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Competitive effects between stationary chemical reaction centres: a theory based on off-center monopoles.
The subject of this paper is competitive effects between multiple reaction sinks. A theory based on off-center monopoles is developed for the steady-state diffusion equation and for the convection-diffusion equation with a constant flow field. The dipolar approximation for the diffusion equation with two equal reaction centres is compared with the exact solution. The former turns out to be remarkably accurate, even for two touching spheres. Numerical evidence is presented to show that the same holds for larger clusters (with more than two spheres). The theory is extended to the convection-diffusion equation with a constant flow field. As one increases the convective velocity, the competitive effects between the reactive centres gradually become less significant. This is demonstrated for a number of cluster configurations. At high flow velocities, the current methodology breaks down. Fixing this problem will be the subject of future research. The current method is useful as an easy-to-use tool for the calibration of other more complicated models in mass and/or heat transfer
Control of Spatially Heterogeneous and Time-Varying Cellular Reaction Networks: A New Summation Law
A hallmark of a plethora of intracellular signaling pathways is the spatial
separation of activation and deactivation processes that potentially results in
precipitous gradients of activated proteins. The classical Metabolic Control
Analysis (MCA), which quantifies the influence of an individual process on a
system variable as the control coefficient, cannot be applied to spatially
separated protein networks. The present paper unravels the principles that
govern the control over the fluxes and intermediate concentrations in spatially
heterogeneous reaction networks. Our main results are two types of the control
summation theorems. The first type is a non-trivial generalization of the
classical theorems to systems with spatially and temporally varying
concentrations. In this generalization, the process of diffusion, which enters
as the result of spatial concentration gradients, plays a role similar to other
processes such as chemical reactions and membrane transport. The second
summation theorem is completely novel. It states that the control by the
membrane transport, the diffusion control coefficient multiplied by two, and a
newly introduced control coefficient associated with changes in the spatial
size of a system (e.g., cell), all add up to one and zero for the control over
flux and concentration. Using a simple example of a kinase/phosphatase system
in a spherical cell, we speculate that unless active mechanisms of
intracellular transport are involved, the threshold cell size is limited by the
diffusion control, when it is beginning to exceed the spatial control
coefficient significantly.Comment: 19 pages, AMS-LaTeX, 6 eps figures included with geompsfi.st
CIN4: a software tool for simulation of heterogeneous reactions at a reactor scale based on a micro-meso-macro coupling
National audienceUnderstanding the industrial reactors behavior is a difficult task in the case of solid state reactions such as solid-gas reactions. Indeed the solid phase is a granular medium through which circulate gaseous reactants and products. The properties of such a medium are modified in space and time due to reactions occurring at a microscopic scale. The thermodynamic conditions are driven not only by the operating conditions but also by the heat and mass transfers in the reactor. CIN4, a multiphysic software resulting from the collaboration between ASTEK and EMSE, offers the resolution of the thermohydraulic equations combined with kinetic laws which describe the heterogeneous reactions. The heat and mass transfers terms entering in the balance equations at a macroscopic scale depend on the kinetics evaluated at the microscopic scale. These equations give the temperature and partial pressure in the reactor, which in turn influence the microscopic kinetic behavior
Kinetic modeling of solid-gas reactions at reactor scale: A general approach
International audienceUnderstanding the industrial reactors behavior is a difficult task in the case of solid state reactions such as solid-gas reactions. Indeed the solid phase is a granular medium through which circulate gaseous reactants and products. The properties of such a medium are modified in space and time due to reactions occurring at a microscopic scale. The thermodynamic conditions are driven not only by the operating conditions but also by the heat and mass transfers in the reactor. We propose to numerically resolve the thermohydraulic equations combined with kinetic laws which describe the heterogeneous reactions. The major advantage of this approach is due to the large variety of kinetic models of grains transformation (~40) compared to the usual approach, especially in the case of surface nucleation and growth processes which need to quantitatively describe the grain conversion kinetics at a microscopic scale due to nucleation frequency and growth rate laws obtained in separate isothermal and isobaric experiments. The heat and mass transfers terms entering in the balance equations at a macroscopic scale depend on the kinetics evaluated at the microscopic scale. These equations give the temperature and partial pressure in the reactor, which in turn influence the microscopic kinetic behavior
Nuclear burning and mixing in the first stars: entrainment at a convective boundary using the PPB advection scheme
The evolution of the first generations of stars at zero or extremly low
metallicity, and especially some crucial properties like the primary N14
production, is charactarized by convective-reactive mixing events that are
mostly absent from similar evolution phases at solar-like metallicity. These
episodes occur when unprocessed H-rich material is mixed accross a convective
boundary into C12 rich He-burning material, as for example in He-shell flashes
of extremely-low metallicity AGB stars. In this paper we describe the
astrophysical context of such convective-reactive events, including the
difficulty of current one-dimensional stellar evolution models to correctly
simulate these evolutionary phases. We then describe the requirements and
current state of modeling convective-reactive processes in the first stars
environment. We demonstrate some of the new concepts that we are applying to
this problem, i.e. the highly accurate PPB advection scheme in the framework of
PPM hydrodynamic simulations of mixing accross a very stiff convective
boundary. We show initial results of such simulations that address the first
non-reactive step of this problem, which is the entrainment of H at the top
boundary of the He-shell flash convection zone.Comment: Proceedings paper of First Stars III, 2006, Santa Fe, contributions
by Falk Herwig and Paul Woodward, to appear in AIP Conf. Ser., ed. T. Abel,
A. Heger and B. O'She
Diffusive search for a stochastically-gated target with resetting
In this paper, we analyze the mean first passage time (MFPT) for a single
Brownian particle to find a stochastically-gated target under the additional
condition that the position of the particle is reset to a fixed position \x_r
at a rate . The gate switches between an open and closed state according to
a two-state Markov chain and can only be detected by the searcher in the open
state. One possible example of such a target is a protein switching between
different conformational states. As expected, the MFPT with or without
resetting is an increasing function of the fraction of time that the
gate is closed. However, the interplay between stochastic resetting and
stochastic gating has non-trivial effects with regards the optimization of the
search process under resetting. First, by considering the diffusive search for
a gated target at one end of an interval, we show that the fractional change in
the MFPT under resetting exhibits a non-monotonic dependence on . In
particular, the percentage reduction of the MFPT at the optimal resetting rate
(when it exists) increases with up to some critical value, after which
it decreases and eventually vanishes. Second, in the case of a spherical target
in , the dependence of the MFPT on the spatial dimension is
significantly amplified in the presence of stochastic gating.Comment: 17 pages, 9 figure
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