Transient Fronts Research Articles

Partial melting in polycrystalline ice: pathways identified in 3D neutron tomographic images

Abstract. In frozen cylinders composed of deuterium ice (Tm+3.8 ∘C) and 10 % water ice (Tm 0 ∘C), it is possible to track melt pathways produced by increasing the temperature during deformation. Raising the temperature to +2 ∘C produces water (H2O) which combines with the D2O ice to form mixtures of HDO. As a consequence of deformation, HDO and H2O meltwater are expelled along conjugate shear bands and as compactional melt segregations. Melt segregations are also associated with high-porosity networks related to the location of transient reaction fronts where the passage of melt-enriched fluids is controlled by the localized ductile yielding and lowering of the effective viscosity. Accompanying the softening, the meltwater also changes and weakens the crystallographic fabric development of the ice. Our observations suggest meltwater-enriched compaction and shear band initiation provide instabilities and the driving force for an enhancement of permeability in terrestrial ice sheets and glaciers.

The Design of a Python Library for the Automatic Definition and Simulation of Transient Ionization Fronts

In recent years, the interest in nonthermal plasma dynamics has grown significantly, within both industry and research. This has been driven by the development of several novel cold plasma technologies across a wide range of different fields, for example, for plasma medicine, chemical processing, pollution control, and surface treatment. The optimization of these technologies relies heavily upon the understanding of gas discharge plasmas: their generation, electrical characteristics, and interaction with their surroundings. Moreover, the manifestation of nonthermal plasmas in the form of streamers is of high relevance and critical importance to high voltage insulation technology, and has further significance to geophysical research concerning atmospheric discharges. The present work describes the development of the StrAFE (Streamers on Adaptive Finite Elements) package, a dedicated Python library built atop the popular open-source FEniCS finite element software, designed with the objective to simplify and to automate the solution of ionization front models. The library features support for mesh adaptivity, distributed memory parallelism, and an intuitive programming interface, while providing an exceptionally high level of user configurability. This article presents the software implementation, describes its features, and presents several code verification studies performed within simple and complex domains. It is concluded that the numerical results gained from this open-source framework are comparable to other well-established software in terms of accuracy. Therefore, it further demonstrates the great potential for open-source software to make significant contributions to technologies involving nonthermal plasmas, ionization fronts, and gas discharges.

Submesoscale variability of subsurface chlorophyll-a across eddy-driven fronts by glider observations

Reflecting the local dynamic state of the pelagic system, subsurface chlorophyll maxima (SCM) are ubiquitous in the global ocean. Here, a high-resolution glider survey was conducted in the South China Sea to study the physical and biogeochemical change across the periphery of a mesoscale cyclonic eddy. We found intense vertical dispersion of SCM associated with enhanced diapycnal mixing at the frontal zones on the edge of the eddy. Submesoscale processes were found to play important roles within the eddy-driven fronts despite the general impact of the mesoscale Ekman upwelling and downwelling. Our results also suggested that wind-driven overturns at the frontal zone could result in a stronger gravitational instability at the northern edge of the eddy leading to an enhanced vertical mixing of subsurface Chl-a. However, turbulent mixing induced by symmetric instability with energy extracted from the lateral buoyancy gradient at the northeastern edge of the eddy had caused a divergence of the subsurface chlorophyll-a at the frontal zone. Our results provide direct evidence of the interaction between mesoscale and submesoscale processes at the oceanic frontal zone. These findings also call for tight cooperation between multidisciplinary observational and modeling approaches for a better understanding of the SCM variability and associated biogeochemical transformations at transient oceanic fronts.

Stabilizing spiral structures and population diversity in the asymmetric May–Leonard model through immigration

We study the induction and stabilization of spiral structures for the cyclic three-species stochastic May-Leonard model with asymmetric predation rates on a spatially inhomogeneous two-dimensional toroidal lattice using Monte Carlo simulations. In an isolated setting, strongly asymmetric predation rates lead to rapid extinction from coexistence of all three species to a single surviving population. Even for weakly asymmetric predation rates, only a fraction of ecologies in a statistical ensemble manages to maintain full three-species coexistence. However, when the asymmetric competing system is coupled via diffusive proliferation to a fully symmetric May-Leonard patch, the stable spiral patterns from this region induce transient plane-wave fronts and ultimately quasi-stationary spiral patterns in the vulnerable asymmetric region. Thus the endangered ecological subsystem may effectively become stabilized through immigration from even a much smaller stable region. To describe the stabilization of spiral population structures in the asymmetric region, we compare the increase in the robustness of these topological defects at extreme values of the asymmetric predation rates in the spatially coupled system with the corresponding asymmetric May-Leonard model in isolation. We delineate the quasi-stationary nature of coexistence induced in the asymmetric subsystem by its diffusive coupling to a symmetric May-Leonard patch, and propose a (semi-)quantitative criterion for the spiral oscillations to be sustained in the asymmetric region.

The sensitivity of southeastern United States climate to varying irrigation vigor

AbstractFour regional climate model runs centered on the Southeast United States (SEUS) assuming a crop growing season of May through October are irrigated at 25% (IRR25), 50% (IRR50), 75% (IRR75), and 100% (IRR100) of the root zone porosity to assess the sensitivity of the SEUS climate to irrigation. A fifth run, assuming no irrigation (CTL), is used as the basis for comparison. Across all IRR runs, it is found that there is a general reduction in seasonal mean precipitation over the irrigated cells relative to CTL. This manifests as an increase in dry (0–1 mm/d) days and reduction in > 1 mm/d rainfall events. A comparative moisture budget reveals that area‐averaged precipitation over the irrigated cells displays a reduction in precipitation and runoff in IRR100 with a weaker reduction in IRR25. This is despite an increase in vertically integrated moisture convergence and local evaporation. We find that irrigation increases the lower atmospheric stability, which in turn reduces the convective rainfall over the irrigated areas. Seasonally averaged temperatures reduce over irrigated areas, with the intensity of the reduction increasing with irrigation vigor. This is largely attributed to a repartitioning of sensible heat flux into latent heat flux. There is also, however, a small increase of heat flow to deeper soil layers. Precipitation ahead of transient cold fronts is also reduced by irrigation as they pass over irrigated cells, owing to the increased stability in the lower troposphere. The intensity of this precipitation reduction becomes more intense as irrigation vigor increases. Lastly, heat waves in the SEUS are reduced in intensity over irrigated cells.

Direct observation of dynamic shear jamming in dense suspensions.

Liquid-like at rest, dense suspensions of hard particles can undergo striking transformations in behaviour when agitated or sheared. These phenomena include solidification during rapid impact, as well as strong shear thickening characterized by discontinuous, orders-of-magnitude increases in suspension viscosity. Much of this highly non-Newtonian behaviour has recently been interpreted within the framework of a jamming transition. However, although jamming indeed induces solid-like rigidity, even a strongly shear-thickened state still flows and thus cannot be fully jammed. Furthermore, although suspensions are incompressible, the onset of rigidity in the standard jamming scenario requires an increase in particle density. Finally, whereas shear thickening occurs in the steady state, impact-induced solidification is transient. As a result, it has remained unclear how these dense suspension phenomena are related and how they are connected to jamming. Here we resolve this by systematically exploring both the steady-state and transient regimes with the same experimental system. We demonstrate that a fully jammed, solid-like state can be reached without compression and instead purely with shear, as recently proposed for dry granular systems. This state is created by transient shear-jamming fronts, which we track directly. We also show that shear stress, rather than shear rate, is the key control parameter. From these findings we map out a state diagram with particle density and shear stress as variables. We identify discontinuous shear thickening with a marginally jammed regime just below the onset of full, solid-like jamming. This state diagram provides a unifying framework, compatible with prior experimental and simulation results on dense suspensions, that connects steady-state and transient behaviour in terms of a dynamic shear-jamming process.

Slow Cracklike Dynamics at the Onset of Frictional Sliding

We propose a friction model which incorporates interfacial elasticity and whose steady state sliding relation is characterized by a generic nonmonotonic behavior, including both velocity weakening and strengthening branches. In 1D and upon the application of sideway loading, we demonstrate the existence of transient cracklike fronts whose velocity is independent of sound speed, which we propose to be analogous to the recently discovered slow interfacial rupture fronts. Most importantly, the properties of these transient inhomogeneously loaded fronts are determined by steady state front solutions at the minimum of the sliding friction law, implying the existence of a new velocity scale and a "forbidden gap" of rupture velocities. We highlight the role played by interfacial elasticity and supplement our analysis with 2D scaling arguments.

Segregation and periodic mixing in a fluidized bidisperse suspension

We address the issue of segregation in bidisperse suspensions of glass beads, by using a liquid fluidized bed in the inertialess regime and an acoustic technique for acquiring the axial composition along the column. Fluidization balances the buoyancy of the particles by a constant uniform upward flow, and therefore enables long-time experiments. From the analysis of the transient segregation fronts, we have collected precise measurements on the sedimentation velocities of small and large beads, Us and Ul, in homogeneous suspensions at the same volume fraction, , for both the bead species, and for different size ratios, 1.13⩽λ⩽1.64, and solid concentrations, . Our measurements provide evidence for a difference in the sedimentation velocities, Us and Ul, over all the ranges of λ and covered. These results make one expect that a long-term fluidization should then result in a stationary segregated state, which was indeed always obtained for large enough particle size ratios, λ⩾1.43. However, at high concentration and for particles of close sizes, λ⩽1.41, we observed a surprising pseudo-periodic intermittency of slow segregation and quick mixing phases. The intermittency time is much longer than the batch sedimentation time and becomes noisy at very high concentration, for which metastable states have been observed. The origin of the mixing destabilization remains an open issue, but we note however that the domain of occurrence, λ⩽1.41, also corresponds, in our experiments, to a continuous size distribution of the particles.

Adaptive predictive collocation with a cubic spline interpolation function for convection-dominant fixed-bed processes: Application to a fixed-bed adsorption process

Interference effects and nonlinearities that are inherent in an adsorption equilibrium add challenges to the task of modeling fixed-bed adsorption processes. In particular, the interference effect tends to produce steep transient concentration fronts, which are enhanced by the high convection to dispersion ratio that is found in normal bed operation conditions. This causes spatial stiffness and renders numerical computations difficult and inaccurate. To solve this problem, we propose a novel numerical procedure using gradient-directed adaptive predictive collocation with a cubic spline interpolation function and far-side boundary conditions. The spatial domain is divided into fixed subdomains and the number of collocation points for each subdomain is adaptively adjusted according to the present location and advancement speed of the maximum gradient in each subdomain. The proposed method was applied to a fixed-bed adsorption process capturing CO 2 from a CO 2/N 2 gas mixture and its effectiveness was compared with that of cubic spline and orthogonal collocation methods with fixed interpolation points, respectively. The effects of kinetic rates on the simulation results were also investigated from the viewpoint of accuracy and CPU time.

SpS1-Digging in the solar COmosphere with NAC

The solar “COmosphere” is an enigmatic region of cold gas (temperatures as low as ~3500 K) coexisting in the low chromosphere with plasma much hotter (~7000 K). This zone probably consists of patchy clouds of cool gas, seen readily in off-limb emissions of CO 4667 nm lines, threaded by hot gas entrained in long-lived magnetic filaments as well as transient shock fronts. The COmosphere was not anticipated in classical 1D models of the solar outer atmosphere, but is quite at home in the contemporary 3D highly dynamic view, which one might call the Magnetic Complexity Zone.

Simulation of reactive nanolaminates using reduced models: I. Basic formulation

Development and testing are described of a reduced model for the simulation of transient reactions in Ni/Al nanolaminates. The model relies on a simplified description of local atomic mixing rates, expressed in terms of an evolution equation for a dimensionless time scale that reflects the age of the mixed layer. The latter is obtained from a theoretical analysis of the quasi-1D evolution equation of a conserved scalar, which also yields canonical curves for the mean composition and its relevant moments as function of the local bilayer age. The reduced model is tested against results of the detailed model of Besnoin et al. [E. Besnoin, S. Cerutti, O.M. Knio, T.P. Weihs, J. Appl. Phys. 92 (2002) 5474–5481] for different scenarios, including self-propagating reactions, and ignition thresholds. Results show that the reduced model predictions are in good agreement with those obtained using the detailed model. By achieving order-of-magnitude increase in computation efficiency, the present model reduction formalism offers a path towards modeling transient reaction fronts in multiple space dimensions.

Duration of transient fronts in a bistable reaction-diffusion equation in a one-dimensional bounded domain

The duration of transient fronts in a bistable reaction-diffusion equation in a bounded domain is considered. The speed of the front decreases exponentially with the length of the domain, and the duration increases exponentially with the domain length. The duration of the fronts generated from random initial conditions is distributed in a power-law form up to a cutoff time. The cutoff time then increases exponentially with the domain length so that the power-law distribution dominates for large domains. Further, external noise of intermediate strength increases the mean duration of the fronts. The increases in the duration with the domain length become almost linear, however, in the presence of asymmetry in the cubic nonlinearity.

Turbulence characteristics of a small subtropical estuary during and after some moderate rainfall

In natural estuaries, scalar diffusion and dispersion are driven by turbulence. In the present study, detailed turbulence measurements were conducted in a small subtropical estuary with semi-diurnal tides under neap tide conditions. Three acoustic Doppler velocimeters were installed mid-estuary at fixed locations close together. The units were sampled simultaneously and continuously at relatively high frequency for 50 h. The results illustrated the influence of tidal forcing in the small estuary, although low-frequency longitudinal velocity oscillations were observed and believed to be induced by external resonance. The boundary shear stress data implied that the turbulent shear in the lower flow region was one order of magnitude larger than the boundary shear itself. The observation differed from turbulence data in a laboratory channel, but a key feature of natural estuary flow was the significant three-dimensional effects associated with strong secondary currents including transverse shear events. The velocity covariances and triple correlations, as well as the backscatter intensity and covariances, were calculated for the entire field study. The covariances of the longitudinal velocity component showed some tidal trend, while the covariances of the transverse horizontal velocity component exhibited trends that reflected changes in secondary current patterns between ebb and flood tides. The triple correlation data tended to show some differences between ebb and flood tides. The acoustic backscatter intensity data were characterised by large fluctuations during the entire study, with dimensionless fluctuation intensity I b ′ / I b ¯ between 0.46 and 0.54. An unusual feature of the field study was some moderate rainfall prior to and during the first part of the sampling period. Visual observations showed some surface scars and marked channels, while some mini transient fronts were observed.

Equilibrium Structure and Dynamics of the California Current System

This paper addresses the structure and dynamical mechanisms of regional and mesoscale physical variability in the subtropical northeast Pacific Ocean using the Regional Oceanic Modeling System (ROMS). The model is configured with a U.S. West Coast domain that spans the California Current System (CCS) with a mesoscale horizontal resolution up to as fine as 3.5 km. Its mean-seasonal forcing is by momentum, heat, and water fluxes at the surface and adaptive nudging to gyre-scale fields at the open water boundaries. Its equilibrium solutions show realistic mean and seasonal states and vigorous mesoscale eddies, fronts, and filaments. The level of eddy kinetic energy (EKE) in the model is comparable to drifter and altimeter estimates in the solutions with sufficiently fine resolution. Because the model lacks nonseasonal transient forcing, the authors conclude that the dominant mesoscale variability in the CCS is intrinsic rather than transiently forced. The primary eddy generation mechanism is the baroclinic instability of upwelling, alongshore currents. There is progressive movement of meanseasonal currents and eddy energy offshore and downward into the oceanic interior in an annually recurrent cycle. The associated offshore eddy heat fluxes provide the principal balance against nearshore cooling by mean Ekman transport and upwelling. The currents are highly nonuniform along the coast, with important influences by capes and ridges in both maintaining mean standing eddies and launching transient filaments and fronts.

Material nano joins

Development and testing are described of a reduced model for the simulation of transient reactions in Ni/Al nanolaminates. The model relies on a simplified description of local atomic mixing rates, expressed in terms of an evolution equation for a dimensionless time scale that reflects the age of the mixed layer. The latter is obtained from a theoretical analysis of the quasi-1D evolution equation of a conserved scalar, which also yields canonical curves for the mean composition and its relevant moments as function of the local bilayer age. The reduced model is tested against results of the detailed model of Besnoin et al. [E. Besnoin, S. Cerutti, O.M. Knio, T.P. Weihs, J. Appl. Phys. 92 (2002) 5474–5481] for different scenarios, including self-propagating reactions, and ignition thresholds. Results show that the reduced model predictions are in good agreement with those obtained using the detailed model. By achieving order-of-magnitude increase in computation efficiency, the present model reduction formalism offers a path towards modeling transient reaction fronts in multiple space dimensions.

Experimental and theoretical analysis of acceleration of a gas flame propagating over a dust deposit

In the present work, we studied experimentally and theoretically the propagation of a methane-air flameand its interaction with a coal dust deposit. The experiments were conducted in a horizontal flame-acceleration tube, 1.48 m long and 89×89 mm square cross section. Photodiodes and a digital video camera were used to detect flame propagation. Flame propagation was analyzed with a one-dimensional flow model and a turbulent-burning-rate model. The results indicate that there are three distinguishable stages and an interesting propagation pattern of three transient fronts when a gaseous flame propagates over a dust deposit. In the first stage, the gas flame accelerates gradually until the dust deposits are dispersed into the air and heated by the gaseous products so that the dust ignites. In the second stage, a flame of three transient fronts—a leading gas flame, a forward dust flame, and a backward dust flame—was observed. At the last stage, a complex gas-dust flame forms when the dust flame is integrated into the gaseous flame. The results also show that, under the present conditions, the flame is accelerated slightly by the dust deposits but dramatically by obstacles. The terminal flame speed with obstacles A (blockage ratio, 0.05) or obstacle B (blockage ratio, 0.10) is about 4 to 16 times of that without obstacles. Because the dispersion of the dust is dominated by gas flow, the dust generally flies in the same direction as the flame propagation when the flame propagates from the closed end of the tube. However, the dust flies in the opposite direction when the flame travels from the open end of the tube.

Measurements of Fractal Properties of Premixed Turbulent Flames and Their Relation to Turbulent Burning Velocities

ABSTRACTThe fractal properties of premixed transient flames propagating downwards through a near-isotropic turbulent flow field in a fan-stirred cruciform burner were investigated. The long vertical section of the cruciform burner was used to provide a downward propagating premixed flame at 1 atm. The large horizontal vessel equipped with a pair of counter-rotating fans and perforated plates at each end was used to generate near-isotropic turbulence. Turbulent flame front images were obtained using high-speed laser sheet imaging for both methane-air and propane-air mixtures. The nondimensional turbulent intensity (u′/SL), Reynolds number based on the integral length scale, and turbulent Karlovitz number were varied from 1 to 10, from 698 to 6032, and from 0.05 to 1.43, respectively. Hundreds of runs for each experimental condition were carried out to obtain sufficient images of these turbulent transient flame fronts just in the central uniform region. These images were then processed to extract fractal dimension, inner and outer cutoffs using both the circle and the caliper methods. It was found that the mean fractal dimension is only 2.18, nearly independent of u′/SL, in support of recent Bunsen-flame results found by Gülder and his co-workers. This contradicts the findings of many previous studies in which the fractal dimension may approach asymptotically to a value of 2.33 when u′/SL > 3. The inner (εi) and outer (ε0) cutoffs are found to be nearly constant for all flames studied, where ε0 is an order of magnitude greater than εi and it is smaller than the integral length scale of unreacted turbulence. Finally, the present fractal characteristics cannot predict turbulent burning velocities correctly when the available fractal closure model was used, indicating a limit of the fractal analysis on prediction of turbulent burning velocities.

Elevated chlorophyll a concentrations associated with a transient shelfbreak front in a western boundary current at Sydney, south-eastern Australia

A mid-water phytoplankton bloom, measured by a five-fold increase in chlorophyll a concentrations above background levels, occurred in coastal waters off Sydney at depths of 80–100 m directly above the shelfbreak. The physical feature underpinning the bloom was a transient shelfbreak front that developed in response to the concurrent actions of an offshore-directed wind-forced bottom boundary-layer flow over the shelf and an onshore-directed bottom boundary-layer flow associated with a meso-scale oceanic eddy over the upper slope. Never previously have these transient fronts been shown to result in phytoplankton blooms on this shelf. The observations are significant because processes leading to nutrient enrichment and enhanced pelagic primary production over continental shelves on western boundaries are in many cases not well understood.

Dynamic simulation of simulated-moving-bed chromatographic processes

The simulated-moving-bed process is a powerful tool for continuous separation of multi-component mixtures in which the components have different adsorption affinities. It is suitable for a broad range of preparative or production scale applications and allows the separation of components with separation factors near unity with high resolution, yield and purity. Furthermore, the desorbent rates and the amount of adsorbent required by simulated-moving-bed processes are much lower than those for corresponding batch processes. The SMB process is subject to different parameters specifying plant size, operating conditions and process variables. Precise simulations are therefore necessary for process design and evaluation of experimental results. In this paper a detailed comparison of different modelling approaches is presented. The aim of this project is to develop a simulation system on the basis of a flowsheeting simulator like SPEEDUP TM (AspenTech, Cambridge, U.S.A.). In order to design and optimize either batch or SMB chromatographic processes it is not only necessary to describe the exact position but also the exact shape of the transient concentration fronts as a function of fluid velocity and complex component concentrations. In order to develop a simulation system which meets these demands and which is accurate enough for high-purity chromatography, appropriate models for unit operations have to be selected. Different rigorous models for batch chromatographic separations and continuous chromatography processes in terms of either moving beds with steady-state countercurrent flow or simulated moving beds with periodic fluid port switching have been evaluated. The results demonstrate that because of the real periodic flow SMB processes should be described by rigorous dynamic models with tubular reactor units and fluid port switching. Axial dispersion and mass transfer resistance have to be taken into account in order to obtain a good agreement with experimental results. The rigorous dynamic model which has been developed makes it possible to understand malfunctions of pilot plants, to optimize process conditions and to specify startup procedures for SMB processes.

Meteorological variability and air‐sea fluxes at a closely spaced array of surface moorings

Data from an array of five surface moorings, deployed in a region of transient ocean fronts with separations of between 17 and 55 km, were used to document meteorological variability at an open ocean site and to look for evidence of links between the surface meteorology and air‐sea fluxes and the gradients in sea surface temperatures. Averages over the array were also compared to historical and model‐produced data. Spatial variability in surface meteorology was observed infrequently, and much ofthat was associated with atmospheric fronts and periods of low winds. Air‐sea fluxes computed using the bulk formulae included spatial variability linked to the ocean, but spatial gradients in ocean surface velocities introduced a larger signal in the bulk formulae fluxes than did the gradients in sea surface temperature. The greatest differences between the observations and climatological data were seen in relative humidity and sea surface temperature. The bulk estimate of net longwave radiation was the heat flux component most different from the climatology. Wind stress time series from numerical weather prediction models were in best agreement with the observations at periods of a few days and longer.