity on transport through a heterogeneous medium and, specifically, on dispersion, mobile porosity and tailing. The results show that Flow fluctuations can enhance transverse mixing and mobile porosity. These phenomena transfer more mass to the plume front, which is now more mobile, and to the low permeability inclusions. As a result, the apparent longitudinal macrodispersion coe°cient is substantially increased due to flow fluctuations. Experimental results determined that the effect of flow fluctuations is maximum (enhancement of transverse mixing and macrodispersion) when the kubo number is close to one.
Coastal aquifers are complex zones due to the combined in¿uences of heterogeneity (scale, shape or structure of the aquifer), inland groundwater forces and oceanic oscillations.These produce complex hydrodynamic that e¿ect the location, shape and extent of the dispersion zone. The dynamics (¿uctuations, width, and location) of the mixing zone between freshwater and seawater in coastal aquifers is essential for detailed understanding of seawater intrusion and bio-geo-chemical processes. Yet, results to date are somewhat contradictory.We perform seawater intrusion experiments heterogeneous sand box subject to groundwater flow oscillations. We ¿nd that the coupling of heterogeneity and ¿uctuations leads to complex ¿ow patterns. Freshwater may ¿ow beneath saltwater isolated in low permeability zones when the seawater wedge recedes, but itself become surrounded by seawater when the wedge advances landwards. Fluctuations also disrupt the traditional seawater convection cell, with most of the seawater flow being horizontal, and mixed water displaying some vertical component back to the sea when the seawater wedge recedes seawards. As a result, the mixing zone becomes broad when sea water advances, but narrows down when it recedes. Surprisingly, despite all this complexity the average center of mass of the wedge is very similar to that of the steady-state wedge.
