A new formulation for pore‐network modeling of two‐phase flow

110

The dispersal rates of self-propelled microorganisms affect their spatial interactions and the ecological functioning of microbial communities. Microbial dispersal rates affect risk of contamination of water resources by soil-borne pathogens, the inoculation of plant roots, or the rates of spoilage of food products. In contrast with the wealth of information on microbial dispersal in water replete systems, very little is known about their dispersal rates in unsaturated porous media. The fragmented aqueous phase occupying complex soil pore spaces suppress motility and limits dispersal ranges in unsaturated soil. The primary objective of this study was to systematically evaluate key factors that shape microbial dispersal in model unsaturated porous media to quantify effects of saturation, pore space geometry, and chemotaxis on characteristics of principles that govern motile microbial dispersion in unsaturated soil. We constructed a novel 3-D angular pore network model (PNM) to mimic aqueous pathways in soil for different hydration conditions; within the PNM, we employed an individual-based model that considers physiological and biophysical properties of motile and chemotactic bacteria. The effects of hydration conditions on first passage times in different pore networks were studied showing that fragmentation of aquatic habitats under dry conditions sharply suppresses nutrient transport and microbial dispersal rates in good agreement with limited experimental data. Chemotactically biased mean travel speed of microbial cells across 9 mm saturated PNM was $3 mm/h decreasing exponentially to 0.45 mm/h for the PNM at matric potential of 215 kPa (for 235 kPa, dispersal practically ceases and the mean travel time to traverse the 9 mm PNM exceeds 1 year). Results indicate that chemotaxis enhances dispersal rates by orders of magnitude relative to random (diffusive) motions. Model predictions considering microbial cell sizes relative to available liquid pathways sizes were in good agreement with experimental results for unsaturated soils. The new modeling platform enables quantitative consideration of key biophysical factors (e.g., pore space heterogeneities and hydration conditions) governing microbial interactions in 3-D soil pore spaces.

Section: Representation Of Soil Pore Spaces Using a Pore Network Modementioning

confidence: 99%

Microbial dispersal in unsaturated porous media: Characteristics of motile bacterial cell motions in unsaturated angular pore networks

Ebrahimi

2014

110

“…In the present work, pore bodies are considered to be cubic in shape, whereas pore throats are assigned a variety of cross-sectional shapes including circular, rectangular, and scalene triangular. The detail of assigning different angular cross sections to various pores can be found in Raoof and Hassanizadeh [2012].…”

Section: Determination Of the Pore Cross Section And Corner Half Anglesmentioning

confidence: 99%

“…The discretization is based on the saturation state of a pore as follows. Under saturated conditions, because pore bodies are significantly larger than pore throats, we consider the resistance to the flow within pore throats only [Raoof and Hassanizadeh, 2012]. Moreover, we assign one (average) concentration to a pore body, assuming that it is a fully mixed domain Zhao and Ioannidis, 2011;Li et al, 2007b].…”

Section: Pore-space Discretizationmentioning

confidence: 99%

“…[24] In a drained angular pore throat, edges have different angles, then their flow rates and solute residence times could be significantly different, even up to few orders of magnitude [Raoof and Hassanizadeh, 2012]. Therefore, we consider each edge of a drained pore throat as a pore element with its own flow rate and concentration.…”

Section: Pore-space Discretizationmentioning

confidence: 99%

“…The conductance of each edge within a drained pore needs to be determined as a function of the thickness of the wetting film residing in the edge. This thickness depends on the radius of curvature of the fluid-fluid interface, which is calculated based on the applied capillary pressure [Raoof and Hassanizadeh, 2012]. The formulations for calculating the flow within each pore element and determining relative permeability of the network at different saturation levels are described in Raoof and Hassanizadeh [2012].…”

Section: Fluid Flow Within Drained Poresmentioning

confidence: 99%

See 2 more Smart Citations

Saturation‐dependent solute dispersivity in porous media: Pore‐scale processes

Raoof

Hassanizadeh

2013

Self Cite

[1] It is known that in variably saturated porous media, dispersion coefficient depends on Darcy velocity and water saturation. In one-dimensional flow, it is commonly assumed that the dispersion coefficient is a linear function of velocity. The coefficient of proportionality, called the dispersivity, is considered to depend on saturation. However, there is not much known about its dependence on saturation. In this study, we investigate, using a pore network model, how the longitudinal dispersivity varies nonlinearly with saturation. We schematize the porous medium as a network of pore bodies and pore throats with finite volumes. The pore space is modeled using the multidirectional pore-network concept, which allows for a distribution of pore coordination numbers. This topological property together with the distribution of pore sizes are used to mimic the microstructure of real porous media. The dispersivity is calculated by solving the mass balance equations for solute concentration in all network elements and averaging the concentrations over a large number of pores. We have introduced a new formulation of solute transport within pore space, where we account for different compartments of residual water within drained pores. This formulation makes it possible to capture the effect of limited mixing due to partial filling of the pores under variably saturated conditions. We found that dispersivity increases with the decrease in saturation, it reaches a maximum value, and then decreases with further decrease in saturation. To show the capability of our formulation to properly capture the effect of saturation on solute dispersion, we applied it to model the results of a reported experimental study.Citation: Raoof, A., and S. M. Hassanizadeh (2013), Saturation-dependent solute dispersivity in porous media: Pore-scale processes,

A re‐examination of throats

Kim

Lindquist

2013

[1] We critically re-examine the concept of a throat in a porous medium as a geometric quantity defined independently of an entry meniscus in a drainage process. To maintain the standard notion of a throat as a locally minimum-area cross section in the pore network, we demonstrate with examples that throats must intersect each other. Using flow simulation, we show that these intersecting throats correspond to capillary pressure controlled entry points during drainage. We have designed a throat-finding algorithm that explicitly locates intersecting throats, using a planar approximation for robustness and speed. The capability of the new algorithm was compared against an existing algorithm in the construction of pore-throat networks from X-ray computed tomography images of consolidated sandstones (7.5-22% porosity) and of an unconsolidated sand pack (32.5% porosity). We show that the probability of throat intersection increases significantly with porosity above 20%; in the sand pack, over 1/4 of all throats intersect with another.