290 research outputs found
Modeling and Simulation of Thermo-Fluid-Electrochemical Ion Flow in Biological Channels
In this article we address the study of ion charge transport in the
biological channels separating the intra and extracellular regions of a cell.
The focus of the investigation is devoted to including thermal driving forces
in the well-known velocity-extended Poisson-Nernst-Planck (vPNP)
electrodiffusion model. Two extensions of the vPNP system are proposed: the
velocity-extended Thermo-Hydrodynamic model (vTHD) and the velocity-extended
Electro-Thermal model (vET). Both formulations are based on the principles of
conservation of mass, momentum and energy, and collapse into the vPNP model
under thermodynamical equilibrium conditions. Upon introducing a suitable
one-dimensional geometrical representation of the channel, we discuss
appropriate boundary conditions that depend only on effectively accessible
measurable quantities. Then, we describe the novel models, the solution map
used to iteratively solve them, and the mixed-hybrid flux-conservative
stabilized finite element scheme used to discretize the linearized equations.
Finally, we successfully apply our computational algorithms to the simulation
of two different realistic biological channels: 1) the Gramicidin-A channel
considered in~\cite{JeromeBPJ}; and 2) the bipolar nanofluidic diode considered
in~\cite{Siwy7}
Solution Map Analysis of a Multiscale Drift-Diffusion Model for Organic Solar Cells
In this article we address the theoretical study of a multiscale
drift-diffusion (DD) model for the description of photoconversion mechanisms in
organic solar cells. The multiscale nature of the formulation is based on the
co-presence of light absorption, conversion and diffusion phenomena that occur
in the three-dimensional material bulk, of charge photoconversion phenomena
that occur at the two-dimensional material interface separating acceptor and
donor material phases, and of charge separation and subsequent charge transport
in each three-dimensional material phase to device terminals that are driven by
drift and diffusion electrical forces. The model accounts for the nonlinear
interaction among four species: excitons, polarons, electrons and holes, and
allows to quantitatively predict the electrical current collected at the device
contacts of the cell. Existence and uniqueness of weak solutions of the DD
system, as well as nonnegativity of all species concentrations, are proved in
the stationary regime via a solution map that is a variant of the Gummel
iteration commonly used in the treatment of the DD model for inorganic
semiconductors. The results are established upon assuming suitable restrictions
on the data and some regularity property on the mixed boundary value problem
for the Poisson equation. The theoretical conclusions are numerically validated
on the simulation of three-dimensional problems characterized by realistic
values of the physical parameters
The role of structural viscoelasticity in deformable porous media with incompressible constituents: applications in biomechanics
The main goal of this work is to clarify and quantify, by means of
mathematical analysis, the role of structural viscoelasticity in the
biomechanical response of deformable porous media with incompressible
constituents to sudden changes in external applied loads. Models of deformable
porous media with incompressible constituents are often utilized to describe
the behavior of biological tissues, such as cartilages, bones and engineered
tissue scaffolds, where viscoelastic properties may change with age, disease or
by design. Here, for the first time, we show that the fluid velocity within the
medium could increase tremendously, even up to infinity, should the external
applied load experience sudden changes in time and the structural
viscoelasticity be too small. In particular, we consider a one-dimensional
poro-visco-elastic model for which we derive explicit solutions in the cases
where the external applied load is characterized by a step pulse or a
trapezoidal pulse in time. By means of dimensional analysis, we identify some
dimensionless parameters that can aid the design of structural properties
and/or experimental conditions as to ensure that the fluid velocity within the
medium remains bounded below a certain given threshold, thereby preventing
potential tissue damage. The application to confined compression tests for
biological tissues is discussed in detail. Interestingly, the loss of
viscoelastic tissue properties has been associated with various disease
conditions, such as atherosclerosis, Alzheimer's disease and glaucoma. Thus,
the findings of this work may be relevant to many applications in biology and
medicine
A Computational Model for Biomass Growth Simulation in Tissue Engineering
This article deals with computational modeling of tissue growth under interstitial perfusion inside a polymeric scaffold-based bioreactor. The mathematical model is the result of the application of the volume averaging technique to the fluid, nutrient and cellular subsystems, and is capable to account for the temporal evolution of local matrix porosity, as the sum of a time-invariant component (the porosity of the uncellularized polymer scaffold) and a time-dependent component (due to the growing biomass). The solution algorithm is based on a block Gauss-Seidel iteration procedure that allows to reduce each time level of the simulated culture period into the successive solution of linearized subproblems, whose numerical approximation is carried out using stable and convergent finite elements. Numerical simulations are carried out to investigate the role of the design porosity of the scaffold on nutrient delivery and biomass production
Effectiveness of dismantling strategies on moderated vs. unmoderated online social platforms
Online social networks are the perfect test bed to better understand
large-scale human behavior in interacting contexts. Although they are broadly
used and studied, little is known about how their terms of service and posting
rules affect the way users interact and information spreads. Acknowledging the
relation between network connectivity and functionality, we compare the
robustness of two different online social platforms, Twitter and Gab, with
respect to dismantling strategies based on the recursive censor of users
characterized by social prominence (degree) or intensity of inflammatory
content (sentiment). We find that the moderated (Twitter) vs unmoderated (Gab)
character of the network is not a discriminating factor for intervention
effectiveness. We find, however, that more complex strategies based upon the
combination of topological and content features may be effective for network
dismantling. Our results provide useful indications to design better strategies
for countervailing the production and dissemination of anti-social content in
online social platforms
A Multiscale Thermo-Fluid Computational Model for a Two-Phase Cooling System
In this paper, we describe a mathematical model and a numerical simulation
method for the condenser component of a novel two-phase thermosyphon cooling
system for power electronics applications. The condenser consists of a set of
roll-bonded vertically mounted fins among which air flows by either natural or
forced convection. In order to deepen the understanding of the mechanisms that
determine the performance of the condenser and to facilitate the further
optimization of its industrial design, a multiscale approach is developed to
reduce as much as possible the complexity of the simulation code while
maintaining reasonable predictive accuracy. To this end, heat diffusion in the
fins and its convective transport in air are modeled as 2D processes while the
flow of the two-phase coolant within the fins is modeled as a 1D network of
pipes. For the numerical solution of the resulting equations, a Dual
Mixed-Finite Volume scheme with Exponential Fitting stabilization is used for
2D heat diffusion and convection while a Primal Mixed Finite Element
discretization method with upwind stabilization is used for the 1D coolant
flow. The mathematical model and the numerical method are validated through
extensive simulations of realistic device structures which prove to be in
excellent agreement with available experimental data
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