Coupling the Stokes and Navier-Stokes equations with two scalar nonlinear parabolic equations

This work deals with a system of nonlinear parabolic equations arising in turbulence modelling. The unknowns are the N components of the velocity field u coupled with two scalar quantities θ and ϕ. The system presents nonlinear turbulent viscosity A(θ, ϕ) and nonlinear source terms of the form θ2|∇u| 2 and θϕ|∇u| 2 lying in L1. Some existence results are shown in this paper, including L∞-estimates and positivity for both θ and ϕ.Nous étudions un système non-linéaire d’équations du type parabolique provenant de la modélisation de la turbulence. Les inconnues sont les N composantes du champ des vitesses u couplées avec deux grandeurs scalaires θ et ϕ. Ce système présente un terme de diffusion non-linéaire sous forme matricielle A(θ,ϕ) et les termes sources non-linéaires θ2|∇u| 2 et θϕ|∇u| 2 appartenant à L1. On démontre alors quelques résultats d’existence de solutions, ainsi que des estimations dans L∞ et positivité pour θ et ϕ.Dirección General de Investigación Científica y Técnic

Trajectory optimization for exposure to minimal electromagnetic pollution using genetic algorithms approach: A case study

Low-frequency electromagnetic pollution associated with electricity supplies and electrical appliances creates broad and specific challenges. Among them, knowing the values of this pollution in urban areas to prevent long exposure in the daily life human beings is rising in today's information society. This paper presents a comprehensive approach for, first, mapping electromagnetic pollution of complete urban areas and, second, based on the former data, the trajectories planning of commuting with minimal electromagnetic exposure. In the first stage, the proposed approach reduces the number of necessary measurements for the pollution mapping, estimating their value by optimizing functional criteria using genetic algorithms (GAs) and considering the superposition effect of different sources. In the second stage, a combination of a specifically designed search space and GA as optimization algorithm makes it possible to determine an optimized trajectory that represents a balanced solution between distance and exposure to magnetic fields. The results verify the obtaining of a complete mapping with less error, between 1% and 2.5%, in power lines and medium/low voltage (MV/LV) substations, respectively. The proposed approach obtains optimized trajectories for different types of commuting (pedestrians, bikers, and vehicles), and it can be integrated into mobile applications. Finally, the method was tested on an actual urban area in Malaga (Spain).Financing for open access position: University of Malaga / CBUA

Micropore Filling and Multilayer Formation in Stöber Spheres upon Water Adsorption

The presence of porosity critically affects the performance of solid systems. The pore accessibility to adsorbate molecules and the corresponding adsorption/desorption behavior are crucial aspects to understand the properties of porous materials but are difficult to address, principally when dealing with narrow micropores. A prominent example is colloidal silica (Stöber) spheres whose microporosity, inaccessible for some adsorbates, can be readily filled by water molecules to a large extent but exhibiting a complex adsorption behavior with unexpected hystereses. Here, we perform water adsorption isotherms on Stöber spheres at different temperatures using an original analysis of the Dubinin–Radushkevich representation to examine both the accessibility to the microporosity and the formation of water multilayers on the outer sphere surface. The micropore filling (and emptying) is found to be limited by the kinetic energy of the water molecules, causing low-pressure hysteresis. We further discover that the (temperature-dependent) completion of the micropore filling delays the onset of multilayer adsorption, leading to hysteresis at a high relative pressure. The number of adsorbed water layers is determined, and the adsorption-induced swelling of the spheres is discussed.This work was funded by Spanish MINECO projects MAT2014-58731-JIN and MAT2016-80285-p and Spanish MCIU project RTI2018-093921-B-C41

Determinación de la influencia del contenido de humedad en el coeficiente de rozamiento interno de microesferas de cristal mediante ensayos de corte directo

Las microesferas de sílice tienen muchas aplicaciones como materia prima en muchos procesos de fabricación, aplicándose en adhesivos, masillas, revestimientos, etc., contribuyendo a mejorar la dureza y la resistencia a la abrasión, entre otras cosas. En este trabajo se realizaron ensayos de corte directo para determinar las curvas de tensión-deformación de este material granular en un amplio intervalo de humedades y conocer así cómo varía el ángulo de rozamiento interno del material con la humedad. A diferencia de las mediciones que se realizan habitualmente con los materiales granulares, en los ensayos realizados no se consolidó la muestra previamente, sino que se le aplicó la carga vertical correspondiente durante el tiempo que duró la realización de cada ensayo, una condición que se aproxima bastante a las situaciones más frecuentes que se dan en la naturaleza y la industria. En estas circunstancias se encontró un rozamiento nunca antes referenciado. Mientras que el medio granular seco mostró un comportamiento típico de un corte lineal clásico, alcanzando un ángulo de rozamiento interno constante de 44º para todas las cargas normales aplicadas, en el caso de la muestra húmeda se observaron dos pendientes bien definidas (dos ángulos de rozamiento interno diferentes), alcanzándose el menor ángulo de rozamiento interno a bajas presiones (de hasta 50 a 100 kPa). Este comportamiento de fricción de dos regímenes abarcó todo el rango de humedades analizado hasta la saturación (23% de humedad, aproximadamente), cuando el medio granular recuperó el comportamiento lineal obteniéndose un ángulo de rozamiento interno cercano al alcanzado en seco. Además, los ángulos de rozamiento interno dependían de la cantidad de agua añadida, mostrando dos mínimos que coincidían con los contenidos límite de agua, y un máximo (o zona de meseta) en los contenidos medios. Un modelo semicuantitativo simple basado en la fracción sólida y la resistencia a tracción del material granular, ambos dependientes del contenido de agua, permitió ajustar las mediciones. Los regímenes de rozamiento observados son necesarios para predecir correctamente el comportamiento del medio granular a un contenido de humedad y una tensión determinados. Los resultados obtenidos muestran, por ejemplo, que el coeficiente de rozamiento interno para material seco puede reducirse en un factor cercano a 3 al añadir una cantidad mínima de agua, lo que es relevante en término de estabilidad granular o costes de manejo

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

Solid colloidal ensembles inherently contain water adsorbed from the ambient moisture. This water, confined in the porous network formed by the building submicron spheres, greatly affects the ensemble properties. Inversely, one can benefit from such influence on collective features to explore the water behavior in such nanoconfinements. Recently, novel approaches have been developed to investigate in-depth where and how water is placed in the nanometric pores of self-assembled colloidal crystals. Here, we summarize these advances, along with new ones, that are linked to general interfacial water phenomena like adsorption, capillary forces, and flow. Water-dependent structural properties of the colloidal crystal give clues to the interplay between nanoconfined water and solid fine particles that determines the behavior of ensembles. We elaborate on how the knowledge gained on water in colloidal crystals provides new opportunities for multidisciplinary study of interfacial and nanoconfined liquids and their essential role in the physics of utmost important systems such as particulate media.Ministerio de Economía, Industria y Competitividad MAT2014-58731-JIN y MAT2015-68075-RComunidad de Madrid S2013/MIT-274

Microbial stability and quality of seasoned cracked green Aloreña table olives packed in diverse chloride salt mixtures.

This work was conducted to determine the effect of the partial replacement of NaCl by KCl and CaCl2 (expressed as percentages, wt/vol) on the microbial stability and physicochemical characteristics of seasoned cracked olives using a simplex centroid mixture design. Neither Enterobacteriaceae nor lactic acid bacteria were found during the 50 days that olive packages were monitored. Therefore, microbial instability was considered due to the growth of yeasts, which were the only detected microorganisms; Saccharomyces cerevisiae and Pichia membranifaciens were the most relevant species. Yeasts decreased during the first 21 to 30 days after packing, but their populations rose to 3.5 log CFU/ml by the end of the storage period, clearly causing product deterioration. The partial substitution of NaCl with the other chloride salts slightly altered the phase of microbial inhibition and regrowth. Most of the quality characteristics were not affected by the use of the alternative salt mixtures, but the pH values and Cl(-) concentrations in brine decreased as the CaCl2 concentration increased. Hence, seasoned cracked table olives can be produced using a lower proportion of NaCl without causing significant changes in the shelf life and product quality, although further detailed studies are necessary to guarantee the stability of products packed with specific salt mixtures

Comparison of different constitutive models to characterize the viscoelastic properties of human abdominal adipose tissue. A pilot study

Knowing the mechanical properties of human adipose tissue is key to simulate surgeries such as liposuction, mammoplasty and many plastic surgeries in which the subcutaneous fat is present. One of the most important surgeries, for its incidence, is the breast reconstruction surgery that follows a mastectomy. In this case, achieving a deformed shape similar to the healthy breast is crucial. The reconstruction is most commonly made using autologous tissue, taken from the patient's abdomen. The amount of autologous tissue and its mechanical properties have a strong influence on the shape of the reconstructed breast. In this work, the viscoelastic mechanical properties of the human adipose tissue have been studied. Uniaxial compression stress relaxation tests were performed in adipose tissue specimens extracted from the human abdomen. Two different viscoelastic models were used to fit to the experimental tests: a quasi-linear viscoelastic (QLV) model and an internal variables viscoelastic (IVV) model; each one with four different hyperelastic strain energy density functions to characterise the elastic response: a 5-terms polynomial function, a first order Ogden function, an isotropic Gasser-Ogden-Holzapfel function and a combination of a neoHookean and an exponential function. The IVV model with the Ogden function was the best combination to fit the experimental tests. The viscoelastic properties are not important in the simulation of the static deformed shape of the breast, but they are needed in a relaxation test performed under finite strain rate, particularly, to derive the long-term behaviour (as time tends to infinity), needed to estimate the static deformed shape of the breast. The so obtained stiffness was compared with previous results given in the literature for adipose tissue of different regions, which exhibited a wide dispersion.Ministerio de Economía y Competitividad DPI2011-2808