371 research outputs found
Bulk Mediated Surface Diffusion: Non Markovian Desorption with Finite First Moment
Here we address a fundamental issue in surface physics: the dynamics of
adsorbed molecules. We study this problem when the particle's desorption is
characterized by a non Markovian process, while the particle's adsorption and
its motion in the bulk are governed by a Markovian dynamics. We study the
diffusion of particles in a semi-infinite cubic lattice, and focus on the
effective diffusion process at the interface . We calculate analytically
the conditional probability to find the particle on the plane as well as
the surface dispersion as functions of time. The comparison of these results
with Monte Carlo simulations show an excellent agreement.Comment: 16 pages, 7 figs. European Physical Journal B (in press
Variational Formulation for the KPZ and Related Kinetic Equations
We present a variational formulation for the Kardar-Parisi-Zhang (KPZ)
equation that leads to a thermodynamic-like potential for the KPZ as well as
for other related kinetic equations. For the KPZ case, with the knowledge of
such a potential we prove some global shift invariance properties previously
conjectured by other authors. We also show a few results about the form of the
stationary probability distribution function for arbitrary dimensions. The
procedure used for KPZ was extended in order to derive more general forms of
such a functional leading to other nonlinear kinetic equations, as well as
cases with density dependent surface tension.Comment: RevTex, 8pgs, double colum
Discretization-related issues in the KPZ equation: Consistency, Galilean-invariance violation, and fluctuation--dissipation relation
In order to perform numerical simulations of the KPZ equation, in any
dimensionality, a spatial discretization scheme must be prescribed. The known
fact that the KPZ equation can be obtained as a result of a Hopf--Cole
transformation applied to a diffusion equation (with \emph{multiplicative}
noise) is shown here to strongly restrict the arbitrariness in the choice of
spatial discretization schemes. On one hand, the discretization prescriptions
for the Laplacian and the nonlinear (KPZ) term cannot be independently chosen.
On the other hand, since the discretization is an operation performed on
\emph{space} and the Hopf--Cole transformation is \emph{local} both in space
and time, the former should be the same regardless of the field to which it is
applied. It is shown that whereas some discretization schemes pass both
consistency tests, known examples in the literature do not. The requirement of
consistency for the discretization of Lyapunov functionals is argued to be a
natural and safe starting point in choosing spatial discretization schemes. We
also analyze the relation between real-space and pseudo-spectral discrete
representations. In addition we discuss the relevance of the Galilean
invariance violation in these consistent discretization schemes, and the
alleged conflict of standard discretization with the fluctuation--dissipation
theorem, peculiar of 1D.Comment: RevTex, 23pgs, 2 figures, submitted to Phys. Rev.
Modelling investigation of HF CW response to sudden and sustained organic and hydraulic overloads
INTRODUCTION
Constructed wetlands (CWs) are typically designed assuming idealized steady-state influent loads. However, CWs might face sporadic periods of overloading during their lifespan, due to an increase either in the volume of wastewater to treat or in the pollutant concentrations in wastewater (or both). Although this technology is well known for its buffering capacity, the mechanisms behind it are not well understood. In this study we aim to improve the understanding of the internal processes that make horizontal flow constructed wetlands (HF CWs) able to cope with sudden contaminant and/or hydraulic overloads, and also to investigate if and how sustained overloading affects the long-term performance of these systems.
METHODS
In this study, we employ the BIO_PORE model (Samsó and GarcÃa, 2013a), which simulates the hydraulics (Darcian flow), biochemistry (CWM1 biokinetic model, Langergraber et al., 2009), plant effects (nutrient uptake and oxygen release) and the interactions between bacteria and accumulated solids in HF CWs.
The effect of organic overloads is studied using the same HF CW configuration and influent pollutant loads as those considered by Samsó and GarcÃa (2013b) (10.3 m long and 5.3 m wide CW – COD and TN effluent concentrations validated by Samsó and GarcÃa (2013a)). The final state of the simulation carried out by Samsó and GarcÃa (2013b), which corresponds to the end of the 3rd year of operation of the wetland, is used as initial condition of the simulations developed in this work, which are one year long and reproduce the functioning of the HF CW subject to overloads. These overloads are simulated by increasing inflow COD concentrations, hydraulic loads, or both at the same time.
For the increases in organic loads, three overloading scenarios are tested: +10%, +30% and +50% of influent COD concentration while keeping influent N-NH4 concentrations constant. Additionally, the effect of the HRT is tested by comparing simulations with +30% increase in the organic load, +30% increase in the hydraulic load, and combined +15% increase in both hydraulic and organic loads.
RESULTS AND DISCUSSION
Both the increase in influent COD concentration and different HRT promote a change in bacterial community distribution, which exhibits the same zonation shown by Samsó and GarcÃa (2013b), but with differences in the relative amount of biomass of each bacterial group with respect to the total biomass. Comparison of simulations with similar total biomass reveals how variations in COD removal efficiency for different hydraulic and organic loads are controlled by changes in HRTs and influent concentrations, respectively.Increasing influent COD concentrations stimulates bacterial growth, with total biomass (TB) that tends be more abundant and to occupy more CW space towards the CW outlet (Figure 1, top). Moreover, the portion of TB near the inlet shifts towards the outlet as well due to the higher inert material that is accumulated near the inlet (Figure 1, bottom). HF CW shows a good buffer capacity for organic overloads, with COD removal efficiencies even higher at the end of the simulated year compared to the beginning of the simulation (not shown). However, the response time is long due to the low growth rate of anaerobic bacteria. This results in a long transition phase (almost six months) in which COD removal efficiency is lower (from 91.3% at normal loadings to 80% for +50% organic overloading) (not shown). Additionally, feeding HF CW with higher organic loads reduces their lifespan due to higher accumulation of inert material (Figure 1).
COD removal efficiencies are also influenced by HRT. COD removal efficiency is higher in the first four months when only hydraulic loads are increased, while higher concentrations promote a higher removal efficiency in the last 8 months of the simulation.
Fig. 1. Spatial distribution of total microbial biomass (upper panels) and accumulated solids (lower panels) at the 360th day of simulation (end of the fourth year of HF-CW functioning) for different organic overloads. Values are expressed in kg m−3.
CONCLUSIONS
HF CWs guarantee a good but slow buffering capacity of COD removal in response to organic overloads. This buffering capacity is achieved through changes on the total biomass and on the relative concentration of the different bacterial groups within the granular media. Moreover, we demonstrate that organic and/or hydraulic overloads reduce HF CW’s lifespan
Construction of an evidence-based integrated morphology cleavage embryo score for implantation potential of embryos scored and transferred on day 2 after oocyte retrieval
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Economic evaluation of Medically Assisted Reproduction: An educational overview of methods and applications for healthcare professionals
Economic evaluations of the value-for-money of Medically Assisted Reproduction (MAR) interventions are increasingly important due to growing pressure on healthcare budgets. Although such evaluations are commonplace in the published literature, the number/methodological complexity of different evaluations available, and the challenges specific to MAR interventions, can complicate the interpretation of such analyses for fertility treatments. This article aims to serve as an educational resource and provide context on the design/interpretation of economic analyses for MAR interventions. Several areas are relevant for first-line providers and decision makers: scope of analysis, comparator used, perspective/time horizon considered, outcomes used to measure success, and how results from cost-effectiveness studies can be summarised and used in clinical practice. We aim to help clinicians better understand the strengths/weaknesses of economic analyses, to enable the best use of the evidence in practice, so resources available for MAR interventions can provide maximum value to patients and society
Resonant phenomena in extended chaotic systems subject to external noise: the Lorenz'96 model case
We investigate the effects of a time-correlated noise on an extended chaotic
system. The chosen model is the Lorenz'96, a kind of "toy" model used for
climate studies. Through the analysis of the system's time evolution and its
time and space correlations, we have obtained numerical evidence for two
stochastic resonance-like behavior. Such behavior is seen when both, the usual
and a generalized signal-to-noise ratio function are depicted as a function of
the external noise intensity or the system size. The underlying mechanism seems
to be associated to a "noise-induced chaos reduction". The possible relevance
of these and other findings for an "optimal" climate prediction are discussed.Comment: Submitted to Europhysics Letters (LaTex, 12 pgs, 5 figures
A system of ODEs for a Perturbation of a Minimal Mass Soliton
We study soliton solutions to a nonlinear Schrodinger equation with a
saturated nonlinearity. Such nonlinearities are known to possess minimal mass
soliton solutions. We consider a small perturbation of a minimal mass soliton,
and identify a system of ODEs similar to those from Comech and Pelinovsky
(2003), which model the behavior of the perturbation for short times. We then
provide numerical evidence that under this system of ODEs there are two
possible dynamical outcomes, which is in accord with the conclusions of
Pelinovsky, Afanasjev, and Kivshar (1996). For initial data which supports a
soliton structure, a generic initial perturbation oscillates around the stable
family of solitons. For initial data which is expected to disperse, the finite
dimensional dynamics follow the unstable portion of the soliton curve.Comment: Minor edit
Fingerprints of Kitaev physics in the magnetic excitations of honeycomb iridates
In the quest for realizations of quantum spin liquids, the exploration of
Kitaev materials - spin-orbit entangled Mott insulators with strong
bond-directional exchanges - has taken center stage. However, in these
materials the local spin-orbital j=1/2 moments typically show long-range
magnetic order at low temperature, thus defying the formation of a spin-liquid
ground state. Using resonant inelastic x-ray scattering (RIXS), we here report
on a proximate spin liquid regime with clear fingerprints of Kitaev physics in
the magnetic excitations of the honeycomb iridates alpha-Li2IrO3 and Na2IrO3.
We observe a broad continuum of magnetic excitations that persists up to at
least 300K, more than an order of magnitude larger than the magnetic ordering
temperatures. We prove the magnetic character of this continuum by an analysis
of the resonance behavior. RIXS measurements of the dynamical structure factor
for energies within the continuum show that dynamical spin-spin correlations
are restricted to nearest neighbors. Notably, these spectroscopic observations
are also present in the magnetically ordered state for excitation energies
above the conventional magnon excitations. Phenomenologically, our data agree
with inelastic neutron scattering results on the related honeycomb compound
RuCl3, establishing a common ground for a proximate Kitaev spin-liquid regime
in these materials.Comment: 13 pages, 14 figure
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