Academic literature on the topic 'Diffusion timescales'

In the statistical quasilinear theory of weak plasma turbulence, charged particles moving in electrostatic fluctuations diffuse in velocity, i.e. the velocity variance 〈Δv2(t)〉 increases linearly with time t, for times long compared with the auto-correlation time τac of the field, which may be estimated as the reciprocal of the spectral width of the fluctuations. Recent test-particle simulations have revealed a new regime at very long timescales t[Gt ]τac where quasilinear theory breaks down, for intermediate field amplitudes. As this behaviour is not consistent with a diffusion on quasilinear timescales, the problem of the motion of particles in a broadband wave field, for the case of a slowly growing field, is considered here from a purely dynamical point of view, introducing no statistics on the field and no restriction on the amplitude of this field. By determining, on a given timescale, and in the frame of wave–particle interaction, the spectral width over which waves interact efficiently with a particle, a new timescale is found: the nonlinear time of wave–particle interaction τNL∝ (spectral density of energy)−1/3[Gt ]τac. This is the correlation time of the dynamics. For times shorter than τNL, the particles trajectories remain globally regular, and do not separate: they follow a quasifractal set of dimension 2. For times long compared with τNL, there appears a ‘true’ diffusive regime with mixing and decorrelation, due to nonlinear mixing in phase space and the localization of the wave–particle interaction. These theoretical results are confirmed by a numerical study of the velocity variance as a function of time. In particular, the particle dynamics really do become diffusive on timescales several orders of magnitude longer than that predicted by quasilinear theory (namely [Gt ]τNL[Gt ]τac). Finally, deviations from the quasilinear value of the diffusion coefficient and wave growth rate, discussed in the literature, are explained.

Protein concentration gradients organize cells and tissues and commonly form through diffusion away from a local source of protein. Interestingly, during the asymmetric division of the Caenorhabditis elegans zygote, the RNA-binding proteins MEX-5 and PIE-1 form opposing concentration gradients in the absence of a local source. In this study, we use near-total internal reflection fluorescence (TIRF) imaging and single-particle tracking to characterize the reaction/diffusion dynamics that maintain the MEX-5 and PIE-1 gradients. Our findings suggest that both proteins interconvert between fast-diffusing and slow-diffusing states on timescales that are much shorter (seconds) than the timescale of gradient formation (minutes). The kinetics of diffusion-state switching are strongly polarized along the anterior/posterior (A/P) axis by the PAR polarity system such that fast-diffusing MEX-5 and PIE-1 particles are approximately symmetrically distributed, whereas slow-diffusing particles are highly enriched in the anterior and posterior cytoplasm, respectively. Using mathematical modeling, we show that local differences in the kinetics of diffusion-state switching can rapidly generate stable concentration gradients over a broad range of spatial and temporal scales.

Intrinsic timescales of activity fluctuations vary hierarchically across the brain. This variation reflects a broad gradient of functional specialization in information storage and processing, with integrative association areas displaying slower timescales that are thought to reflect longer temporal processing windows. The organization of timescales is associated with cognitive function, distinctive between individuals, and disrupted in disease, but we do not yet understand how the temporal properties of activity dynamics are shaped by the brain’s underlying structural connectivity network. Using resting-state fMRI and diffusion MRI data from 100 healthy individuals from the Human Connectome Project, here we show that the timescale of resting-state fMRI dynamics increases with structural connectivity strength, matching recent results in the mouse brain. Our results hold at the level of individuals, are robust to parcellation schemes, and are conserved across a range of different timescale- related statistics. We establish a comprehensive BOLD dynamical signature of structural connectivity strength by comparing over 6,000 time series features, highlighting a range of new temporal features for characterizing BOLD dynamics, including measures of stationarity and symbolic motif frequencies. Our findings indicate a conserved property of mouse and human brain organization in which a brain region’s spontaneous activity fluctuations are closely related to their surrounding structural scaffold.

Rotational motion of molecules plays an important role in determining NMR spin relaxation properties of liquids. The textbook theory of NMR spin relaxation predominantly uses the assumption that the reorientational dynamics of molecules is described by a continuous time rotational diffusion random walk with a single rotational diffusion coefficient. Previously we and others have shown that reorientation of water molecules on the timescales of picoseconds is not consistent with the Debye rotational-diffusion model. In particular, multiple timescales of molecular reorientation were observed in liquid water. This was attributed to the hydrogen bonding network in water and the consequent presence of collective rearrangements of the molecular network. In order to better understand the origins of the complex reorientational behaviour of water molecules, we carried out molecular dynamics (MD) simulations of a liquid that has a similar molecular geometry to water but does not form hydrogen bonds: hydrogen sulfide. These simulations were carried out at T=208K and p=1 atm (~5K below the boiling point). Ensemble-averaged Legendre polynomial functions of hydrogen sulfide exhibited a Gaussian decay on the sub-picosecond timescale but, unlike water, did not exhibit oscillatory behaviour. We attribute these differences to hydrogen sulfide’s absence of hydrogen bonding.

AbstractA numerical solution to the problem of self-consistent diffusive shock acceleration is presented. The cosmic rays are scattered, accelerated and exert a back-reaction on the gas through their interaction with turbulence frozen into the local fluid frame. Using a grid with a hierarchical spacetime structure the physically interesting limit of Bohm diffusion (к ∝ pv), which introduces a wide range of diffusion lengthscales and acceleration timescales, can be studied. Some implications for modified shocks and particle acceleration are presented.Subject headings: acceleration of particles — cosmic rays — diffusion — shock waves

Les grandes éruptions caldériques sont parmi les phénomènes les plus destructeurs de la Terre, mais les processus à l’origine des grands réservoirs de magma siliceux et pauvre en cristaux qui alimentent ces éruptions ne sont pas bien compris. Le temps de stockage de ces réservoirs dans la croûte supérieure a un intérêt particulier. De longs temps de stockage—jusqu’à 105 ans—ont été estimés en utilisant les temps de repos entre les éruptions et les âges radiométriques des cristaux qui se trouvent dans les produits éruptifs. Par contre, des travaux récents sur la diffusion dans des cristaux suggèrent que les réservoirs qui alimentent même les plus grandes éruptions peuvent se mettre en place pendant une période beaucoup plus courte—101–102 ans. Afin de répondre à cette question, j’ai étudié l’éruption dacitique de Cape Riva de Santorin, Grèce (>10km3, 22 ka). Pendant les 18.000 ans précédant cette éruption, une série de dômes et de coulées dacitiques a été émise, alternant avec des dépôts de ponce dacitique (le complexe de dômes de Therasia). Ces dacites ont des compositions similaires à celle qui a été émise pendant l’éruption de Cape Riva, et ont été décrites précédemment comme des « fuites » provenant du réservoir de Cape Riva pendant sa croissance. Cependant, le magma de Cape Riva est appauvri en éléments incompatibles (tels que K, Zr, La, Ce) par rapport au magma de Therasia, une différence qui apparaît également dans les cristaux de plagioclase. Cette différence ne peut pas être expliquée par des processus peu profonds, tels que la cristallisation fractionnée ou l’assimilation de la croûte, ce qui suggère que les magmas de Cape Riva et Therasia ont des origines différentes. En outre, il existe des arguments tendant à montrer que les dacites de Therasia n’ont pas été alimentées par un réservoir majoritairement liquide ayant eu une longue durée de vie. Il y a des variations non systématiques dans la composition du magma, les compostions des bords ainsi que les caractéristiques des cristaux de plagioclase tout au long de la séquence. De plus, les temps de résidence à haute température des cristaux de plagioclase et d’orthopyroxène estimés par des modèles de diffusion sont 101–102 ans. Ces temps sont courts par rapport au temps moyen entre éruptions (1.500 ans), ce qui suggère que les cristaux observés dans chaque coulée ne se sont formés que peu de temps avant l’éruption. Les différentes teneurs en éléments incompatibles indiquent qu’un nouveau magma s’est mis en place dans le système volcanique superficiel peu de temps avant l’éruption de Cape Riva. Cet apport de magma a eu lieu après la dernière éruption de Therasia, qui s’est produite <2.800±1.400 ans avant l’éruption de Cape Riva selon les âges 40Ar/39Ar. Les périphéries des cristaux de plagioclase présents dans la dacite de Cape Riva sont en équilibre avec une rhyodacite, avec une composition similaire à celui du verre de l’éruption. Cependant, les zonations dans les éléments majeurs et traces enregistrent des changements dans la composition du liquide magmatique pendant la croissance des cristaux. La composition du centre de la plupart des cristaux de plagioclase est la même que celle des bords ; toutefois ces cristaux sont souvent partiellement résorbés, et la croissance a repris avec du plagioclase plus calcique. Ces cycles se répètent jusqu’à trois fois. La relation étroite entre la teneur en anorthite, Sr et Ti des différentes zones suggère que la composition des plagioclases est corrélé avec la composition du liquide, allant de liquides dacitiques à rhyodacitiques. Des cristaux d’orthopyroxène révèlent une séquence similaire. Les motifs de zonation sont interprétés comme un témoin de la formation du réservoir de Cape Riva dans la croûte supérieure par le mélange de plusieurs magmas ayant des compositions diverses. Des modèles de diffusion de Mg dans le plagioclase et de Fe–Mg dans l’orthopyroxène suggèrent que ce mélange a eu lieu 101–102 ans avant l’éruption
Large, explosive, caldera-forming eruptions are amongst the most destructive phenomena on the planet, but the processes that allow the large bodies of crystal-poor silicic magma that feed them to assemble in the shallow crust are still poorly understood. Of particular interest is the timescales over which these reservoirs exist prior to eruption. Long storage times—up to 105 y—have previously been estimated using the repose times between eruptions and radiometric dating of crystals found within the eruptive products. However, more recent work modelling diffusion within single crystals has been used to argue that the reservoirs that feed even the largest eruptions are assembled over much shorter periods—101–102 y. In order to address this question, I studied the >10km3, 22-ka, dacitic Cape Riva eruption of Santorini, Greece. Over the 18 ky preceding the Cape Riva eruption a series of dacitic lava dome and coulées were erupted, and these lavas are interspersed with occasional dacitic pumice fall deposits (the Therasia dome complex). These dacites have similar major element contents to the dacite that was erupted during the Cape Riva eruption, and have previously been described as “precursory leaks” from the growing Cape Riva magma reservoir. However, the Cape Riva magma is depleted in incompatible elements (such as K, Zr, La, Ce) relative to the Therasia magma, as are the plagioclase crystals in the respective magmas. This difference cannot be explained using shallow processes such as fractional crystallisation or crustal assimilation, which suggests that the Cape Riva and Therasia magmas are separate batches. Furthermore, there is evidence that the Therasia dacites were not fed from a long-lived, melt-dominated reservoir. There are non-systematic variations in melt composition, plagioclase rim compositions, and plagioclase textures throughout the sequence. In addition, high-temperature residence times of plagioclase and orthopyroxene crystals from the Therasia dacites estimated using diffusion chronometry are 101–102 y. This is short compared to the average time between eruptions (1,500 y), which suggests the crystals in each lava grew only shortly before eruption. The different incompatible element contents of the Cape Riva and Therasia magmas and plagioclase crystals suggest that a new batch of incompatible-depleted silicic magma arrived in the shallow volcanic plumbing system shortly before the Cape Riva eruption. This influx must have taken place after the last Therasia eruption, which 40Ar/39Ar dates show occurred less than 2,800±1,400 years before the Cape Riva eruption. The rims of the plagioclase crystals found in the Cape Riva dacite are in equilibrium with a rhyodacite, with a similar composition to the Cape Riva glass. However, the major and trace element zoning patterns of the crystals record variations in the melt composition during their growth. The compositions at the centre of most crystals are the same as the rims; however, these crystals are often partially resorbed and overgrown by more calcic plagioclase. The plagioclase then grades normally back to rim compositions. This cycle is repeated up to three times. The tight relationships between the anorthite, Sr and Ti contents of the different zones suggests that the composition of the plagioclase crystals correlates with the composition of the melt from which theygrew. The different plagioclase compositions correspond to dacitic and rhyodacitic melt compositions. The orthopyroxene crystals reveal a similar sequence, although they only record one cycle. These zoning patterns are interpreted to document the assembly of the Cape Riva reservoir in the shallow crust through the amalgamation of multiple batches of compositionally diverse magma. Models of magnesium diffusion in plagioclase and Fe–Mg interdiffusion in orthopyroxene suggest that this amalgamation took place within 101–102 y of the Cape Riva eruption

La croissance de la phase vésiculée, moteur de l'éruption, est contrôlée par les processus de diffusion qui permettent la migration des gaz (et notamment des gaz rares) dans les bulles. On utilise la haute volatilité des gaz rares comme traceur géochimique de l'évolution d'une phase gazeuse sans interaction chimique. Ainsi, documenter précisément les mécanismes de diffusion des différents gaz rares (He, Ne, Ar) lors de l'éruption (c'est-à-dire en fonction de la chute de température et de pression du système), permet de quantifier les phénomènes de fractionnement de la phase gazeuse. La compréhension des processus de fractionnements cinétiques, permet dès lors de prédire le temps nécessaire pour atteindre une certaine quantité de gaz rares dans une bulle (située au sein d'un système magmatique), lors de l'éjection des laves. Pour cela, la compréhension de l'influence de la température et de la structure du réseau silicaté sur les coefficients de diffusion est nécessaire. Cependant, la compréhension physique des processus de diffusion ainsi que l'évolution des coefficients de diffusion en fonction de la température, n'est pas suffisante pour dériver des temps caractéristiques d'une éruption volcanique de type Plinian. La complexité symptomatique de tels systèmes, nécessite une résolution numérique des équations de diffusion prenant en compte la dépendance des coefficients de diffusion à la température. Plusieurs verres synthétiques et naturels de composition basaltique ont été fabriqués dans le but de déterminer la vitesse de diffusion des gaz rares. Les données de diffusivités expérimentales mesurées sur ces systèmes, depuis l'état vitreux de basse température (T = 423 K) jusqu'à des températures sur-liquidus (T = 1823 K), documentent nos connaissances des processus physiques de diffusion dans ces milieux. Un modèle numérique intègre ces données et permet de suivre en continue la variation des coefficients de diffusion lors de la trempe d'une lave. On a pu ainsi montré : - La relation particulière entre la structure du milieu diffusif et les espèces diffusantes. La quantité de formateurs de réseaux (SiO2) et de modificateurs (CaO - MgO - etc.), joue sur la connectivité des chemins de diffusions de chaque gaz rare, avec un effet antagoniste entre l'ouverture globale du réseau et la connexion des tétraèdres de la structure. - La présence de comportements non-arrheniens des gaz rares proches de la Tg, due à la relaxation du réseau silicate. - L'importance des données expérimentales dans l'étude des mécanismes de dégazage des magmas basaltiques. En effet, les études précédentes utilisent des extrapolations des coefficients de diffusions, mesurés dans le verre pour extrapoler les diffusivités dans le liquide silicaté. Nos données montrent que le fractionnement cinétique des gaz rares pendant le dégazage de lave basaltique, est surestimé par ces extrapolations basées sur les vitesses de diffusions aux basses températures (T << Tg)
Noble gas geochemistry is an important tool for constraining the history of the volatile phase during magmatic eruptions. Degassing processes control the gas flux from liquid to bubble, leading to solubility- or kinetic-control of the fractionation mechanisms. Noble gases have no chemical interactions at magmatic conditions and are therefore well adapted to tracing gas fractionation mechanisms during the evolution of the gas phase. Well constrained diffusion coefficients, and their dependence on temperature, of several noble gases are critical for estimating the timescale of a plinian eruption, for example. During the quench phase of the lava ejected in the plume, atmospheric noble gases will diffuse through the liquid/glass shell surrounding gas bubbles. Diffusion of these atmospheric gases determine the gas content measured in the eruption products, which are therefore a function of the timescale of the eruption, the initial and final temperatures, the glass/liquid shell thickness and the cooling rate of the magma. Therefore, it should be possible to calculate plinian eruption timescales from noble gas fractionation patterns trapped in pumice. However, in order to perform the diffusion calculations, it is first necessary to model the diffusive system: a numerical resolution of the diffusion equations for hollow sphere geometry is required as there are no analytical solutions (for complex thermal histories such as for a plinian ash column). In order to constrain the diffusion mechanisms (He, Ne and Ar) in silicate glasses and liquids, several synthetic basaltic glasses were produced. Diffusion coefficients were measured from low temperatures (423 K) to the Tg (glass transition temperature) of the system (1005 K). These experiments allowed us to investigate the physical processes that limit diffusion in glassy media: He, Ne and Ar diffusion in silicate glasses show non-Arrhenian behavior as the Tg is approached thought to be due to structural relaxation of the silicate network itself. Complementary diffusion experiments (on He and Ar) at super-liquidus conditions (1673 K and 1823 K) provide important information on the temperature dependency of He/Ar fractionation in silicate liquids. These diffusion measurements required that a new experimental protocol was developed in order to investigate noble gas diffusivities in silicate melts. The results show that relative He and Ar diffusion (i.e. DHe/DAr) decreases with temperature, from 165 at temperatures close to the Tg to 3.2 at high (>1823K) temperature. The measured coefficient diffusions are incorporated to a numerical model of the diffusion equations for a hollow sphere geometry that were developed as a MatLab code as part of this thesis work. This enabled us to determine the likely timescales of plinian eruptions from existing noble gas measurements. These results also have important implications for mechanisms of degassing in basaltic magmas: previous work used diffusivities measured on glasses in order to extrapolate to noble gas diffusivities at magmatic temperatures. Our measurements show that kinetic fractionation of noble gases during degassing of basaltic magmas has likely been overstated because noble gas diffusion in the glass cannot be extrapolated to the liquid state

abstract: Volcanic eruptions are serious geological hazards; the aftermath of the explosive eruptions produced at high-silica volcanic systems often results in long-term threats to climate, travel, farming, and human life. To construct models for eruption forecasting, the timescales of events leading up to eruption must be accurately quantified. In the field of igneous petrology, the timing of these events (e.g. periods of magma formation, duration of recharge events) and their influence on eruptive timescales are still poorly constrained. In this dissertation, I discuss how the new tools and methods I have developed are helping to improve our understanding of these magmatic events. I have developed a method to calculate more accurate timescales for these events from the diffusive relaxation of chemical zoning in individual mineral crystals (i.e., diffusion chronometry), and I use this technique to compare the times recorded by different minerals from the same Yellowstone lava flow, the Scaup Lake rhyolite. I have also derived a new geothermometer to calculate magma temperature from the compositions of the mineral clinopyroxene and the surrounding liquid. This empirically-derived geothermometer is calibrated for the high FeOtot (Mg# = 56) and low Al2O3 (0.53–0.73 wt%) clinopyroxene found in the Scaup Lake rhyolite and other high-silica igneous systems. A determination of accurate mineral temperatures is crucial to calculate magmatic heat budgets and to use methods such as diffusion chronometry. Together, these tools allow me to paint a more accurate picture of the conditions and tempo of events inside a magma body in the millennia to months leading up to eruption. Additionally, I conducted petrological experiments to determine the composition of hypothetical exoplanet partial mantle melts, which could become these planets’ new crust, and therefore new surface. Understanding the composition of an exoplanet’s crust is the first step to understanding chemical weathering, surface-atmosphere chemical interactions, the volcanic contribution to any atmosphere present, and biological processes, as life depends on these surfaces for nutrients. The data I have produced can be used to predict differences in crust compositions of exoplanets with similar bulk compositions to those explored herein, as well as to calibrate future exoplanet petrologic models.
Dissertation/Thesis
Doctoral Dissertation Geological Sciences 2020

This work presents a series of experimental and numerical studies of macromolecular organic, inorganic and biological structures, in all instances focusing on the behavior characteristic of individual discrete molecular elements. Chapters 1 and 2, beginning on pages 1 and 31, respectively, describe fabrication, use and numerical analysis of of single-walled carbon nanotube probes for amplitude-modulation atomic force microscopy. These studies reach the conclusion that the molecular structure and nanoscale surface interaction potential unique to carbon nanotubes collectively give rise to atomic force microscopy imaging artifacts manifesting as apparent lateral topographic resolution significantly better than that predicted by the probe and sample structures.

Chapter 3 (p. 61) presents a brief review of single-molecule microscopy, describes a generalized mathematical formalism for focusing polarized visible-spectrum electromagnetic radiation, and delineates the construction of a custom two-channel scanning confocal fluorescence microscope system with single-photon detection capability for spectral- and polarization-resolved studies of individual mobile fluorophores. This Chapter includes a theory-based optical analysis of the confocal probe volume structure and photoluminescence collection efficiency from 3D-polarized single-dipole emitters. The latter analysis was aided by introducing a modified Jones formalism using non-square matrix representation for polarization state changes in the specific context of confocal optics. Proper calculation of the expected confocal probe volume dimensions was essential for accurately interpreting experimental data in the following chapter. Additionally, the quantitative understanding that followed from analysis of 3D polarization state measurement by orthogonally polarized detection channels was critical to both the interpretation of experimental data and the numerical generation of simulated data in Chapter 5.

Chapter 4 (p. 125) presents a generalized formalism for correlation analysis of the fluorescence signal collected using the two-channel microscopy system described in Chapter 3. Particular focus was directed toward the theoretical auto- and cross-correlation traces anticipated from polarization-sensitive bivariate time series of photoluminescence emission from freely-rotating transition dipoles. Chapter 4 also presents population-resolved data collected from single F¨orster resonance energy transfer fluorphore pairs conjugated to DNA oligomers as they undergo cleavage by restriction endonucleases. The endonuclease enzyme Michaelis constants KM measured for EcoRI and BglI via fluorescence burst analysis were in agreement with prior literature. The success of these experiments provide concrete confirmation of the microscope’s fluorescence emission sensitivity and detection channel selectivity in the context of single-molecule experiments.

Chapter 5 describes a polarized fluorescence correlation spectroscopy (PFCS) investigation of liquid phase rotational diffusion by colloidal CdSe semiconductor nanocrystals possessing two-dimensional nondegenerate photoluminescence transition dipoles, as well as red fluorescent protein (monomeric DsRed) and rhodamine-labeled phospholipids that possess more conventional one-dimensional fluorescence transition dipoles. The experimental PFCS data collected from these samples is in close agreement with simulated PFCS data produced by a Monte Carlo rotational diffusion numerical routine that incorporates the microscope 3D polarization state sensitivity calculated in Chapter 3.

Appendices beginning on page 221 include a matrix-based description of arbitrary 3D rotation that was used in the rotational diffusion simulations, Matlab code transcripts (p. 227), and an additional mathematical formalism based on information theoretic precepts (p. 242) for assessing directed causal relationships in bivariate time series data.

The fundamentals and best practices of passive microrheology using dynamic light scattering and diffusing wave spectroscopy are discussed. The principles of light scattering are introduced and applied in both the single and multiple scattering regimes, including derivations of the light and field autocorrelation functions. Applications to high-frequency microrheology and polymer dynamics are presented, including inertial corrections. Methods to treat gels and other non-ergodic samples, including multi-speckle and optical mixing designs are discussed. Dynamic light scattering (DLS) is a well established method for measuring the motion of colloids, proteins and macromolecules. Light scattering has several advantages for microrheology, especially given the availability of commercial instruments, the relatively large sample volumes that average over many probes, and the sensitivity of the measurement to small particle displacements, which can extend the range of length and timescales probed beyond those typically accessed by the methods of multiple particle tracking and bulk rheology.

Abstract A counterflow diffusion flame for supercritical CO2 combustion is investigated at various CO2 dilution levels and pressures by accounting for realgas effects into both thermal and transport properties. The UCF 1.1 24-species mechanism is used to account the chemistry. The nature of important non-premixed combustion characteristics such as Prandtl number, thermal diffusivity, Lewis number, stoichiometric scalar dissipation rate, flame thickness, and Damköhler number are investigated with respect to CO2 dilution and pressure. The result show that, the aforementioned parameters are influenced by both dilution and pressure; the dilution effect is more dominant. Further, result shows that Prandtl number increases with CO2 dilution and at ninety percent CO2 dilution, the difference between the Prandtl number of the inlet jets and the flame is minimal. Also, the common assumption of unity Lewis number in the theory and modeling of non-premixed combustion does not hold reasonable for sCO2 applications due to large difference of Lewis number across the flame and the Lewis number on the flame drop significantly with increase in the CO2 dilution. An interesting relation between Lewis number and CO2 dilution is observed. The Lewis number of species drops by 15% when increasing the CO2 dilution by 30%. Increasing the CO2 dilution increases both the flow and chemical timescales; however chemical timescale increases faster than the flow time scales. The magnitudes of the Damköhler number signifies the need to consider finite rate chemistry for sCO2 applications. Further, the Damköhler numbers at 90% sCO2 dilution are very small, hence laminar flamelet assumptions in turbulent combustion simulations are not physically correct for this application. Also, it is observed that the Damköhler number drops non-linearly with increasing CO2 dilution in the oxidizer stream. This is a very important observation for the operation of sCO2 combustors. Further, the flame thickness is found to increase with CO2 dilution and reduce with pressure.

Oxygen reduction in a solid oxide fuel cell (SOFC) cathode involves a non-equilibrium process of coupled mass and heat diffusion and electrochemical and chemical reactions. These phenomena occur at multiple temporal and spatial scales, from the mesoscopic to the atomistic level, making the modeling, especially in the transient regime, very difficult. Nonetheless, multi-scale models are needed to improve an understanding of oxygen reduction and guide fuel cell cathode design. Existing methods are typically phenomenological or empirical in nature so their application is limited to the continuum realm and quantum effects are not captured. Steepest-entropy-ascent quantum thermodynamics (SEAQT) can be used to model non-equilibrium processes (even those far-from equilibrium) from the atomistic to the macroscopic level. The non-equilibrium relaxation is characterized by the entropy generation, and the study of coupled heat and mass diffusion as well as electrochemical and chemical activity are unified into a single framework. This framework is used here to study the transient and steady state behavior of oxygen reduction in an SOFC cathode system. The result reveals the effects on performance of the different timescales of the varied phenomena involved and their coupling. In addition, the influence of cathode microstructure changes on performance is captured.

Hot-wire and hot-film anemometry are widely used in steady flows for instantaneous velocity measurements, and their use has been extended to velocity and wall shear stress measurements in unsteady flows. The technique of hot-film anemometry relies on the Reynolds analogy which relates the diffusion of heat to the momentum exchange. The paper investigates the applicability of the analogy in linearly varying flows. The investigation is a combination of CFD analyses using the Transition SST model and experimental measurements. Results show that, in a linearly accelerating flow, while wall shear stress increases immediately upon the onset of acceleration, heat transfer indicates a relative lag in response. A quantitative analysis of the effects of flow parameters shows that the deviant behaviour is especially pronounced with increasing acceleration and/or reduced initial flow Reynolds number. The initial deviation can be predicted using a non-dimensional parameter based on turbulence timescales and acceleration rate, thereby providing a possible solution to correcting wall shear stress measurements using hot-film anemometry in fast accelerating flows.