Recent Simulation Studies Research Articles

Structure and rheology of soft hybrid systems of magnetic nanoparticles in liquid-crystalline matrices: results from particle-resolved computer simulations

Abstract Hybrid mixtures composed of magnetic nanoparticles (MNP) in liquid crystalline (LC) matrices are a fascinating class of soft materials with intriguing physical properties and a wide range of potential applications, e.g., as stimuli-responsive and adaptive materials. Already in the absence of an external stimulus, these systems can display various types of orientationally disordered and ordered phases, which are enriched by self-assembled structures formed by the MNPs. In the presence of external fields, one typically observes highly nonlinear macroscopic behavior. However, an understanding of the structure and dynamics of such systems on the particle level has, so far, remained elusive. In the present paper we review recent computer simulation studies targeting the structure, equilibrium dynamics and rheology of LC-MNP systems, in which the particle sizes of the two components are comparable. As a numerically tractable model system we consider mixtures of soft spherical or elongated particles with a permanent magnetic dipole moment and ellipsoidal non-magnetic particles interacting via a Gay-Berne potential. We address, first, equilibrium aspects such as structural organization and self-assembly (cluster formation) of the MNPs in dependence of the orientational state of the matrix, the role of the size ratio, the impact of an external magnetic field, and the translational and orientational diffusion of the two components. Second, we discuss the non-equilibrium dynamics of LC-MNP mixtures under planar shear flow, considering both, spherical and non-spherical MNPs. Our results contribute to a detailed understanding of these intriguing hybrid materials, and they may serve as a guide for future experiments.

Internal dose assessment of 148Gd using isotope ratios of gamma-emitting 146Gd or 153Gd in accidently released spallation target particles

The pure alpha emitter 148Gd may have a significant radiological impact in terms of internal dose to exposed humans in case of accidental releases from a spallation source using a tungsten target, such as the one to be used in the European Spallation Source (ESS). In this work we aim to present an approach to indirectly estimate the whole-body burden of 148Gd and the associated committed effective dose in exposed humans, by means of high-resolution gamma spectrometry of the gamma-emitting radiogadolinium isotopes 146Gd and 153Gd that are accompanied by 148Gd generated from the operation of the tungsten target. Theoretical minimum detectable whole-body activity (MDA) and associated internal doses from 148Gd are calculated using a combination of existing biokinetic models and recent computer simulation studies on the generated isotope ratios of 146Gd/148Gd and 153Gd/148Gd in the ESS target. Of the two gamma-emitting gadolinium isotopes, 146Gd is initially the most sensitive indicator of the presence of 148Gd if whole-body counting is performed within a month after the release, using the twin photo peaks of 146Gd centered at 115.4 keV (MDA < 1 Bq for ingested 148Gd, and < 25 Bq for inhaled 148Gd). The corresponding minimum detectable committed effective doses will be less than 1 µSv for ingested 148Gd, but substantially higher for inhaled 148Gd (up to 0.3 mSv), depending on operation time of the target prior to the release. However, a few months after an atmospheric release, 153Gd becomes a much more sensitive indicator of body burdens of 148Gd, with a minimum detectable committed effective doses ranging from 18 to 77 µSv for chronic ingestion and between 0.65 to 2.7 mSv for acute inhalation in connection to the release. The main issue with this indirect method for 148Gd internal dose estimation, is whether the primary photon peaks from 146 and 153Gd can be detected undisturbed. Preliminary simulations show that nuclides such as 182Ta may potentially create perturbations that could impair this evaluation method, and which impact needs to be further studied in future safety assessments of accidental target releases.

Is the near-spherical shape the “new black” for smoke?

Abstract. We examine the capability of near-spherical-shaped particles to reproduce the triple-wavelength particle linear depolarization ratio (PLDR) and lidar ratio (LR) values measured over Europe for stratospheric smoke originating from Canadian wildfires. The smoke layers were detected both in the troposphere and the stratosphere, though in the latter case the particles presented PLDR values of almost 18 % at 532 nm as well as a strong spectral dependence from the UV to the near-IR wavelength. Although recent simulation studies of rather complicated smoke particle morphologies have shown that heavily coated smoke aggregates can produce large PLDR, herein we propose a much simpler model of compact near-spherical smoke particles. This assumption allows for the reproduction of the observed intensive optical properties of stratospheric smoke, as well as their spectral dependence. We further examine whether an extension of the current Aerosol Robotic Network (AERONET) scattering model to include the near-spherical shapes could be of benefit to the AERONET retrieval for stratospheric smoke cases associated with enhanced PLDR. Results of our study illustrate the fact that triple-wavelength PLDR and LR lidar measurements can provide us with additional insight when it comes to particle characterization.

Postural stability in human running with step-down perturbations: an experimental and numerical study.

Postural stability is one of the most crucial elements in bipedal locomotion. Bipeds are dynamically unstable and need to maintain their trunk upright against the rotations induced by the ground reaction forces (GRFs), especially when running. Gait studies report that the GRF vectors focus around a virtual point above the centre of mass (VPA), while the trunk moves forward in pitch axis during the stance phase of human running. However, a recent simulation study suggests that a virtual point below the centre of mass (VPB) might be present in human running, because a VPA yields backward trunk rotation during the stance phase. In this work, we perform a gait analysis to investigate the existence and location of the VP in human running at 5 m s−1, and support our findings numerically using the spring-loaded inverted pendulum model with a trunk. We extend our analysis to include perturbations in terrain height (visible and camouflaged), and investigate the response of the VP mechanism to step-down perturbations both experimentally and numerically. Our experimental results show that the human running gait displays a VPB of ≈−30 cm and a forward trunk motion during the stance phase. The camouflaged step-down perturbations affect the location of the VPB. Our simulation results suggest that the VPB is able to encounter the step-down perturbations and bring the system back to its initial equilibrium state.

On the estimation of inbreeding depression using different measures of inbreeding from molecular markers.

The inbreeding coefficient (F) of individuals can be estimated from molecular marker data, such as SNPs, using measures of homozygosity of individual markers or runs of homozygosity (ROH) across the genome. These different measures of F can then be used to estimate the rate of inbreeding depression (ID) for quantitative traits. Some recent simulation studies have investigated the accuracy of this estimation with contradictory results. Whereas some studies suggest that estimates of inbreeding from ROH account more accurately for ID, others suggest that inbreeding measures from SNP‐by‐SNP homozygosity giving a large weight to rare alleles are more accurate. Here, we try to give more light on this issue by carrying out a set of computer simulations considering a range of population genetic parameters and population sizes. Our results show that the previous studies are indeed not contradictory. In populations with low effective size, where relationships are more tight and selection is relatively less intense, F measures based on ROH provide very accurate estimates of ID whereas SNP‐by‐SNP‐based F measures with high weight to rare alleles can show substantial upwardly biased estimates of ID. However, in populations of large effective size, with more intense selection and trait allele frequencies expected to be low if they are deleterious for fitness because of purifying selection, average estimates of ID from SNP‐by‐SNP‐based F values become unbiased or slightly downwardly biased and those from ROH‐based F values become slightly downwardly biased. The noise attached to all these estimates, nevertheless, can be very high in large‐sized populations. We also investigate the relationship between the different F measures and the homozygous mutation load, which has been suggested as a proxy of inbreeding depression.

Transport and Interfacial Properties of Mixed Molten Carbonate/Hydroxide Electrolytes by Molecular Dynamics Simulations

We use molecular dynamics simulations based on a recently developed force field to obtain the viscosity, ionic conductivity, and liquid–vapor surface tension of molten alkali-metal carbonate–hydroxide mixtures over a range of cation and hydroxide compositions. Recent experimental and simulation studies have suggested that molten carbonates contain non-negligible amounts of hydroxide ions in the presence of water at low partial pressures of CO2. However, due to the high temperatures (≃600 °C or higher) required to melt pure alkali carbonates and their mixtures, there is a lack of experimental thermodynamic and transport data for these molten phases. Here, we deploy a recently parametrized force field for molten alkali carbonates and hydroxides [ J. Chem. Theory Comput. 2020, 16, 5736−5746, DOI: 10.1021/acs.jctc.0c00285] to simulate some physical properties using atomistic molecular dynamics. Our predictions show a consistent decrease in viscosity and an increase in ionic conductivity with increasing hydroxide fractions, whereas a higher Li+ mole fraction leads to an increase in both viscosity and ionic conductivity. The computed surface tension values exhibit an upward trend with higher asymmetry in cation and anion sizes. Structural analysis suggests that in the carbonate–hydroxide melts the smaller/harder ions, OH– and Li+, are more favored at the interface than the larger/softer ions, CO32– and K+/Na+. The current approach provides a systematic route to obtaining physicochemical properties of molten alkali carbonate and hydroxide mixtures over a large domain of chemical compositions and thus can steer future experimental research for molten carbonates and their applications.

Hydrophobic dewetting in gating and regulation of transmembrane protein ion channels.

Water is at the heart of almost all biological phenomena, without which no life that we know of would have been possible. It is a misleadingly complex liquid that exists in near coexistence with the vapor phase under ambient conditions. Confinement within a hydrophobic cavity can tip this balance enough to drive a cooperative dewetting transition. For a nanometer-scale pore, the dewetting transition leads to a stable dry state that is physically open but impermeable to ions. This phenomenon is often referred to as hydrophobic gating. Numerous transmembrane protein ion channels have now been observed to utilize hydrophobic gating in their activation and regulation. Here, we review recent theoretical, simulation, and experimental studies that together have started to establish the principles of hydrophobic gating and discuss how channels of various sizes, topologies, and biological functions can utilize these principles to control the thermodynamic properties of water within their interior pores for gating and regulation. Exciting opportunities remain in multiple areas, particularly on direct experimental detection of hydrophobic dewetting in biological channels and on understanding how the cell may control the hydrophobic gating in regulation of ion channels.

Instantaneous vs. non-instantaneous diver-operated stereo-video (DOV) surveys of highly mobile sharks in the Galápagos Marine Reserve

Effective management of wild populations of animals relies on having accurate methods for estimating their densities from field surveys. Surveying sharks and other highly mobile marine predators is particularly difficult, requiring specialised methods that can entail a range of potential biases. Recent simulation studies have suggested that non-instantaneous visual and video survey methods may overestimate densities of highly mobile fishes. A non-instantaneous method is one where each point in space is surveyed for a non-negligible period of time, rather than in an instantaneous ‘snapshot’, allowing animals to move into the survey space during the survey period. We conducted instantaneous and non-instantaneous surveys of sharks (Sphyrna lewini and Carcharhinus galapagensis) around Darwin and Wolf Islands (Galápagos Islands, Ecuador) using diver-operated stereo-video (stereo-DOV) systems with towed GPS. We provide the first empirical evidence of non-instantaneous bias and show that the degree of bias is negatively related to diver swimming speed. At a speed of 40 m min-1, the non-instantaneous method recorded twice as many sharks as the instantaneous method. The bias may be as high as 10 fold at slow speeds (23 m min-1) and negligible at very fast speeds (65 m min-1). Moreover, we used GPS data to produce fine-scale distribution maps, revealing high densities in areas of strong currents along the south-eastern sides of both islands. We recommend the use of stereo-DOV systems with GPS and fast swimming speeds for surveying highly mobile sharks, to produce more realistic estimates of densities on which to base management of these species.

Thermal Chains and Entrainment in Cumulus Updrafts. Part I: Theoretical Description

AbstractRecent studies have shown that cumulus updrafts often consist of a succession of discrete rising thermals with spherical vortex-like circulations. In this paper, a theory is developed for why this “thermal chain” structure occurs. Theoretical expressions are obtained for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. Analysis of these expressions suggests that the thermal chain structure arises from enhanced lateral mixing associated with intrusions of dry environmental air below an updraft’s vertical velocity maximum. This dry-air entrainment reduces buoyancy locally. Consequently, the updraft flow above levels of locally reduced buoyancy separates from below, leading to a breakdown of the updraft into successive discrete thermals. The range of conditions in which thermal chains exist is also analyzed from the theoretical expressions. A transition in updraft structure from isolated rising thermal, to thermal chain, to starting plume occurs with increases in updraft width, environmental relative humidity, and/or convective available potential energy. Corresponding expressions for the bulk fractional entrainment rate ε are also obtained. These expressions indicate rather complicated entrainment behavior of ascending updrafts, with local enhancement of ε up to a factor of ~2 associated with the aforementioned environmental-air intrusions, consistent with recent large-eddy simulation (LES) studies. These locally large entrainment rates contribute significantly to overall updraft dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Part II of this study compares results from the theoretical expressions to idealized numerical simulations and LES.

Molecular simulation of double layer expansion mechanism during low-salinity waterflooding

Low-salinity waterflooding is an improved oil recovery technique whose mechanisms are still elusive. Double Layer Expansion (DLE) is one of the main proposed mechanisms which have been settled in the petroleum literature. However, recent molecular simulation studies have questioned its validity. In this study, we discuss the current understanding of DLE mechanism and design molecular simulations to investigate it at the molecular level. We used kaolinite and montmorillonite substrates in contact with water having varying concentrations of monovalent and divalent ions. Our results confirm that the location of the adsorption planes is independent of the ionic strength. However, the electric potential developed over these surfaces and how it decays depends on both the ionic strength and ion type. A shrinkage is observed in the double layer for the case of low salinity, supported by both film thickness estimations and interaction energy analysis. This shrinkage, which contradicts the prevailing assumption, is consistent with molecular simulation literature, and casts some doubts on the efficiency of double layer expansion as a mechanism for improved oil recovery observed during low-salinity waterflooding.

Estimating outcome-specific effects in meta-analyses of multiple outcomes: A simulation study.

In meta-analysis, primary studies often include multiple, dependent effect sizes. Several methods address this dependency, such as the multivariate approach, three-level models, and the robust variance estimation (RVE) method. As for today, most simulation studies that explore the performance of these methods have focused on the estimation of the overall effect size. However, researchers are sometimes interested in obtaining separate effect size estimates for different types of outcomes. A recent simulation study (Park & Beretvas, 2019) has compared the performance of the three-level approach and the RVE method in estimating outcome-specific effects when several effect sizes are reported for different types of outcomes within studies. The goal of this paper is to extend that study by incorporating additional simulation conditions and by exploring the performance of additional models, such as the multivariate model, a three-level model that specifies different study-effects for each type of outcome, a three-level model that specifies a common study-effect for all outcomes, and separate three-level models for each type of outcome. Additionally, we also tested whether the a posteriori application of the RV correction improves the standard error estimates and the 95% confidence intervals. Results show that the application of separate three-level models for each type of outcome is the only approach that consistently gives adequate standard error estimates. Also, the a posteriori application of the RV correction results in correct 95% confidence intervals in all models, even if they are misspecified, meaning that Type I error rate is adequate when the RV correction is implemented.

Movement History Influences Pendulum Test Kinematics in Children With Spastic Cerebral Palsy.

The pendulum test assesses quadriceps spasticity by dropping the lower leg of a relaxed patient from the horizontal position and observing limb movement. The first swing excursion (FS) decreases with increasing spasticity severity. Our recent simulation study suggests that the reduced initial swing results from muscle short-range stiffness and its interaction with reflex hyper-excitability. Short-range stiffness emerges from the thixotropic behavior of muscles where fiber stiffness upon stretch increases when the muscle is held isometric. Fiber stiffness might thus be higher during the first swing of the pendulum test than during consecutive swings. In addition, it has recently been suggested that muscle spindle firing reflects fiber force rather than velocity and therefore, reflex activity might depend on fiber stiffness. If this hypothesized mechanism is true, we expect to observe larger first swing excursions and reduced reflex muscle activity when the leg is moved rather than kept isometric before release, especially in patients with increased reflex activity. We performed the pendulum test in 15 children with cerebral palsy (CP) and 15 age-matched typically developing (TD) children in two conditions. In the hold condition, the leg was kept isometric in the extended position before release. In the movement condition, the leg was moved up and down before release to reduce the contribution of short-range stiffness. Knee kinematics and muscle activity were recorded. Moving the leg before release increased first swing excursion (p < 0.001) and this increase was larger in children with CP (21°) than in TD children (8°) (p < 0.005). In addition, pre-movement delayed reflex onset by 87 ms (p < 0.05) and reduced reflex activity as assessed through the area under the curve of rectus femoris electromyography (p < 0.05) in children with CP. The movement history dependence of pendulum kinematics and reflex activity supports our hypothesis that muscle short-range stiffness and its interaction with reflex hyper-excitability contribute to joint hyper-resistance in spastic CP. Our results have implications for standardizing movement history in clinical tests of spasticity and for understanding the role of spasticity in functional movements, where movement history differs from movement history in clinical tests.

Simulations of activities, solubilities, transport properties, and nucleation rates for aqueous electrolyte solutions.

This article reviews recent molecular simulation studies of "collective" properties of aqueous electrolyte solutions, specifically free energies and activity coefficients, solubilities, nucleation rates of crystals, and transport coefficients. These are important fundamental properties for biology and geoscience, but also relevant for many technological applications. Their determination from molecular-scale calculations requires large systems and long sampling times, as well as specialized sampling algorithms. As a result, such properties have not typically been taken into account during optimization of force field parameters; thus, they provide stringent tests for the transferability and range of applicability of proposed molecular models. There has been significant progress on simulation algorithms to enable the determination of these properties with good statistical uncertainties. Comparisons of simulation results to experimental data reveal deficiencies shared by many commonly used models. Moreover, there appear to exist specific tradeoffs within existing modeling frameworks so that good prediction of some properties is linked to poor prediction for specific other properties. For example, non-polarizable models that utilize full charges on the ions generally fail to predict accurately both activity coefficients and solubilities; the concentration dependence of viscosity and diffusivity for these models is also incorrect. Scaled-charge models improve the dynamic properties and could also perform well for solubilities but fail in the prediction of nucleation rates. Even models that do well at room temperature for some properties generally fail to capture their experimentally observed temperature dependence. The main conclusion from the present review is that qualitatively new physics will need to be incorporated in future models of electrolyte solutions to allow the description of collective properties for broad ranges of concentrations, temperatures, and solvent conditions.

Generalised Eddy Dissipation Concept for MILD combustion regime at low local Reynolds and Damköhler numbers. Part 2: Validation of the model

The generalised Eddy Dissipation Concept (EDC) developed in the first part of this article is thoroughly validated against twelve flames from the Delft and Adelaide jet-in-hot-coflow (JHC) burners. These flames emulate Moderate or Intense Low Oxygen Dilution (MILD) conditions. Modelling of turbulence-chemistry interactions in this regime is a non trivial problem and many standard combustion models may fail. Recent Direct Numerical Simulation studies revealed a distributed appearance of the reaction zone indicating non-flamelet regime, which justified the use of reactor type modelling approaches. Those kind of models are of empirical nature and are sometimes criticized for being dependent on a number of tunable parameters. Also, most of new concepts are validated against a limited number of experiments. In this study, using the same modelling setup, twelve flames with different jet Reynolds number, level of oxidizer dilution with various fuel mixture were simulated. It turned out that the generalised EDC model considerably improved predictions with respect to the standard model for all the considered flames. Even though the predictions of the other EDC extensions provided better results in some regions, only the proposed generalised approach could cover the broad tange of operating conditions, proving its “universality” and reliability.

Leveraging Posterity’s Prosperity?

We critically review studies by Blanchard (B) and Rachel and Summers (RS). By the standard fiscal-gap measure, the US government is in dire fiscal shape thanks to constantly enlarging its postwar, take-as-you-go Ponzi scheme. Yet B and RS seemingly rationalize its expansion. Their arguments rest on the safe rate being very low. But almost all households face high safe rates--the rates available from pre-paying their loans. We also question modeling assumptions that help drive key B and RS results and reference recent simulation studies, which reach strongly opposite conclusions to B's.

Integration of mitochondrial energetics in heart with mathematical modelling.

It has been an unsolved question how cardiac mitochondrial energetics is regulated during working transition. Mathematical modelling is a powerful tool for exploring the complicated networks of mitochondrial metabolism. We summarize the recent progress and remaining questions about mitochondrial energetics in heart, especially focusing on approaches utilizing mathematical modelling. Feedback activation by ADP and/or inorganic phosphate is an old but still attractive hypothesis for explaining the regulation mechanisms of cardiac mitochondrial energetics. However, this hypothesis has not been fully validated by experiments because rises of ADP and/or inorganic phosphate concentrations during cardiac workload increase have not been detected in many experiments. The hypothesis of intracellular energetic units is an extended version of feedback activation, which has a similar problem. The each-step activation hypothesis beautifully reproduces metabolite constancy, although such master regulators have not been identified yet. Ca2+ has been the most plausible candidate because some of the mitochondrial dehydrogenases are activated by it. Recent experimental and simulation studies, however, throw doubt on its physiological relevance. Finally, we discuss issues to be solved to obtain a better view of cardiac mitochondrial energetics.

Periodic dynamic induction control of wind farms: proving the potential in simulations and wind tunnel experiments

Abstract. As wind turbines in a wind farm interact with each other, a control problem arises that has been extensively studied in the literature: how can we optimize the power production of a wind farm as a whole? A traditional approach to this problem is called induction control, in which the power capture of an upstream turbine is lowered for the benefit of downstream machines. In recent simulation studies, an alternative approach, where the induction factor is varied over time, has shown promising results. In this paper, the potential of this dynamic induction control (DIC) approach is further investigated. Only periodic variations, where the input is a sinusoid, are studied. A proof of concept for this periodic DIC approach will be given by the execution of scaled wind tunnel experiments, showing for the first time that this approach can yield power gains in real-world wind farms. Furthermore, the effects on the damage equivalent loads (DEL) of the turbine are evaluated in a simulation environment. These indicate that the increase in DEL on the excited turbine is limited.

Morphological Phylogenetics Evaluated Using Novel Evolutionary Simulations.

Evolutionary inferences require reliable phylogenies. Morphological data have traditionally been analyzed using maximum parsimony, but recent simulation studies have suggested that Bayesian analyses yield more accurate trees. This debate is ongoing, in part, because of ambiguity over modes of morphological evolution and a lack of appropriate models. Here, we investigate phylogenetic methods using two novel simulation models—one in which morphological characters evolve stochastically along lineages and another in which individuals undergo selection. Both models generate character data and lineage splitting simultaneously: the resulting trees are an emergent property, rather than a fixed parameter. Standard consensus methods for Bayesian searches (Mki) yield fewer incorrect nodes and quartets than the standard consensus trees recovered using equal weighting and implied weighting parsimony searches. Distances between the pool of derived trees (most parsimonious or posterior distribution) and the true trees—measured using Robinson-Foulds (RF), subtree prune and regraft (SPR), and tree bisection reconnection (TBR) metrics—demonstrate that this is related to the search strategy and consensus method of each technique. The amount and structure of homoplasy in character data differ between models. Morphological coherence, which has previously not been considered in this context, proves to be a more important factor for phylogenetic accuracy than homoplasy. Selection-based models exhibit relatively lower homoplasy, lower morphological coherence, and higher inaccuracy in inferred trees. Selection is a dominant driver of morphological evolution, but we demonstrate that it has a confounding effect on numerous character properties which are fundamental to phylogenetic inference. We suggest that the current debate should move beyond considerations of parsimony versus Bayesian, toward identifying modes of morphological evolution and using these to build models for probabilistic search methods. [Bayesian; evolution; morphology; parsimony; phylogenetics; selection; simulation.]

Improving parameter recovery for conflict drift-diffusion models

Several drift-diffusion models have been developed to account for the performance in conflict tasks. Although a common characteristic of these models is that the drift rate changes within a trial, their architecture is rather different. Comparative studies usually examine which model fits the data best. However, a good fit does not guarantee good parameter recovery, which is a necessary condition for a valid interpretation of any fit. A recent simulation study revealed that recovery performance varies largely between models and individual parameters. Moreover, recovery was generally not very impressive. Therefore, the aim of the present study was to introduce and test an improved fit procedure. It is based on a grid search for determining the initial parameter values and on a specific criterion for assessing the goodness of fit. Simulations show that not only the fit performance but also parameter recovery improved substantially by applying this procedure, compared to the standard one. The improvement was largest for the most complex model.

The effect of fuel bed height in grass fire spread: addressing the findings and recommendations of Moinuddin et al. (2018)

A recent numerical simulation study by Moinuddin et al. (2018) determined that over a specific range of Froude numbers defined by them as ‘plume mode’, grass fuel height has a strong inverse effect on the rate of fire spread in grasslands. They then suggested that a relationship for effect of fuel height derived from their simulation results could be used to support fire management decision-making. The present analysis used fire spread measurement data from two outdoor experimental burning studies in grass fuels where an explicit control of fuel height was imposed to verify the realism of their results. It was found that a reduction in grass height, with or without removal of the cut fuel and regardless of the Froude number, led to a significant reduction in rate of fire spread, a result opposite to the simulations obtained by Moinuddin et al. (2018).