Percolation Research Papers - Academia.edu (original) (raw)

Vegetation cover is a heterogeneous medium composed of different kinds of fuels and non-combustible parts. Some properties of real fires arise from this heterogeneity. Creating heterogeneous fuel areas may be useful both in land... more

Vegetation cover is a heterogeneous medium composed of different kinds of fuels and non-combustible parts. Some properties of real fires arise from this heterogeneity. Creating heterogeneous fuel areas may be useful both in land management and in firefighting by reducing fire intensity and fire rate of spread. The spreading of a fire through a heterogeneous medium was studied with a two-dimensional reaction–diffusion physical model of fire spread. Randomly distributed combustible and non-combustible square elements constituted the heterogeneous fuel. Two main characteristics of the fire were directly computed by the model: the size of the zone influenced by the heat transferred from the fire front and the ignition condition of vegetation. The model was able to provide rate of fire spread, temperature distribution and energy transfers. The influence on the fire properties of the ratio between the amount of combustible elements and the total amount of elements was studied. The results...

Despite seven decades of intense research, the generally accepted insertion mechanism (IM) has generated many open questions. It is shown in this paper why IM is incorrect and nonsense. We reexamined the fundamental presumption of this... more

Despite seven decades of intense research, the generally accepted insertion mechanism (IM) has generated many open questions. It is shown in this paper why IM is incorrect and nonsense. We reexamined the fundamental presumption of this polymerization, evaluated the roles of each basic component and proposed an alternative mechanism by electric charge percolation (CPM).

Composites of poly(methyl methacrylate) (PMMA) with multi-walled carbon nanotubes (MWCNT) of varying aspect ratio and carboxylic acid functionality were prepared using melt mixing. The extent of dispersion and distribution of the MWCNTs... more

Composites of poly(methyl methacrylate) (PMMA) with multi-walled carbon nanotubes (MWCNT) of varying aspect ratio and carboxylic acid functionality were prepared using melt mixing. The extent of dispersion and distribution of the MWCNTs in the PMMA matrix was investigated using a combination of high-resolution transmission electron microscopy (HRTEM), wide-angle X-ray diffraction (XRD) and Raman spectroscopy. The electrical resistivity and oscillatory shear rheological properties of the composites were measured as a function of MWCNT geometry, functionality, and concentration. The fundamental ballistic conductance of the pristine free-standing MWCNTs was investigated using a mechanically controlled break-junction method. The electrical conductivity of PMMA was enhanced by up to 11 orders of magnitude for MWCNT concentrations below 0.5 wt.%. MWCNTs having higher aspect ratio, above 500, or functionalized with carboxylic acid groups readily formed rheological percolated networks with thresholds, determined from a power law relationship, of 1.52 and 2.06 wt.%, respectively. The onset of pseudo-solid-like behaviour and network formation is observed as G′, η∗, and tan δ−1 are independent of frequency as MWCNT loading increased. Sufficiently long and/or functionalized tubes are required to physically bridge or provide interfacial interactions with PMMA to alter polymer chain dynamics. Carboxylic acid functionalization disrupts the crystalline order of MWCNTs due to a loss of π-conjugation and electron de-localisation of sp2 C–C bonds resulting in non-ballistic electron transport in these tubes, irrespective of how highly dispersed they are in the PMMA matrix.

Saline melt water from road salt applications that has percolated into a fine sandy soil in winter is rinsed out of the soil by infiltrating rainwater in the following warmer seasons. This sequence of saturated and unsaturated flow... more

Saline melt water from road salt applications that has percolated into a fine sandy soil in winter is rinsed out of the soil by infiltrating rainwater in the following warmer seasons. This sequence of saturated and unsaturated flow processes associated with saline water transport in a fine sandy soil was studied by simulation and exploratory laboratory experiments. Experiments in soil columns of 300-μm sand revealed that two rinses of pure water, each of one pore volume, were sufficient to reduce the salt concentration by 99% of its original value in the soil column. Simulated time variations of salt concentration in the effluent from the column agreed with experimental results. Based on simulated and experimental results, a sandy soil must become saturated to experience pore water flow in order to efficiently rinse saline snowmelt water. Depending on the saturated hydraulic conductivity and the soil depth, days, weeks, or months of freshwater infiltration in summer are needed to rinse saline melt water from an unsaturated sandy soil after road salt applications in winter. This explains findings of significant salt concentrations in surface and shallow groundwater during summer months, long after road salt application and infiltration has ceased.

Percolation can be defined as the flow of fluids through a porous media (filter). Infiltration rate may be defined as the meters per unit time of the entry of water into the soil surface regardless of the types or values of forces or... more

Percolation can be defined as the flow of fluids through a porous media (filter). Infiltration rate may be defined as the meters per unit time of the entry of water into the soil surface regardless of the types or values of forces or gradients. Water entry into the soil is caused by matric and gravitational forces. Infiltration normally refers to the downward movement.

This report describes the results of a theoretical investigation into the nonlinear optical response of granular composites. In particular, we have investigated the degree to which the cubic nonlinear response can be enhanced in such... more

This report describes the results of a theoretical investigation into the nonlinear optical response of granular composites. In particular, we have investigated the degree to which the cubic nonlinear response can be enhanced in such composites, relative to the same response in pure materials. Cubic nonlinear response is potentially of great practical importance in developing novel materials with a strongly intensity-dependent transmission coefficient, index of refraction, etc. We find that there are many types of composite geometries which give rise to such strong enhancements. These include composites near the percolation threshold, composites in which one of the components is arranged in the form of a fractal cluster, and nonlinear coatings (such as dyes) adsorbed onto the surface of particles which have a resonant optical response at an appropriate frequency. We have carried out this investigation using both analytical approximations and a variety of approximate numerical techniques, including large-scale computer simulations. Finally, we briefly describe some work in progress, in which a composite composed of materials with a purely guardratic response can exhibit an enhanced curbic response.

Abstract. The application of percolation theory to porous media is closely tied to network mod-els. A network model is a detailed model of a porous medium, generally incorporating pore-scale descriptions of the medium and the physics of... more

Abstract. The application of percolation theory to porous media is closely tied to network mod-els. A network model is a detailed model of a porous medium, generally incorporating pore-scale descriptions of the medium and the physics of pore-scale events. Network ...

Feral American mink (Mustela vison) are emerging as one of the biggest threats to biodiversity in northern Europe. Here we summarize responses of migratory seabirds, island small mammals and amphibians to 15 years of large-scale,... more

Feral American mink (Mustela vison) are emerging as one of the biggest threats to biodiversity in northern Europe. Here we summarize responses of migratory seabirds, island small mammals and amphibians to 15 years of large-scale, experimental mink removal in the outer Finnish archipelago. Mink control led to increases in breeding populations of 14 of 22 species of seabirds, which were generally of smaller body size and later breeders as compared with those species which showed no responses. Mink also suppressed the natural summer increases of bank vole (Clethrionomys glareolus) and field vole (Microtus agrestis) populations but only during years of good rainfall. Impacts on common frogs (Rana temporaria) took seven breeding seasons to appear probably because of delayed maturation of frogs; the less palatable common toads (Bufo bufo) seemed unaffected. It appears then that the disruptive effects of mink impacts may cover the entire island vertebrate community with cascading consequen...

We investigate the flow of various non-Newtonian fluids through three-dimensional disordered porous media by direct numerical simulation of momentum transport and continuity equations. Remarkably, our results for power-law (PL) fluids... more

We investigate the flow of various non-Newtonian fluids through three-dimensional disordered porous media by direct numerical simulation of momentum transport and continuity equations. Remarkably, our results for power-law (PL) fluids indicate that the flow, when quantified in terms of a properly modified permeability-like index and Reynolds number, can be successfully described by a single (universal) curve over a broad range of Reynolds conditions and power-law exponents. We also study the flow behavior of Bingham fluids described in terms of the Herschel-Bulkley model. In this case, our simulations reveal that the interplay of ( i) the disordered geometry of the pore space, ( ii) the fluid rheological properties, and ( iii) the inertial effects on the flow is responsible for a substantial enhancement of the macroscopic hydraulic conductance of the system at intermediate Reynolds conditions. This anomalous condition of "enhanced transport" represents a novel feature for ...

The basis for a theory of porous electrodes is the percolation problem. Its solution would make it possible to answer the question of how effectively the structure of a porous electrode is used for ensuring the occurrence of... more

The basis for a theory of porous electrodes is the percolation problem. Its solution would make it possible to answer the question of how effectively the structure of a porous electrode is used for ensuring the occurrence of electrochemical processes on its inner surface, for bringing together all its participants (reagents, ions, electrons, catalyst particles), and for removing the formed products. For the solution of this problem, it is necessary that both the elements of the structure of a porous electrode and the substances filling it should be connected. In the case of hydrophobized electrodes, these are exemplified by a mixture of agglomerated grains of polytetrafluoroethylene and catalyst, the gaseous reagent entering the electrons, and the electrolyte. In this publication, we describe a general method intended for the calculation of thresholds and lines of percolation and present particular estimates for an important class of lattices modeling porous media – rectangular: plane, square lattice, three-dimensional cubic, and their analogues in space of any number of measurements.

In the hard-rock terrains of peninsular India groundwater resource management cannot be generally achieved through policy and regulatory actions by government, given the extremely large numbers of individually small users and the limited... more

In the hard-rock terrains of peninsular India groundwater resource management cannot be generally achieved through policy and regulatory actions by government, given the extremely large numbers of individually small users and the limited institutional capacity which needs to be focused on that small proportion of critical aquifers at risk of irreversible degradation. There are, however, just a few examples of long-standing community self-regulation of groundwater resource use for agricultural irrigation, which need to be understood and replicated by any available mechanism. Hivre Bazaar (1,140 population), in the drought-prone part of the elevated Deccan traps country of Maharashtra state, is one such example and has received particular attention because of its apparent success at increasing farmer incomes and social well-being. It is the subject of this study to assess its socio-economic dynamics, agricultural evolution and hydrogeological sustainability, which was carried out by G...